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DOI: 10.1039/D0NR00306A (Communication) Nanoscale, 2020, 12, 6227-6232

Single-photon upconversion in 6-pentaceneone crystal from bulk to ultrathin flakes†

Yue-E Huang‡ a, Xing-Zhi Wang‡ e, Peng Hu‡ c, Xing-Hui Qi b, Xiao-Ying Huang b, Christian Kloc d, Xiaohui Wu *a and Ke-Zhao Du *a
aCollege of Chemistry and Materials Science, Fujian Provincial Key Laboratory of Advanced Materials Oriented Chemical Engineering, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, China. E-mail: sherrywu@fjnu.edu.cn; duke@fjnu.edu.cn
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People's Republic of China
cSchool of Physics, Northwest University, Xi'an 710069, China
dSchool of Materials Science & Engineering, Nanyang Technological University, 639798, Singapore
eDivision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore

Received 11th January 2020 , Accepted 19th February 2020

First published on 19th February 2020

Abstract

Phonon-assisted single-photon upconversion, which was not previously reported in organic materials, has been demonstrated in the 6-pentaceneone crystal through the linear pumping power dependent anti-Stokes photoluminescence (ASPL), nanoseconds PL lifetime and quenched ASPL at low temperature. Furthermore, the 6-pentaceneone crystal can be mechanically exfoliated to ultrathin flakes and it exhibits thickness-dependent photoluminescence.

In the anti-Stokes photoluminescence (ASPL) (or photon upconversion) process photons of higher energy than the excitation photons would be emitted. This feature makes the ASPL technologically critical for diverse applications such as photothermal therapy,1 biological imaging2 and extended range of photon absorption in solar cells.3 Because of the design flexibility and feasible integration of organic molecules, the organic ASPL materials have attracted considerable interest.4 Several mechanisms are involved in the organic ASPL process, consisting of triplet–triplet annihilation (TTA) in the sensitized systems,5 two-photon absorption (TPA) in rubrene6,7 and multi-photon absorption (MPA) in (E)-3-(4-(2-(1-hexyl-4-methyl-1H-imidazol-5-yl)vinyl)pyridinium-1-yl)propyl sulphate.8 The phonon-assisted absorption is a unique ASPL mechanism. In comparison with TTA, TPA and MPA, the phonon-assisted ASPL only needs single-photon absorption. The single photon absorption requires a much lower pumping power and holds a higher theoretical quantum efficiency, which would be beneficial for applications including biological imaging with low laser damage9 and optical refrigeration.10,11

However, the phonon-assisted ASPL was only reported in inorganic crystalline materials before.12–14 It has not been found in the organic fluorophores including the disc-shaped organic system with aggregation-caused quenching and rotor-like organic system with aggregation-induced emission.15 Because the condensed disc-shaped organic molecules would tend to form an excimer resulting in aggregation-caused quenching,16 dispersion into matrices (solvents,17 polymers18 and metal–organic frameworks19) is necessary for the organic molecules to exhibit ASPL. Even for the rotor-like organic system, phonon-assisted process is still absent in its ASPL mechanisms, where TPA, MPA and TTA are the dominant processes.16,20 As mentioned above, the ASPL of organic fluorophores is mostly studied in the dispersion system.9 In comparison with the dispersion system, the advantage of using a crystalline solid for ASPL study is that it can increase the molecular packing to the maximum extent with minimum molecular motions, and then lead to a higher quantum yield when the excimer effect can be neglected.21 Furthermore, the crystalline material can provide the exact information about the atom arrangement and molecular packing, which would facilitate the material design under the guidance of the structure–property relationship. Thus, phonon-assisted ASPL in the organic crystalline solid has a promising research prospect.

Acenes with linearly fused benzene rings are important organic materials in electronic and optical applications.22–24 For example, pentacene is a classic organic semiconductor, which could be used as a superior field-effect transistor25,26 and an attractive photovoltaic architecture due to its efficient singlet fission.27,28 However, pentacene is very sensitive to light due to its narrow bandgap (1.8 eV) and susceptible to oxygen because of its high HOMO energy level (−5.0 eV).26 The electronic and optical properties can be greatly modified by functionalizing pentacene.29–31 For example, oxidized pentacenes with improved stability (i.e., 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O)) have been applied as photocatalysts for water splitting32 and as cathodes in lithium batteries33. The disadvantage of pentacene-quinone is its large bandgap (close to the insulator), which is unfavorable for the desired semiconductor applications. Thus, pentaceneone (e.g., 6-pentaceneone (P1O)) instead of pentacene-quinone might demonstrate a moderate bandgap and enhanced stability. So far, the P1O molecule has been synthesized as an intermediate product during the synthesis of peripentacene.34 Nevertheless, the structure and fluorescence property of the P1O crystal are still undeveloped.

Herein, the dark green P1O crystal (Fig. 1A) with determined crystal structure was obtained through physical vapor transport (Fig. S1†). Without any sensitizers, the P1O crystal shows a strong ASPL, which might arise from the phonon-assisted process based on the study of the transient- and steady-state photoluminescence (PL). In addition, the organic bulk P1O crystal can be mechanically exfoliated to ultrathin flakes with an atomically flat surface, and the thickness effect on the PL was investigated.


image file: d0nr00306a-f1.tif
Fig. 1 (A) Images of a 1O crystal under natural light (left) and under white LED irradiation (right); (B) the overall packing structure of the P1O crystal. (C)The Hirshfeld surfaces (dnorm) of the P1O surrounded by neighboring molecules; the C⋯C (D) and O⋯H (E) interaction in the 2D fingerprint plots for P1O according to the Hirshfeld surface calculations.

As a result of the oxygen- and photo-sensitive character of pentacene, a green impurity is usually found on the surface of the commercial pentacene35 and it has a great impact on the property of pentacene.36,37 However, there is no report on the topic of green materials so far. We have collected the dark green crystal for further study by purifying the commercial pentacene through the physical vapor transport method. Through characterization by single crystal X-ray diffraction, we found that the dark green crystal is composed of P1O molecules crystallized in the P21 space group (CCDC 1907538, Table S1†). There are two independent P1O molecules in the asymmetric unit. The molecular packing of the P1O crystal is herringbone motif in the unit cell, which is similar to the packing motif of the pentacene crystal (herringbone), but quite different from the those of the P2O crystal (slip-stacked/crisscross) and the P4O crystal (slip-stacked).38 Generally, the P1O molecules in the structure can be divided into two groups (Fig. 1B). In one group, the molecules line up in parallel with an intermolecular distance of 3.38 Å (Fig. 1B). In the other one, the molecules stack crosswise with an intersection angle of 49.2° (Fig. 1B). C–H⋯O hydrogen bonds, connecting the two independent P1O molecules into a molecular pair, can be found within the structure (Table S2 and Fig. S2A†). In addition, the Hirshfeld surface calculations are performed to analyze the molecular interactions in detail (Fig. 1C).39,40 The distance from a point on the Hirshfeld surface to the nearest nucleus outside the Hirshfeld surface is defined as de, while the distance from a point on the Hirshfeld surface to the nearest nucleus inside the Hirshfeld surface is defined as di. Thus, the strongest C⋯C interaction (di = de around 1.8 Å) in the 2D fingerprint plot should occur between the parallel molecules (d = 3.38 Å) (Fig. 1B and D). The two horns in the O⋯H fingerprint plot are consistent with the hydrogen bond interaction given in Table S2† (Fig. 1E). Finally, the P1O molecules are assembled into a van der Waals’ crystal. The experimental powder X-ray diffaraction (PXRD) pattern of the P1O crystal can match well with the theoretical one, confirming the phase purity of the title compound (Fig. S3†). The preferred orientation peaks (0l0) suggest that the organic layers should pack along the b-axis as shown in Fig. S2B.†

The deep color of the crystalline P1O is in agreement with the intense optical absorption ranging from 300 to 660 nm and the weak absorption ranging from 660 to 850 nm, which can be ascribed to the inter- and sub-band transition, respectively (Fig. 2A). In the optical absorption spectrum of P1O, three equidistant peaks including 618 nm (2.00 eV), 572 nm (2.17 eV) and 532 nm (2.33 eV) can be attributed to the 0–0, 0 → 1 and 0 → 2 transition, respectively. Under white LED irradiation, the change in the crystal color from dark-green to red (Fig. 1A) obviously indicates the PL property of the P1O crystal. The strong emission spectrum of P1O can be fit into three equidistant peaks as well (i.e., 645 nm (1.92 eV), 680 nm (1.82 eV), and 716 nm (1.73 eV) in Fig. 2A), which are contributed by 0–0, 1 ← 0 and 2 ← 0 transition corresponding to the three absorption peaks, respectively. Furthermore, the temperature dependent PL has been studied (Fig. S4†). As the temperature increases, the PL intensity of the 0–0 peak becomes lower while those of the replica PL peaks (i.e. 1 ← 0 and 2 ← 0 emission) remain almost constant (Fig. 2B). This phenomenon is the same as that in the case of the tetracene crystal with the herringbone packing structure, which is attributed to the different thermal scattering effects on the different PL peaks (i.e., 0–0, 1 ← 0 and 2 ← 0).41 Because the PL measurements were performed in the high-temperature range (>60 K), the intensity of 0–0 emission could be described by the Arrhenius equation:42

 
image file: d0nr00306a-t1.tif(1)
where I0 represents the initial PL intensity of 0–0 emission at low temperature, k is the Boltzmann constant, a is the process rate parameter and EA is the activation energy representing the free exciton luminescence quenching. EA is calculated to be 16.9 meV through fitting the curve of temperature dependent 0–0 peak intensity based on the deformed Arrhenius equation (inserted in the graph shown in Fig. 2C), which is comparable with that of the inorganic semiconductor GaN (21–27 meV).43


image file: d0nr00306a-f2.tif
Fig. 2 (A) Optical absorption and PL spectra of the P1O crystal excited by 514 nm laser. The dash lines are the fit curves using a Gaussian function; (B) the temperature dependent PL intensity of the 0–0, 1 ← 0 and 2 ← 0 peaks (532 nm excitation with 0.05 mW). The details of fit curves are shown in Fig. S4;† (C) the temperature dependent intensity I(T) of 0–0 emission in P1O fit by deformed Arrhenius equation that is inserted in the graph.

Following the molecular energy level study by optical absorption and temperature-dependent PL, the excitation-wavelength-dependent spectra were also obtained as shown in Fig. 3A. It should be noted that an obvious ASPL phenomenon is found to occur in the P1O crystal. The ASPL spectra excited by 671 nm, 693 nm, 700 nm and 720 nm lasers contain the same peaks composed of 0–0, 1 ← 0 and 2 ← 0 as the Stokes PL excited by 532 nm laser. A new peak around 800 nm begins to appear under 693 nm excitation and becomes more distinct, and dominant under the red-shifting excitation wavelength. The emerging PL peak centered at 800 nm might relate to the sub-band emission, which can be confirmed by the small bulging curve in the absorption spectrum (from 660 nm to 850 nm) as shown in Fig. 2A. To understand the ASPL mechanism of the P1O crystal, we performed the pumping power dependent ASPL using 671 nm, 693 nm and 700 nm lasers at room temperature (Fig. 3B–D). Based on a simple rate equation model,44,45 the ASPL intensity IPL is proportional to the pumping power IPU with power n, i.e., IPL ∝ IPUn. However, a nonintegral value of n will be expected because this model does not reckon the depletion of the ground-state populations in. The values of n are determined separately as 0.92 for 671 nm excitation (below 5.34 mW), 0.98 for 693 nm excitation (below 2.33 mW) and 1.03 for 700 nm excitation (below 3.44 mW). After performing the above-mentioned laser experiments, the surface appearance of the sample did not change at all, suggesting good light stability under measurement conditions (Fig. S5†). According to the linear pumping power dependent ASPL intensity of P1O, the TPA and MPA mechanisms for ASPL can be excluded in the low pumping power region.


image file: d0nr00306a-f3.tif
Fig. 3 (A). The normalized PL spectra of the P1O crystal with different excitation wavelengths varying from 532 nm to 784 nm at ambient temperature; (B)–(D): log–log plot of the integrated PL intensity of P1O crystal versus pumping power of 671 nm laser (B), 693 nm laser (C) and 700 nm laser (D).

Moreover, the PL decay curve of the crystalline P1O is fit by a single exponential equation with 2.329 ns PL lifetime (Fig. 4A). Because the TTA diagram contains the triplet excited state with a long lifetime process, the typical decay time in TTA is of the order of several microseconds or beyond.46 Thus, the short PL lifetime of the crystalline P1O should not be derived from TTA. In addition, we performed the ASPL measurement of the P1O crystal at 77 K (Fig. 4B), but no obvious ASPL phenomenon was found to occur. The quenched ASPL at low temperature could be due to the less phonons at low temperature, which resembles the case of an inorganic semiconductor.13 Inferred from all the results above (i.e., linear pumping power dependent ASPL, nanoseconds PL lifetime and quenched ASPL at low temperature), the ASPL in the crystalline P1O should involve with the phonon-assisted mechanism at low power intensity. It should be noted that a similar single-photon upconversion mechanism named hot-band absorption has been reported in a rhodamine 101 solution.47 However, the single-photon hot-band absorption has only been reported in the dye-dispersed systems (e.g., solution and polymer).


image file: d0nr00306a-f4.tif
Fig. 4 (A) Time-resolved PL curve of the P1O crystal; (B) the ASPL intensity of P1O excited by 671 nm laser at 293 K and 77 K. The ASPL intensity at 77 K is magnified 50 times; (C) the scheme of absorption, PL and ASPL processes in the P1O crystal.

Overall, the phonon-assisted single-photon ASPL process in the P1O crystal is proposed in Fig. 4C: the phonon assists the electrons in internal conversion from the lowest vibration-rotation level 0 to the higher levels (1 and 2) in the singlet-ground-state S0; subsequently, the electrons are excited from the high vibration-rotation levels of S0 to the singlet-excited-state S1 by incident photons; finally, the recombination of excitons results in the emission of a strong ASPL. Thus, the ASPL in the P1O crystal would lead to phonon annihilation, which indicates that laser cooling is possible under high enough quantum yields.

All the above-mentioned research was performed using bulk crystals. To extend the application of organic semiconductors in the miniaturized device, the thickness of bulk crystals should be reduced conveniently. Mechanical cleavage, which can be used to obtain clean samples with high crystallinity and atomically flat surface, is a typical method for fabricating micro- or nano-sized materials from the two-dimensional inorganic crystals including graphite and black phosphorus. Though mechanical exfoliation has also been introduced into the molecular crystals,48 only the crystal with small cleavage energy can be exfoliated to ultrathin samples.49 Fortunately, the bulk P1O crystal in our work can be exfoliated on the Si substrates with 285 nm SiO2 using Scotch tape, indicating the low cleavage energy of the bulk P1O crystal. The mechanical exfoliation makes it feasible to evaluate the impact of thickness on the PL property of the P1O crystal. As shown in Fig. 5A, five platelets with different thickness exhibit extremely distinct colors under an optical microscope. Through characterization by atomic force microscopy (AFM), we determined the average height to be 1.2 μm for region d, 317 nm for region c, 212 nm for region b and 104 nm for region a. The thickness of region e is much greater than 1.2 μm which is out of the optimal detection range of AFM. The signal-to-noise ratio for the ASPL is much weaker than that for the PL, which is particularly obvious in the thin sample as shown in Fig. S6.† Therefore, the thickness-dependent PL has been studied to understand the thickness effect on the optical property. As shown in Fig. 5B, the PL intensity becomes higher with the increasing thickness of the five regions, which should be attributed to the larger optical absorption of the thicker sample. In addition, there is an obvious red shift (6 nm) of the highest PL peak between the 104 nm sample and the bulk sample because of the reabsorption, which is contributed by the overlap between the optical absorption and emission.50–52 As a derivative of pentacene, the peelable P1O crystal might be a potential candidate for further application in electro-optical micro-devices, for example light-emitting field-effect transistor.53


image file: d0nr00306a-f5.tif
Fig. 5 (A) Optical image of exfoliated P1O crystal flakes. The AFM images of a, b, c and d are the enlarged parts corresponding to the four regions marked in image A, while the thickness of region e is much greater than 1.2 μm which is out of the optimal detection range of AFM; (B) the PL spectra measured in the five regions marked in image A under 532 nm excitation.

Conclusions

In conclusion, the dark-green P1O organic crystal with determined structure was synthesized by the physical vapor transport method for the first time. Remarkably, the P1O bulk crystal exhibits a strong ASPL without any sensitizers. The linear pumping power dependent ASPL intensity, nanoseconds PL lifetime, and quenched ASPL at low temperature suggest that the ASPL of P1O crystal should arise from the phonon-assisted process, which has not been reported in the solid organic crystals. Furthermore, the bulk P1O crystal can be mechanically exfoliated to ultrathin flakes with high crystalline quality, which shows thickness-dependent PL performance. The low cleavage energy and unique PL property of the P1O crystal would extend its application to photon-related micro-devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work at Fujian Normal University was supported by the National Natural Science Foundation of China (NSFC) (No. 51802039 and 21871048), NSF of Fujian Province (2019J01266), the State Key Laboratory Program of Structural Chemistry in China (20190014).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1907538. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0nr00306a
‡ These authors contributed equally to this work.
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