Development of BODIPY-based dyes with ICT and AIE characteristics for dual-channel CO₂ detection in ionic liquid optical sensors†

Woo Jin Choi‡ ^a, Jun Ho Yoon‡^a, Tae Gyu Hwang^b, Suhyeon Kim^a, Hyun Kyu Lee^c, Wan Soo Kim^a, Seong Hyun Jang^ad, Yoo Sang Kim^a, Dong Jun Lee^a, Sang Goo Lee^b, Byeongjun Park^be and Jae Pil Kim*^af
aLab. of Organic Photo-functional Materials, Department of Materials Science and Engineering Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea. E-mail: jaepil@snu.ac.kr
^bInterface Materials and Chemical Engineering Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea
^cMaterial & Component Convergence R&D Department, Korea Institute of Industrial Technology (KITECH), Ansan, 15588, Republic of Korea
^dUser Convenience Technology R&D Department, Korea Institute of Industrial Technology (KITECH), Ansan, 15588, Republic of Korea
^eAdvanced Materials and Chemical Engineering, University of Science and Technology, Daejeon 34113, Republic of Korea
^fResearch Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

First published on 7th August 2025

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Introduction

In recent years, carbon dioxide (CO₂), a prominent greenhouse gas, has garnered increasing attention across diverse research disciplines. Its accumulation in the atmosphere contributes to climate change, making CO₂ monitoring essential in many sectors. In industrial applications, the regulation of CO₂ emissions is critical to controlling environmental pollution.¹ In the food packaging industry, elevated CO₂ concentrations inhibit microbial activity, thereby extending the shelf life of food products.² Furthermore, CO₂ concentrations are vital indicators in medical diagnostics, where they are monitored to assess physiological and pathological conditions.³ Environmental and earth sciences also rely on CO₂ detection to track changes in atmospheric and oceanic CO₂ concentrations, which influence climate systems.⁴ These wide-ranging applications highlight the need for accurate, efficient, and versatile CO₂ detection technologies.

Conventional methods of CO₂ detection, including infrared (IR) spectroscopy, gas chromatography-mass spectrometry (GC-MS), electrochemical techniques, and Severinghaus-type potentiometric sensors, have proven reliable. However, these techniques face several limitations, including bulky equipment, high energy requirements, costly maintenance, and frequent recalibration. Environmental factors such as humidity can further reduce detection accuracy.^5,6 As an alternative, optical sensors based on organic dyes have gained attention due to their portability, low power consumption, and ability to provide real-time monitoring. These sensors are also relatively inexpensive and simpler to fabricate compared to conventional devices, making them promising candidates for CO₂ detection.⁷

Research efforts have explored various strategies for CO₂ detection using optical sensors based on organic materials. One approach involves utilizing dyes that respond to pH changes caused by CO₂ dissolution in water, forming bicarbonate ions (HCO₃⁻). For example, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) has been employed to detect pH-induced optical changes.⁸ Additionally, sensors using strongly electron-withdrawing anions, such as fluorine, induce proton abstraction from dye molecules, forming lone electron pairs that interact with CO₂ and alter the optical properties of the sensor.⁹ Metal–organic frameworks (MOFs), which combine organic and inorganic components, have also been developed for CO₂ detection due to their structural versatility.¹⁰ Despite these advancements, existing sensors are limited by their reliance on auxiliary agents for CO₂ detection and their ability to detect CO₂ primarily through fluorescence channels, which often requires UV light sources.

To address these limitations, this study employs ionic liquids to create a dual-channel CO₂ sensor capable of detecting CO₂ through both colorimetric and fluorescence channels. Ionic liquids, characterized by their high electrical conductivity, thermal stability, low volatility, and ease of functionalization^11,12 have found extensive applications across a wide range of fields, including electrochemistry,¹³ chemical processing,¹⁴ food science,¹⁵ analytical chemistry,¹⁶ and as additives.¹⁷ Notably, ionic liquids exhibit a high solubility for CO₂ and possess the unique ability to interact with CO₂ without requiring external stimuli. These properties have rendered ionic liquids highly valuable in CO₂-related research and applications.¹⁸

As shown in Fig. 1, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and aliphatic alcohols typically exist in a molecular liquid form under standard conditions. Upon exposure to CO₂, DBU abstracts a proton from the alcohol, resulting in positively charged DBU and negatively charged CO₂-bound alcohol. This chemical transformation creates an ionic liquid that exhibits increased polarity and viscosity.¹⁹ Dyes sensitive to polarity and viscosity changes, such as Nile red²⁰ and Reichardt's dye,²¹ have been used to develop optical sensors based on this mechanism.²² However, many existing sensors remain limited by single-channel detection, prompting the need for improved dual-channel sensors (Fig. 2).

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is a representative example of an organic dye complexed with boron. BODIPY is known for its high molar absorptivity and strong fluorescence, attributed to its rigid molecular structure. Additionally, it shows a narrow full width at half maximum (FWHM), excellent thermal and chemical stability, ease of structural modification, and outstanding photophysical and chemical properties.²³ BODIPY has been applied in fields such as bioimaging,²⁴ organic light-emitting devices (OLEDs),²⁵ triplet–triplet annihilation upconversion (TTA-UC) sensitizers,^26,27 photodynamic therapy (PDT),²⁸ photocatalysis,²⁹ energy storage devices,³⁰ and solar cells.³¹ However, optical properties of BODIPY are insensitive to environmental polarity,³² and the dye is prone to aggregation-caused quenching (ACQ) in high-viscosity environments due to π–π interactions and excimer formation. These challenges limit effectiveness of BODIPY in optical sensors.³³ To enhance the performance of BODIPY, we designed a series of derivatives with donor–acceptor (ED–EA) structures by introducing triphenylamine (TPA) moieties with varying electron-donating strengths. This modification enhances intramolecular charge transfer (ICT), allowing the dyes to respond to polarity changes.²⁴ Additionally, when the TPA moiety possesses a rotatable structure, the dye can exhibit aggregation-induced emission (AIE), where fluorescence intensity increases due to restricted molecular motion in the aggregated state or high-viscosity environments.³⁴

In this study, we analyzed the optical properties of BODIPY dyes modified with TPA moieties with varying electron-donating powers and further investigated their CO₂ detection capabilities within an ionic liquid environment. To this end, we designed and synthesized four BODIPY derivatives (TPABDP, PTPABDP, TBTPABDP, and MTPABDP), each featuring a distinct TPA moiety, and their molecular structures are summarized in Fig. 3. To compare the ICT characteristics of the synthesized dyes, theoretical calculations, as well as absorption and emission spectra, were measured and analyzed in various solvent environments. Additionally, the AIE properties of the dyes were investigated by analyzing their fluorescence behavior in response to aggregation and environmental viscosity. Moreover, to evaluate their potential for dual-channel CO₂ detection (visual and fluorescence), the dyes were incorporated into ionic liquids, and their absorption and fluorescence spectra were measured at varying CO₂ concentrations. Color coordinate analysis was also conducted to assess visible color changes. We believe these findings provide a strong foundation for the development of high-performance molecular systems for ionic liquid-based CO₂ optical sensors.

Experimental section

Materials

The reagents used for the synthesis were purchased from Tokyo Chemical Industry (TCI) (bis(4-biphenylyl)amine: purity > 98%, 4-bromobenzaldehyde: purity > 97%, bis(4-(tert-butyl)phenyl)amine: purity > 90%; 4-(N,N-diphenylamino)benzaldehyde: purity > 98%; 4-(bis(4-methoxyphenyl)amino)benzaldehyde: purity > 98%; pyrrole: purity > 99%; triethylamine: purity > 99%; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone: purity > 97%; trifluoroacetic acid: purity > 99%) and Sigma Aldrich (Tris(dibenzylideneacetone)dipalladium(0): purity > 97%; 1,1′-ferrocenediyl-bis(diphenylphosphine): purity > 97%; sodium t-butoxide: purity > 97%; boron trifluoride diethyl etherate) used without further purification. All other organic solvents were purchased from Samchun Chemicals and used without further purification. The detailed synthetic procedures and characterization of compounds are provided in the ESI.†

Instruments

The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance III spectrometer at 600 MHz with chloroform-d as the solvent. The molecular masses of the dyes were measured using gas chromatography high-resolution time of flight mass spectrometry (GC-HRTOFMS) mass spectra collected using a JMS-T2000GC instrument. Fourier transform infrared (FT-IR) spectroscopy was performed using a Bruker TENSOR27 spectrometer equipped with an attenuated total reflection (ATR) accessory. Differential scanning calorimetry (DSC) measurements were carried out using a PerkinElmer DSC 4000 instrument. Thermal analyses were performed under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. UV-vis absorption, fluorescence spectra, and quantum yield (Φ) were recorded using a Shimadzu UV-1900i UV-vis spectrophotometer, a PerkinElmer LS 55 fluorescence spectrometer, and an Otsuka Electronics QE-1100 equipped with an integrating sphere, respectively. Time-resolved fluorescence decay analysis was conducted using a PicoQuant FluoTime 300 system, which featured a time-correlated single-photon counting (TCSPC) board and a picosecond laser (λ = 405 nm) as the excitation source. Dynamic light scattering (DLS) analysis was performed with an Anton Paar Litesizer 500. Measurements were conducted at room temperature (25 °C) and repeated three times to ensure accuracy and reproducibility. Field emission scanning electron microscopy (FE-SEM) images were obtained using a JEOL JSM-7800F Prime equipped with Pt coating, performed with a Zeiss MERLIN Compact. Color coordinates for visual color and fluorescence color were measured using a Scinco Colormate color spectrophotometer and a smartphone application (Color Picker), respectively.

Computational analysis

Time-dependent density functional theory (TD-DFT) calculations were performed using the Gaussian 16 software, employing the 6-31G(d,p) basis set and the B3LYP hybrid function. Geometry optimization, electron density distributions of frontier molecular orbitals, and energy levels of the ground state (S₀) and excited state (S₁) were analyzed using the integral equation formalism polarizable continuum model (IEFPCM) with n-hexane (Hx) and dimethylsulfoxide (DMSO) as solvents. The orbital overlap integral and hole–electron analysis were calculated using the Multiwfn 3.7 program package.³⁵

Fabrication of ionic liquid type CO₂ optical sensor

The reagents for the fabrication of the ionic liquid type CO₂ optical sensor were procured from Sigma Aldrich (ethanolamine, purity > 98%; 1-propanol, purity > 99.5%) and TCI (DBU, purity > 98%) and were used without further purification. An ionic liquid mixture was prepared by combining DBU, 1-propanol, and ethanolamine in a molar ratio of 0.3:0.3:0.1, respectively. The CO₂ optical sensor was formulated by dissolving the synthesized BODIPY dyes at a concentration of 10⁻⁴ M. CO₂ was introduced from a gas-phase source and delivered into the system through a 27G needle (15 mm length) with the tip bent at a 5° angle. The CO₂ concentration inside the sensor was measured using a portable CO₂ detector AZ 7755.

Results and discussion

Molecular design and synthesis

Previous studies reported the introduction of an electron donor (ED) at the meso position of BODIPY modifies its optical properties in response to environmental polarity.³⁶ In this study, four BODIPY derivatives—TPABDP, PTPABDP, TBTPABDP, and MTPABDP—were synthesized by incorporating electron-donating groups, including 4-(diphenylamino)benzaldehyde, 4-(di([1,1′-biphenyl]-4-yl)amino)benzaldehyde, 4-(bis(4-(tert-butyl)phenyl)amino)benzaldehyde, and 4-(bis(4-methoxyphenyl)amino)benzaldehyde, at the meso position of the BODIPY core (Fig. 3). In these structures, the electron-rich TPA moiety acts as an electron donor (ED), while the electron-deficient BODIPY core functions as an electron acceptor (EA).³⁷ The ED–EA structure present in all four dyes is expected to promote strong electronic coupling, enabling ICT behavior where optical properties change with variations in solvent polarity. Additionally, previous studies demonstrated the suppression of intramolecular rotation or motion (RIR/RIM) when a rotatable moiety is coupled to BODIPY, leading to enhanced fluorescence intensity as viscosity increases.³⁸ Since all four synthesized BODIPY dyes include a rotatable TPA unit, their fluorescence properties are anticipated to respond to viscosity changes.

Solvatochromic properties

The photophysical properties of the synthesized dyes were analyzed by measuring absorption and fluorescence spectra, Φ, and TCSPC values. Each sample was prepared by dissolving the dye at a concentration of 10⁻⁵ M. The selected solvents, in increasing order of polarity, were n-hexane (Hx), toluene (Tol), 1,4-dioxane (Dio), chloroform (CF), tetrahydrofuran (THF), dichloromethane (MC), and dimethyl sulfoxide (DMSO). Additionally, to compare the magnitude of CT absorption among the synthesized dyes, the transition dipole moment for CT absorption (M_abs) and the electronic coupling matrix (V_DA) were calculated and summarized in Table S4 (ESI†).

Fig. 4(a)–(d) illustrates the absorption spectra and optical images of the BODIPY dyes in relation to solvent polarity. The absorption spectra reveal peaks in the UV region (300–400 nm) and around 500 nm, regardless of solvent polarity. The absorption in the UV region is associated with the π–π* transition of the TPA moiety linked to the BODIPY core,³⁹ while the peak near 500 nm originates from the BODIPY core itself.⁴⁰ Except for MTPABDP, all dyes exhibited sharp and intense absorption spectra in the nonpolar solvent Hx, which became broader and weaker as solvent polarity increased. These spectral changes were primarily observed in the long-wavelength region (550–600 nm) and are attributed to CT absorption. The increased absorbance in this region with higher solvent polarity indicates strong electronic coupling between the ED and EA moieties within the molecules, confirming the occurrence of CT transitions.⁴¹ MTPABDP exhibited CT absorption even in the Hx solvent, indicating stronger coupling between the ED and EA compared to the other dyes. The changes in the absorption spectrum were accompanied by noticeable color variations in the optical images. In Hx, the dye displayed colors in the shorter wavelength region, such as yellow or orange. However, as the solvent polarity increased, CT absorption in the longer wavelength region resulted in colors shifting to red or purple.

To compare and analyze the S₀ → ¹CT band transition based on the donating power of the ED moieties of the dye, the M_abs was calculated using eqn (1).⁴²

	(1)

In the equation, h [J s] represents Planck's constant, c [cm s⁻¹] is the speed of light, N_A [mol⁻¹] is Avogadro's number, n is the refractive index of the solvent, _abs [cm⁻¹] denotes the maximum absorption of the S₀ → ¹CT band, and ε() [M⁻¹ cm⁻¹] is the molar absorptivity on the wavenumber scale. Since CT absorption was not observed for TPABDP in the Hx solvent, M_abs could not be determined. For all synthesized dyes, CT absorption increased in proportion to solvent polarity, leading to a corresponding increase in M_abs (Table S4, ESI†). This observation implies stabilization of the ¹CT state relative to the ground state with increasing solvent polarity enhances electron transitions to the ¹CT state. The M_abs values followed the same trend as the CT absorption data shown in Fig. 4, increasing in the order TPABDP < PTPABDP, TBTPABDP < MTPABDP. This trend indicates an increase in CT transitions in proportion to the donating power of the ED moiety.

To compare and analyze the degree of electronic coupling between the ¹CT state and the S₀ state as a function of the donating power of the ED moieties, the electronic coupling matrix element V_DA was calculated using CT absorption data and eqn (2).⁴³

	(2)

In the equation, R [Å] represents the distance between the centers of the ED and EA moieties, determined via geometry optimization using density functional theory (DFT) calculations. While the values varied slightly across different solvents, these differences were not significant enough to markedly influence the V_DA values. The parameter ε^CT_max [M⁻¹ cm⁻¹] denotes the maximum molar absorptivity of the S₀ → ¹CT band, v^−CT_max [cm⁻¹] is the maximum absorption wavenumber of the S₀ → ¹CT band, and Δv^−CT_1/2 [cm⁻¹] represents FWHM of the CT absorption. Since CT absorption was not observed for TPABDP in the Hx solvent, V_DA was not calculated for this case. The V_DA values for the four synthesized dyes increased proportionally with solvent polarity. In DMSO, TPABDP exhibited a maximum V_DA value of 0.26 eV, while PTPABDP, TBTPABDP, and MTPABDP showed higher values of 0.30, 0.27, and 0.35 eV, respectively (Table S4, ESI†). This trend shows how increasing solvent polarity and the donating power of the ED moiety enhance the coupling between ED and EA. The calculated M_abs and V_DA values, when correlated with the measured UV-vis spectra, reveal an increase in the donating power of the ED moiety in the order of TPABDP < PTPABDP < TBTPABDP < MTPABDP. As a result, solvent polarity-induced changes in optical properties became more pronounced following this sequence.

Fig. 5(a)–(d) presents the fluorescence spectra and optical images of the BODIPY dyes in response to varying solvent polarities. Similar to the absorption spectra, the fluorescence spectra exhibited distinct optical properties influenced by solvent polarity. In the nonpolar solvent Hx, all four dyes displayed sharp and intense locally excited (LE) emission in the shorter wavelength region, with mirror-like shapes corresponding to their absorption spectra. However, as solvent polarity increased (Tol < Dio < CF < THF < MC < DMSO), the emission shifted to the longer wavelength region, becoming broader and weaker due to CT emission. In MC or DMSO, the fluorescence intensity was too weak to detect an emission peak. The optical images, consistent with the spectral data, exhibited strong fluorescence in the short-wavelength region under nonpolar solvent conditions such as Hx. As the solvent polarity increased, the emission color gradually red-shifted and eventually underwent quenching, resulting in a transparent appearance. The Φ of TPABDP, PTPABDP, TBTPABDP, and MTPABDP in Hx were measured to be 16.50%, 60.00%, 68.87%, and 39.08%, respectively, indicating relatively high emission efficiencies. With increasing solvent polarity, Φ values showed a decreasing trend. In highly polar solvents such as MC and DMSO, fluorescence quenching was observed, and Φ values could not be determined due to the absence of measurable emission, consistent with the spectral results (Table 1).

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The reasons for the red-shift in the emission spectrum and fluorescence quenching with increasing solvent polarity are as follows. As shown in Fig. 6, dyes with an ED–EA structure typically exhibit strong fluorescence in the short-wavelength region due to LE emission in nonpolar solvents. However, as solvent polarity increases, intramolecular charge separation leads to molecular structural distortion. This process consumes significant energy, stabilizing the ¹CT state.⁴⁴ Through this process, the energy bandgap decreases, resulting in the emission of weak fluorescence in the long-wavelength region. As the electron-donating power of the ED moiety increases, the coupling between the ED and EA becomes stronger, leading to a greater reduction in the bandgap. Consequently, the degree of bathochromic shift in response to solvent polarity increases in the order of TPABDP < PTPABDP, TBTPABDP < MTPABDP, as observed.⁴⁵

As shown in Fig. S23 (ESI†), the TCSPC values of the four dyes exhibited distinct trends depending on solvent polarity. TCSPC measurements in DMSO were excluded from the analysis due to extremely weak fluorescence intensity. For TPABDP, PTPABDP, and TBTPABDP, the TCSPC data followed a single-exponential decay pattern in low-polarity solvents (Hx, Tol, and Dio), while in high-polarity solvents such as CF, THF and MC, the data exhibited a bi-exponential decay behavior. This change in the order of the exponential decay suggests alterations in the energy decay pathways of the excited state.⁴⁶ In the case of MTPABDP, a mono-exponential decay appeared only in Hx, while bi-exponential decay profiles were observed in other solvents. This behavior suggests a much stronger ICT character relative to the other dyes, leading to increased non-radiative decay into non-emissive states as solvent polarity rises. The τ values of all synthesized dyes were higher in Hx and decreased with increasing solvent polarity (from Hx to DMSO) (Table 1). To investigate the energy decay behavior, the radiative (k_r = Φ/τ) and non-radiative (k_nr = (1 − Φ)/τ) rate constants were calculated using the Φ and τ. As solvent polarity increased, k_r decreased while k_nr increased, resulting in a reduced k_r/k_nr ratio. These results indicate enhanced non-radiative energy loss in more polar solvents, which explains the observed fluorescence quenching. The photophysical properties confirmed an intramolecular push–pull interaction between the TPA unit and the BODIPY core, revealing strong ICT characteristics in the synthesized dyes.

Lippert–Mataga plot and dipole moment of excited states

In general, ED–EA structures are excited to either a LE state or a CT state, depending on the solvent polarity. While the distinction between LE and CT states can be inferred from absorption and fluorescence spectra, a more detailed understanding requires determination of the excited-state dipole moment (μ_e). When ED–EA structures undergo changes in electron density between S₀ and S₁ states, a difference between the ground-state (μ_g) and excited-state dipole moments arises. This difference can be quantitatively analyzed using the Lippert–Mataga plot.³⁶

Fig. 7 illustrates the Lippert–Mataga plots for the four synthesized dyes, constructed by plotting the Stokes shifts observed in five solvents of varying polarities against their respective solvent polarity parameters. The slope of each plot was used to calculate the difference in dipole moments between the ground and excited states. For all dyes, data from DMSO, where fluorescence was not observed, were excluded from the analysis. Additionally, for MTPABDP, the results obtained in MC solvent were also excluded from the Lippert–Mataga plot. The Lippert–Mataga equation is presented in eqn (3), and the detailed calculation process is provided in the ESI.†

	(3)

In the equation, [cm⁻¹] represents the maximum absorption wavelength, and [cm⁻¹] represents the maximum fluorescence wavelength, with their difference, [cm⁻¹], corresponding to the Stokes shift. h [J s] is Planck's constant, c [cm s⁻¹] is the speed of light, ε₀ [C² J⁻¹ m⁻¹] is the permittivity of free space, a³ [ų] is the Onsager cavity radius of the solvent, and f(ε,n) is the orientational polarizability of the solvent. Here, ε and n represent the dielectric constant and refractive index of the solvents, respectively.

Fig. 7(a) illustrates the increasing Stokes shift for all synthesized dyes as solvent polarity rises, indicating the CT nature of the S₁ state. The Stokes shift order, TPABDP < PTPABDP < TBTPABDP < MTPABDP, observed at the same solvent polarity, implies an increasing difference in dipole moments between the ground and excited states in this sequence. Onsager cavity radii, calculated based on molecular volume, were 4.50 Å for TPABDP, 5.01 Å for PTPABDP, 4.99 Å for TBTPABDP, and 4.70 Å for MTPABDP (Table S1, ESI†). The slopes of the Lippert–Mataga plots were determined as 7962, 8174, 9008, and 8193 for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively (Table S3, ESI†). These values, substituted into eqn (3), produced μ_e of 16.11 D, 17.72 D, 19.06 D, and 18.86 D for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively (Fig. 7(b)). The μ_e exceeding 10 Debye (D) typically indicates a CT state with sensitivity to solvent polarity.⁴⁷ The calculated μ_e values for all four dyes exceeded 10 D, confirming the CT nature of the S₁ state.

Theoretical analysis

To analyze the effect of solvent polarity on the charge distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states, as well as the photophysical properties of BODIPY dyes with different ED, computational studies were conducted.

Fig. 8 illustrates the energy diagram of the HOMO and LUMO levels, the energy bandgap (E_g), and the electron distribution of the frontier molecular orbitals for the optimized structures of the synthesized dyes. Under Hx solvent conditions, excitation from HOMO to LUMO exhibited CT transitions for all dyes except TPABDP, which showed an LE transition with orbitals localized on the EA moiety. The HOMO level rose with increasing ED donating strength (TPABDP → MTPABDP), reflecting the enhanced electron-donating ability of the TPA units. Accordingly, the E_g decreased from 3.11 to 2.47 eV across TPABDP, PTPABDP, TBTPABDP, and MTPABDP, indicating a clear trend of bandgap narrowing.

In DMSO, all four dyes exhibited CT transitions, with electron orbitals shifting from the ED to the EA moiety upon excitation from the HOMO to the LUMO. As in Hx, the HOMO levels increased progressively from TPABDP to MTPABDP, reflecting the enhanced electron-donating ability of the ED units. Consequently, the E_g values were calculated as 2.52, 2.43, 2.40, and 2.33 eV for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively—each lower than the corresponding values in Hx. These results demonstrate that both increased ED strength and solvent polarity contribute to E_g reduction and the observed red shift in optical spectra. The DFT calculations are consistent with the experimental data presented in Fig. 4 and 5.

To analyze the transition type of the excited state within the molecule, the orbital overlap integral between the HOMO and LUMO levels (S_HL) was calculated for each dye. Fig. S24 (ESI†) presents the S_HL values and orbital overlap topologies between the HOMO and LUMO levels of the four synthesized dyes as a function of solvent polarity. In Hx, S_HL values were calculated as 0.706, 0.305, 0.268, and 0.119 a.u. for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively. TPABDP, exhibiting LE characteristics, showed substantial HOMO–LUMO overlap and a high S_HL value, with orbital topology broadly distributed over the EA moiety (BODIPY core). In contrast, the other dyes exhibited CT behavior, resulting in lower S_HL values and more spatially separated topologies. In DMSO, S_HL values decreased further to 0.175, 0.149, 0.126, and 0.107 a.u., respectively, maintaining the same trend. The overall reduction in S_HL values in DMSO compared to Hx is attributed to the enhanced charge separation induced by the high solvent polarity, which diminishes orbital overlap.

Additionally, to investigate the electron excitation characteristics based on solvent polarity and ED donating power in the optimized S₁ state of the molecules, the overlap integral of hole and electron (S_r), charge transfer length (D), the average degree of extension of hole and electron distributions (H), and the separation degree of hole and electron (t) were analyzed. The results are summarized in Table S5 (ESI†).

The S_r values for TPABDP, PTPABDP, TBTPABDP, and MTPABDP were calculated as 0.447, 0.314, 0.252, and 0.157 a.u. in Hx and 0.169, 0.159, 0.131, and 0.112 a.u. in DMSO, respectively. These values exhibit a decreasing trend inversely proportional to the donating power of the ED moiety. The decrease in S_r values indicates an increase in the separation between the hole and electron.⁴⁸ The D index increased with both solvent polarity and the donating power of the ED moiety, reflecting a greater degree of separation between the hole and electron within the molecule as these parameters rise. In contrast, the H index exhibited a downward trend, suggesting that higher solvent polarity and stronger donating power cause the hole and electron to become more confined to narrower regions of the molecule. For all calculated dyes, the t index remained greater than zero, verifying the spatial separation of the hole and electron within the molecule and emphasizing the CT characteristics of the excited state for each dye.

To investigate the hole–electron coherence and the spatial distribution and overlap between hole and electron in the S₁ state, two-dimensional transition density matrix (TDM) plots and distribution heat maps were visualized. In addition, charge density difference (CDD) plots were created to visually represent the hole–electron separation. These results are illustrated in Fig. 9. Calculations were performed under two solvent conditions: (a–d) for Hx and (e–h) for DMSO.

In the isosurface of CDD presented in Fig. 9, blue and green regions represent the hole and electron distributions, respectively, with the isovalue set to 0.002. In Hx, TBTPABDP and MTPABDP (Fig. 9(c) and (d)) showed complete separation between the hole and electron distributions, while TPABDP and PTPABDP (Fig. 9(a) and (b)) showed partial overlap. In contrast, under DMSO conditions, all four dyes exhibited perfectly separated distributions, with the hole localized on the ED moiety and the electron on the EA moiety. The CDD analysis showed an increase in hole–electron separation with both higher ED donating power (TPABDP → MTPABDP) and greater solvent polarity.

In the TDM, the X- and Y-axes represent the atom indices of the analyzed dye, corresponding to atomic positions shown in the molecular structure of the CDD. Hydrogen atoms were excluded due to their negligible contribution to electronic transitions. The X-axis denotes the hole position, while the Y-axis indicates the electron position. For all four dyes, regardless of solvent polarity or the electron-donating strength of the ED moiety, the TDM consistently showed near-zero (dark blue) values along the diagonal and prominent off-diagonal features. This pattern indicates that no single atom simultaneously participates in both hole and electron transitions, evidencing a spatial separation between the hole and electron within the molecule.

In the heat map, the X-axis represents the atom numbers, which can be identified from the molecular structure shown in the CDD. In Hx, for all dyes except TPABDP, the hole was predominantly localized on the ED and the electron on the EA, resulting in minimal overlap. Consequently, the overlap axis displayed dark blue regions close to zero. For TPABDP, as observed in the CDD, partial local overlap of the hole and electron occurred, leading to blue-cyan regions in the EA segment (atom number > 20) of the heat map. In DMSO, all dyes showed overlap values near zero, reflecting strong charge separation due to the high solvent polarity. Theoretical calculations of solvent polarity effects on the S₀ and S₁ for all four synthesized dyes demonstrated CT transition characteristics for each dye.

Optical properties in the binary mixture (THF/H₂O and EtOH/glycerol)

To examine the AIE properties of the synthesized dyes in aggregated states and under different viscosity environments, the dyes were prepared at a concentration of 10⁻⁴ M in THF/H₂O and EtOH/glycerol binary mixtures. Fluorescence spectra and Φ and TCSPC data were measured while varying the volumetric water fraction (f_w) and glycerol fraction (f_g).

Fig. 10(a)–(d) presents the fluorescence spectra of the synthesized dyes as a function of f_w, with insets showing plots of I (fluorescence intensity in the THF/H₂O mixture) relative to I₀ (fluorescence intensity in pure THF) against f_w. In pure THF, TPABDP, PTPABDP, and TBTPABDP exhibited weak emission with maximum wavelengths of 714, 731, and 746 nm, respectively (Table S6, ESI†). MTPABDP displayed complete fluorescence quenching under the same condition, yielding no detectable spectrum. As f_w increased from 10% to 60%, all dyes experienced significant quenching, resulting in I/I₀ values approaching zero. This initial decrease is attributed to increased solvent polarity, where the excited-state energy dissipates primarily through vibrational relaxation processes.⁴⁹ However, beyond f_w = 70%, a pronounced enhancement in fluorescence intensity was observed near 700 nm. At f_w = 90%, the intensities of TPABDP, PTPABDP, TBTPABDP, and MTPABDP increased by approximately 9-fold, 10-fold, 40-fold, and 30-fold, respectively. The corresponding Φ values also rose proportionally with f_w (Table S6, ESI†). This transition from quenching to strong emission suggests AIE behavior. In highly aqueous mixtures, dye aggregation restricts intramolecular motions, thereby favoring radiative decay pathways.⁵⁰ Optical images visually confirmed this behavior, with fluorescence fading and then reappearing as bright red emission as f_w increased—consistent with the spectral observations (Fig. S25(a–d), ESI†).

To confirm dye aggregation at high f_w, DLS and SEM analyses were performed. As shown in Fig. S26 (ESI†), no particle size distribution was detected in pure THF for any of the dyes, indicating complete dissolution and molecular dispersion. At f_w = 90%, clear particle size distributions emerged, with average sizes of 230.57 nm, 147.41 nm, 187.49 nm, and 212.06 nm for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively. SEM images (Fig. S27, ESI†) further supported these findings, revealing nanoaggregates for all four dyes. These results confirm that aggregation is responsible for the observed changes in fluorescence behavior at higher f_w values.

Fig. S28(a–c) (ESI†) show multi-exponential decay for TPABDP, PTPABDP, and TBTPABDP in pure THF, with a reduced number of decay components observed as f_w increased to 90%, indicating simplified relaxation pathways. In contrast, MTPABDP (Fig. S28(d), ESI†) maintained a consistent multi-exponential profile regardless of f_w, reflecting persistent non-radiative decay. The τ values for all dyes generally increased with f_w, showing enhancements up to 5.37-fold (Table S6, ESI†), consistent with suppression of non-radiative pathways in aggregated states.⁵¹ However, MTPABDP exhibited an opposing trend, with τ decreasing from 1.148 ns (THF) to 0.296 ns (f_w = 90%), indicating dominant non-radiative decay despite aggregation. k_r and k_nr values further supported this, with TPABDP, PTPABDP, and TBTPABDP exhibiting increasing k_r and decreasing k_nr at high f_w, thus enhancing the k_r/k_nr ratio and confirming improved fluorescence via RIM. In contrast, MTPABDP showed the opposite trend, reflecting its stronger ICT nature and higher non-radiative energy loss.

The fluorescence spectra and photophysical properties of the dyes in EtOH/glycerol mixtures were evaluated as a function of f_g, as shown in Fig. 11 and Table S7 (ESI†). Insets in the graphs in Fig. 11 display plots of I (fluorescence intensity in the EtOH/glycerol mixture) normalized by I₀ (fluorescence intensity in pure EtOH) versus the f_g. All dyes exhibited fluorescence quenching within the f_g = 0–60% range, attributed to unrestricted intramolecular motion promoting non-radiative decay.⁵² The I/I₀ plots also approached zero in this range, confirming the quenching behavior. However, for f_g > 70%, fluorescence intensity increased markedly in the 600–800 nm region, peaking at f_g = 90%. At this point, I/I₀ enhancements of 45-fold (TPABDP), 100-fold (PTPABDP), 170-fold (TBTPABDP), and 16-fold (MTPABDP) were observed. The strong viscosity sensitivity of PTPABDP and TBTPABDP was attributed to the increased number of rotatable sites in their ED moieties.⁵³ Although MTPABDP also possessed rotatable ED units, its strong ICT behavior likely suppressed fluorescence. Φ values followed similar increasing trends with f_g (Table S7, ESI†). Optical images (Fig. S25(e–h), ESI†) confirmed these spectral results, showing bright red fluorescence at higher f_g values.

Fig. S29 (ESI†) presents the TCSPC graphs of each dye as a function of f_g. In Fig. S29 (ESI†), all four dyes exhibited multi-exponential decay profiles in pure EtOH. Increasing viscosity caused a shift to lower-order exponential profiles, indicating changes in energy decay pathways. The τ values for all dyes decreased inversely with rising f_g (Table S7, ESI†). In pure EtOH, the absence of measurable Φ prevented k_r calculations, confirming the release of most excited energy through non-radiative decay pathways. As f_g increased, k_r showed an upward trend, k_nr decreased, and the k_r/k_nr ratio increased. These results indicate increased solution viscosity enhances RIM, enabling relaxation through radiative decay processes.⁵⁴ The findings demonstrate fluorescence enhancement through RIM induced by aggregation or viscosity increase, validating the AIE characteristics of all four dyes.

Photophysical properties of the CO₂ sensor

To evaluate the CO₂ sensing performance of the synthesized dyes, optical sensors were fabricated by dissolving each dye (10⁻⁴ M) in an ionic liquid mixture. CO₂ gas was introduced incrementally in 1000 ppm steps, and corresponding absorption and fluorescence spectra, Φ, τ, and TCSPC data were recorded.

Fig. 12(a)–(d) presents the absorption spectra of the sensors as a function of CO₂ concentration. Upon CO₂ exposure, all sensors exhibited a red-shift of approximately 20 nm in the main absorption peak, accompanied by an increase in peak intensity. Specifically, the absorption intensities increased by factors of 6.2 (TPABDP), 6.2 (PTPABDP), 5.8 (TBTPABDP), and 4.9 (MTPABDP). In the 520–600 nm region, CT absorption also intensified. These spectral changes were visually reflected by a transition in sensor color from pale yellow to vivid red. The red-shift and intensity increase were more pronounced in dyes with stronger ICT character (MTPABDP > TBTPABDP > PTPABDP > TPABDP), consistent with greater molecular planarity and enhanced intermolecular interactions under increased viscosity conditions in the ionic liquid mixture.^55–57 To assess the CO₂ concentration dependency, the CT band absorption intensity (A) was normalized to the CT band absorption intensity of the pristine state (A₀) and plotted against CO₂ concentration (see Fig. S30, ESI†). A linear relationship was observed across all sensors, with maximum A/A₀ values of 19.7, 11.8, 10.0, and 7.7 for TPABDP, PTPABDP, TBTPABDP, and MTPABDP, respectively. Notably, dyes with stronger ED donating power exhibited smaller A/A₀ increments, likely due to their initially elevated CT absorption in the absence of CO₂. The linear correlation coefficients (R²) ranging from 0.8 to 0.9 indicate a strong linear dependence of CT absorption enhancement on CO₂ concentration. These results demonstrate the potential of ICT-type BODIPY dyes in ionic liquid systems as high-performance, tunable optical sensors for CO₂ detection.

Fig. 12(e)–(h) presents the fluorescence spectra and optical images of the sensors under various CO₂ concentrations. In the absence of CO₂, all sensors showed emission peaks in the 400–450 nm range with bright blue fluorescence in the optical images. As CO₂ concentration increased, no significant spectral shifts occurred initially; however, fluorescence intensity decreased due to the ICT nature of the dyes. This change manifested as a reduction in brightness in the optical images without noticeable color variation. For TPABDP, PTPABDP, and TBTPABDP sensors, distinct spectral transitions appeared at certain CO₂ concentrations. The TPABDP-based sensor exhibited a new emission peak at 550 nm starting at 4000 ppm, with intensity increasing proportionally to CO₂ concentration, accompanied by a visual change to bright green fluorescence. The PTPABDP and TBTPABDP sensors showed new peaks at 500 nm and 550 nm, respectively, emerging at 6000 ppm and 5000 ppm. These were accompanied by proportional increases in emission intensity and visible transitions to bright cyan and light green fluorescence. MTPABDP showed spectral changes only above 7000 ppm. Below this threshold, fluorescence quenching dominated without peak shifts. A new emission peak appeared at 530 nm, but strong ICT-induced quenching resulted in weak fluorescence, limiting visual detectability. To evaluate the relationship between fluorescence response and CO₂ concentration, fluorescence intensity (I) was normalized to the pristine state (I₀), and linear fitting was applied (Fig. S31, ESI†). TPABDP, PTPABDP, and TBTPABDP sensors showed initial decreases in I/I₀ up to 3000, 6000, and 5000 ppm, respectively, followed by increases attributable to aggregation-induced emission (AIE) driven by viscosity changes in the ionic liquid. Conversely, MTPABDP continued to exhibit decreasing I/I₀ values due to its strong ICT effect, which suppressed AIE even at high CO₂ concentrations. All sensors yielded R² values between 0.8 and 0.9, indicating strong linear correlations between CO₂ concentration and fluorescence response.

Fig. S32 (ESI†) presents TCSPC waveforms of dye-based sensors under increasing CO₂ concentrations. For TPABDP, PTPABDP, and TBTPABDP sensors (Fig. S32(a–c), ESI†), multi-exponential decay profiles observed in the pristine state gradually shifted to single-exponential decay as CO₂ concentrations increased This change indicates a reduction in energy decay pathways as CO₂ concentration increases. In contrast, MTPABDP (Fig. S32(d), ESI†) retained a multi-exponential profile even at elevated CO₂ concentrations, suggesting dominance of non-radiative decay pathways due to strong ICT behavior despite increased viscosity. The τ values of TPABDP, PTPABDP, TBTPABDP, and MTPABDP increased from 1.9–2.0 ns in the pristine state to 2.8–3.4 ns with rising CO₂ concentrations. Φ values also increased by 1.6-, 2.5-, 1.5-, and 4.5-fold, respectively. For TPABDP and TBTPABDP, small changes in Φ led to a decrease in k_r, but a more substantial reduction in k_nr caused the k_r/k_nr ratio to rise. For PTPABDP and MTPABDP, increasing k_r and decreasing k_nr resulted in a stronger enhancement in the k_r/k_nr ratio (Table S8, ESI†). Overall, enhanced fluorescence properties were confirmed across all sensors as CO₂ concentration increased. This behavior is attributed to RIM effects in the viscous sensor environment, which suppress non-radiative pathways and promote radiative emission. Optical and photophysical analyses confirmed the responsiveness of BODIPY-based dyes to CO₂ through polarity- and viscosity-induced changes in the ionic liquid mixture.

Color studies of the CO₂ sensor

To quantitatively investigate color changes of the sensors in both visual and fluorescence channels as a function of CO₂ concentration, color coordinates (x,y) corresponding to each CO₂ concentration were plotted for each dye on the Commission Internationale de L’Eclairage (CIE) 1931 chromaticity diagram. The results are depicted in Fig. S33 (ESI†).

Fig. S33(a–d) (ESI†) depict the visual color coordinates of the sensors as a function of CO₂ concentration. In their pristine states, all four sensors exhibited coordinates within x = 0.33–0.36 and y = 0.35–0.38, corresponding to white or pale yellow regions. Upon exposure to increasing CO₂ levels, the coordinates gradually shifted toward the red region. Specifically, TPABDP shifted to x = 0.3858, y = 0.3613; PTPABDP to x = 0.4120, y = 0.3565; TBTPABDP to x = 0.4686, y = 0.3496; and MTPABDP to x = 0.4601, y = 0.3296. The magnitude of the shift correlated with the electron-donating strength of the substituent groups, with stronger donors producing greater shifts. These visual color shifts corresponded well with the optical images shown in Fig. 12(a)–(d), confirming a strong relationship between CO₂ concentration and sensor color in the visual channel.

Fig. S33(e–h) (ESI†) shows the chromaticity coordinates derived from the fluorescence spectra. Initially, all sensors exhibited blue emission, with coordinates located in the blue region of the diagram. As CO₂ concentration increased, the coordinates shifted toward the center. Except for MTPABDP, the dyes displayed shifts extending into the green region. At high CO₂ concentrations, TPABDP and TBTPABDP exhibited greenish-yellow fluorescence (x = 0.24–0.25, y = 0.44), while PTPABDP emitted bright cyan-to-sky-blue fluorescence (x = 0.12, y = 0.36). MTPABDP showed relatively small shifts due to fluorescence quenching, with coordinates moving to the white region (x = 0.24, y = 0.30). These fluorescence color changes aligned well with the visual fluorescence images in Fig. 12(e)–(h). Collectively, these results confirm consistent and distinguishable CO₂-responsive color changes across both visual and fluorescence channels.

To evaluate the color variation induced by the dyes added to the ionic liquid, the color difference value (ΔE), representing the relative difference between two colors, was calculated using eqn (4).⁵⁸ The results are presented in Fig. 13.

	(4)

In this equation, ΔL, Δa, and Δb represent the differences in brightness, red/green values, and yellow/blue values, respectively, between two colors. A ΔE indicates complete visual distinction, allowing the two colors to be perceived as entirely different.⁵⁹

In the visual channel, ΔE values between the pristine state and 10000 ppm CO₂ exceeded the perceptibility threshold of 3.3 for all sensors, confirming distinct color changes upon CO₂ exposure. The ΔE values increased with the electron-donating strength of the ED moiety, with MTPABDP showing the highest ΔE of 56.22, attributed to its strong CT absorption. In the fluorescence channel, all sensors also exhibited ΔE values above 3.3, indicating clear emission color changes. However, as the electron-donating ability increased, fluorescence quenching became more pronounced due to increased environmental polarity. As a result, TPABDP exhibited the highest ΔE, while MTPABDP showed the lowest, reversing the trend observed in the visual channel. These results provide a quantitative comparison of CO₂-induced colorimetric and fluorescent responses and highlight the potential of these dyes as dual-mode CO₂ sensors.

Conclusion

This study focused on developing and optimizing BODIPY-based dyes for dual-channel CO₂ detection in ionic liquid sensors, enabling both visual and fluorescence-based detection. Four derivatives—TPABDP, PTPABDP, TBTPABDP, and MTPABDP—were synthesized by incorporating electron-donating groups with varying strengths. Experimental and computational analyses revealed strong CT characteristics in all dyes, with increased coupling strength and CT absorption shifts correlating with the donating power of the electron donor. The dyes exhibited AIE properties, with optical behavior influenced by aggregation state and environmental viscosity. CO₂ sensing performance demonstrated pronounced color changes in the visual channel for MTPABDP due to its strong ICT characteristics, while TBTPABDP and TPABDP showed the greatest fluorescence enhancement in the fluorescence channel. Colorimetric and fluorescence analyses indicated improved visual detection with stronger ICT properties, though fluorescence quenching reduced sensitivity in the fluorescence channel. This work highlights the importance of electron donor strength in balancing detection performance across channels. Future studies will focus on enhancing fluorescence detection through improved AIE characteristics and developing sensor films for broader applications. These findings provide a foundation for advancing dual-channel ionic liquid-based CO₂ sensors for industrial use.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work was supported by Samsung Display Co., Ltd as part of the industry-academia cooperation project (0414-20240108). This ware also supported by the Korea Planning & Evaluation Institute of Industrial Technology (KEIT) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (RS-2023 00267510), and supported by Korea Research Institute of Chemical Technology (KRICT) Basic Research Fund (project number KS2521-30). We thank R. D. Lee of the Research Facilities Center at the Catholic University for assistance with the DSC measurements. The Institute of Engineering Research at Seoul National University provided research facilities for this work. Fluorescence lifetime measurements facilities for this work were provided by the Research Institute of Advanced Materials (RIAM) of Seoul National University. We also thank H. N. Bae, S. H. Shin, and S. Y. Kim of the National Center for Inter-University Research Facilities (NCIRF) for assistance with NMR, GC-HRTOFMS, and SEM image measurements.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00568j

‡ These authors contributed equally to this work.

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