Synthesis, Photoswitching Behavior and Nonlinear Optical Properties of Substituted Tribenzo[a,d,g]coronene

A family of tribenzocoronene derivatives bearing various substituents (3) were constructed through the Diels–Alder reaction, followed by the Scholl oxidation, where the molecular structure of 3b was determined via single crystal X-ray diffraction analysis. The effect of substitution on the optical and electrochemical property was systematically investigated, with the assistance of theoretical calculations. Moreover, the thin films of the resulting molecules 3b and 3e complexed with fullerene produced strong photocurrent response upon irradiation of white light. In addition, 3b and 3e exhibit a positive nonlinear optical response resulting from the two-photon absorption and excited state absorption processes.


Introduction
The construction of structurally defined polycyclic aromatic hydrocarbons (PAHs) has attracted substantial interest during the past several decades because such molecules can be usually seen as a segmental model for defects of graphene possessing interesting physical properties and can be used in organic electronics including laser, photodetectors, organic light emitting diodes, organic field effect transistors and organic solar cells [1][2][3][4]. Among them, curved π-conjugated derivatives provide us with more room to deepen our understanding of the anomalous hexagon arrays, which can be obtained through the implementation of armchair, cove, fjord regions, and the embedment of four-, five-seven-and eight-membered rings [5][6][7][8]. Undoubtedly, the edge and size can affect the optoelectronic and magnetic properties to a great extent, leading to different aromaticity, energy levels and band gaps. Meanwhile, if the heteroatoms or heterorings were doped into the parent frameworks, the resulting heteroarenes exhibit some appealing behaviors such as ease of synthesis, tailoring physical property and molecular stability [9][10][11][12]. More interestingly, the introduction of some functional groups including electron-withdrawing groups and electron-donating groups into the π-systems is a straightforward method for selective modification and functionalization. As a highly symmetric (D 6h ) molecule, coronene is the subject of considerable investigations owing to its tailoring optoelectronic properties. The self-assembly of a single coronene can form regular nanowires used for optoelectronic devices [13]. More strikingly, this "standard" six-membered ring-fused molecule cocrystallizes with different acceptors, including 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), 1,2,4,5-tetracyanobenzene (TCNB), napthalenetetracarboxylic diimide via a weak interaction [14][15][16][17][18]. In addition, the functionalization of appropriate precursors can generate expanded coronene derivatives. All the observations stimulate us to prepare novel arenes bearing a coronene unit and to investigate the physical properties. panded coronene derivatives. All the observations stimulate us to prepare nove bearing a coronene unit and to investigate the physical properties.
The wide application of laser in optic devices provides great convenience in and civil aspects [19][20][21]. However, the big injury risk of laser asks researchers to excellent optical limiting materials to decrease the laser intensity in order to pro eyes and optical devices. At present, organic materials such as fullerene deri phthalocyanine, graphene and metallated graphdiyne can compete with inorgan terparts over flexibility, machinability and quick response [22][23][24][25]. In comparison less graphene, coronene and its derivatives are well defined, as well as possessin trollable physical property and a high charge-carrier mobility. Hirata et al. found estradiol doped with deuterated coronene could present large reverse saturable tion characteristics with sunlight power level [26]. More recently, organic cobased on coronene and naphthalenediimide have exhibited an enhanced nonlinea response and charge transfer with the increase in the intermolecular interactio group of Wang [27]. Until now, the optical limiting property of such functionaliz nene derivatives has been limited.
In this work, we strategically synthesized a family of substituted tribenzo[a,d nene derivatives (3a-3e, Scheme 1). The molecular structure of 5,10-di-tert-butylrotribenzo[a,d,g]coronene (3b) is determined through single crystal X-ray diffrac of them emit green fluorescence. The complexes of 3b/3e-C60 produce a strong p sponse under irradiation of white light. Moreover, 3b and 3e exhibit nonlinear performance and the possible mechanism is caused by the two-photon absorpt excited state absorption processes. Clearly, systematic studies may be instructiv sign and approach coronene-containing derivatives. Scheme 1. Synthetic route to 3a, 3b, 3c, 3d and 3e.
To further prove the molecular structures and examine the arrangement in the solid state, flake-like single crystals of 3b were obtained by slow evaporation of 1,2-dichloroethane (DCE) and acetonitrile solution. It should be noted that no crystals suitable for single crystal X-ray analysis of 3a, 3c, 3d and 3e were formed under a similar operation condition.  (Table S1). As can be seen from Figure 1a, all the benzene rings are not in one plane, which is different from the parent coronene unit [29]. More interestingly, the benzo moieties on the pyrene and the terminal chlorobenzene unit in the horizontal tetracene part bend to the same side, and thus 3b can form a reclining-chair configuration (Figure 1b). Similar architectures were observed in the twistarenes observed in our group [30]. Molecule 3b can stack in column style, where the distance between the naphthalene in the pyrene unit is 3.63 Å (Figure 1c), which implies that π-π stacking interaction is absent [31]. 5,10-di-tert-butyltribenzo[a,d,g]coronene-15-carbonitrile (3e) in an isolated 24% y the new compounds were purified by silica gel column chromatography and ch ized through 1 H NMR, 13 C NMR and HR-MS (Figures S1-S18). More importan resultant derivatives bearing various substituents should provide more room for lective modification and functionalization.
To further prove the molecular structures and examine the arrangement in state, flake-like single crystals of 3b were obtained by slow evaporation of 1,2-d ethane (DCE) and acetonitrile solution. It should be noted that no crystals sui single crystal X-ray analysis of 3a, 3c, 3d and 3e were formed under a similar o condition. Molecule 3b adopts monoclinic space group C2/c with Z = 8. The uni mensions are a = 26.684(3) Å, b = 13.5714(13) Å, c = 19.003(2) Å, β = 100.02(4) o (T As can be seen from Figure 1a, all the benzene rings are not in one plane, which is from the parent coronene unit [29]. More interestingly, the benzo moieties on the and the terminal chlorobenzene unit in the horizontal tetracene part bend to the sa and thus 3b can form a reclining-chair configuration (Figure 1b). Similar arch were observed in the twistarenes observed in our group [30]. Molecule 3b can column style, where the distance between the naphthalene in the pyrene unit i (Figure 1c), which implies that π-π stacking interaction is absent [31]. The optical properties were manifested via UV-visible absorption and fluo spectra in a solution. As shown in Figure 2a, 3a bearing a weak electron-donatin oxyl group presents a broad absorption band centered at 451 nm in the low-energ and 381/362/331/312 nm in the high energy region. In comparison, the other fo pounds 3b-3e display similar absorption profiles, while 3e possesses a bathochro absorption peak probably owing to the increase in the π-conjugation length wit troduction of cyano unit [32][33][34]. Compound 3a exhibits a broad emission peak a and the emission maxima and contours of the other four compounds 3b-3e are al same (Figure 2b,d). The quantum yields are calculated to be 1.7% for 3a, 1.9% for 3 for 3c, 0.27% for 3d, 5.2% for 3e, respectively, by using 9,10-diphenylanthracene as ard [28]. The fluorescence lifetimes (τs) were recorded to be 8.40 ns for 3a, 16.22 n 31.32/5.06 ns for 3c, 2.09/14.29 ns for 3d and 12.50 ns for 3e, respectively, by using resolved fluorescence way ( Figure S19). Clearly, molecules 3c and 3d display two compared with the other three homologues. The low quantum yield of the 3d co iodine atom and the diexponential decay process of 3c and 3d should be ascribe heavy atom effect. The optical properties were manifested via UV-visible absorption and fluorescence spectra in a solution. As shown in Figure 2a, 3a bearing a weak electron-donating methoxyl group presents a broad absorption band centered at 451 nm in the low-energy region and 381/362/331/312 nm in the high energy region. In comparison, the other four compounds 3b-3e display similar absorption profiles, while 3e possesses a bathochromic shift absorption peak probably owing to the increase in the π-conjugation length with the introduction of cyano unit [32][33][34]. Compound 3a exhibits a broad emission peak at 506 nm and the emission maxima and contours of the other four compounds 3b-3e are almost the same (Figure 2b,d). The quantum yields are calculated to be 1.7% for 3a, 1.9% for 3b, 0.51% for 3c, 0.27% for 3d, 5.2% for 3e, respectively, by using 9,10-diphenylanthracene as a standard [28]. The fluorescence lifetimes (τ s ) were recorded to be 8.40 ns for 3a, 16.22 ns for 3b, 31.32/5.06 ns for 3c, 2.09/14.29 ns for 3d and 12.50 ns for 3e, respectively, by using a time-resolved fluorescence way ( Figure S19). Clearly, molecules 3c and 3d display two lifetimes compared with the other three homologues. The low quantum yield of the 3d The electrochemical properties of the functionalized coronene derivatives w amined through cyclic voltammetry in anhydrous and degassed dichlorometh shown in Figure 2c, all of them exhibit one reversible oxidative wave with the p of 0.66 V for 3a, 0.61 V for 3b, 0.59 V for 3c, 0.60 for 3d and 0.72 V for 3e, resp against ferrocene (Fc + /Fc), whereas no reduction waves could be monitored w accessible scanning range in the dichloromethane. Accordingly, the HOMO energ are calculated to be −5.46 eV for 3a, −5.41 eV for 3b, −5.39 eV for 3c, −5.40 eV for 3 eV for 3e on the basis of the first oxidation potentials. Molecular orbital calculatio on the B3lyp/def2SVP indicate that the HOMOs of all the compounds are spre substituted tribenzo[a,d,g]coronene moiety and LUMOs are located on the sub dibenzo[fg,ij]naphtho [1,2,3,4-rst]pentaphene unit ( Figure 3 and Table S2)  observations suggest that the substituents do contribute to the orbitals to a lesser The electrochemical properties of the functionalized coronene derivatives were examined through cyclic voltammetry in anhydrous and degassed dichloromethane. As shown in Figure 2c, all of them exhibit one reversible oxidative wave with the potentials of 0.66 V for 3a, 0.61 V for 3b, 0.59 V for 3c, 0.60 for 3d and 0.72 V for 3e, respectively, against ferrocene (Fc + /Fc), whereas no reduction waves could be monitored within the accessible scanning range in the dichloromethane. Accordingly, the HOMO energy levels are calculated to be −5.46 eV for 3a, −5.41 eV for 3b, −5.39 eV for 3c, −5.40 eV for 3d, −5.52 eV for 3e on the basis of the first oxidation potentials. Molecular orbital calculations based on the B3lyp/def2SVP indicate that the HOMOs of all the compounds are spread over substituted tribenzo[a,d,g]coronene moiety and LUMOs are located on the substituted dibenzo[fg,ij]naphtho [1,2,3,4-rst]pentaphene unit ( Figure 3 and Table S2) [35][36][37][38]. Such observations suggest that the substituents do contribute to the orbitals to a lesser extent.
To examine the photoconductor properties, compounds 3b and 3e mixed with C 60 were used as active layers to fabricate photodetector devices. As observed in Figure 4a,c, the blended systems of 3b-C 60 and 3e-C 60 were subjected to white light at varying illumination intensities, with the photocurrent increasing correspondingly. The maxima data of 0.031 µA for 3b-C 60 and 0.167 µA for 3e-C 60 at 200 mW/cm 2 were generated when the mixture films were switched on and off. It should be stressed that no photocurrent was found that was white-light illumination-free. Such phenomena may be caused by the photoinduced charge transfer in the donor and acceptor systems. Meanwhile, film 3e-C 60 exhibited a higher photocurrent than film 3b-C 60 , being close to the fluorescence spectra. In addition, the photoresponses to ON/OFF cycles were prompt, stable and reducible for both of them (Figure 4b,d). Such features of the tribenzocoronene derivatives endow an opportunity for them to be regarded as fascinating ingredients for a photo-controlled switch and photodetectors. To examine the photoconductor properties, compounds 3b and 3e mixed wit were used as active layers to fabricate photodetector devices. As observed in Figure the blended systems of 3b-C60 and 3e-C60 were subjected to white light at varying ill nation intensities, with the photocurrent increasing correspondingly. The maxima da duced charge transfer in the donor and acceptor systems. Meanwhile, film 3e-C60 exhibited a higher photocurrent than film 3b-C60, being close to the fluorescence spectra. In addition, the photoresponses to ON/OFF cycles were prompt, stable and reducible for both of them (Figure 4b,d). Such features of the tribenzocoronene derivatives endow an opportunity for them to be regarded as fascinating ingredients for a photo-controlled switch and photodetectors. To expand the applications of such materials, the nonlinear optical properties of 3b and 3e were further studied through open aperture Z-scan technology under the Nd: YAG-based 532 nm wavelength nanosecond pulse laser irradiation [39]. Both of them present reverse saturation absorption (RSA) and the curves show symmetric peaks on both sides near the laser focus (Figure 5a,c). The nonlinear absorption coefficients (βeff) of 3b and 3e fluctuate with the energy density at the focal point, indicating that the ESA effect plays a major role in the RSA signal (Table S3) [40]. The minimum normalized transmittance (Tmin) decreases gradually with the increase in incident energy. Tmin of 3b at 20.7 µJ, 40.6 µJ and 60.5 µJ are 90%, 78% and 67%, respectively. Tmin of 3e at 20.7 µJ, 40.6 µJ and 60.5 µJ are 76%, 67% and 59%, respectively. The onset optical limiting threshold (Fon, the incident laser intensity when the normalized transmittance drops to 95%) of 3b are 1.  To expand the applications of such materials, the nonlinear optical properties of 3b and 3e were further studied through open aperture Z-scan technology under the Nd: YAGbased 532 nm wavelength nanosecond pulse laser irradiation [39]. Both of them present reverse saturation absorption (RSA) and the curves show symmetric peaks on both sides near the laser focus (Figure 5a,c). The nonlinear absorption coefficients (β eff ) of 3b and 3e fluctuate with the energy density at the focal point, indicating that the ESA effect plays a major role in the RSA signal (Table S3) [40]. The minimum normalized transmittance (T min ) decreases gradually with the increase in incident energy. T min of 3b at 20.7 µJ, 40.6 µJ and 60.5 µJ are 90%, 78% and 67%, respectively. T min of 3e at 20.7 µJ, 40.6 µJ and 60.5 µJ are 76%, 67% and 59%, respectively. The onset optical limiting threshold (F on , the incident laser intensity when the normalized transmittance drops to 95%) of 3b are 1. The possible mechanism of nonlinear optical processes is examined by measuring the nanosecond transient absorption spectroscopy. As shown in Figure 6, the timescale corresponding to the spectral change is approximately 50-1550 ns, which should be attributed The possible mechanism of nonlinear optical processes is examined by measuring the nanosecond transient absorption spectroscopy. As shown in Figure 6, the timescale corresponding to the spectral change is approximately 50-1550 ns, which should be attributed to the effect of the triple excited state [41]. The peak position of the two compounds changed little with the delay time, indicating that the excited state absorption was generated in the same excited state, and no other processes occurred during the excited state absorption [42][43][44]. Both of them show similar spectral shapes, displaying wide excited state absorption bands after 440 nm and an isolated excited state absorption peak at 400 nm. There is also an isolated excited absorption peak of 3b at 330 nm, but the excited absorption peak of 3e is suppressed at this position, which scarcely shows a positive signal. The attenuation curves of 3b and 3e at the absorption peak of 480 nm are shown in Figure S20, and the attenuation lives of their triple excited states for 3d and 3e were 334.4 ns and 263.2 ns, respectively. On the whole, there was little difference between the two compounds, even though the excited state absorption peak of 3e at 480 nm is slightly stronger than that of 3b. Figure 5. Typical OA Z-scan curves of 3b (a) and 3e (c) at different energies (20.7 µJ, 40.6 µJ and 60.5 µJ). Plot of normalized transmittance versus input fluence of 3b (b) and 3d (d) derived from the Z scan curve. The incident wavelength is 532 nm.
The possible mechanism of nonlinear optical processes is examined by measuring the nanosecond transient absorption spectroscopy. As shown in Figure 6, the timescale corre sponding to the spectral change is approximately 50-1550 ns, which should be attributed to the effect of the triple excited state [41]. The peak position of the two compounds changed little with the delay time, indicating that the excited state absorption was gener ated in the same excited state, and no other processes occurred during the excited state absorption [42][43][44]. Both of them show similar spectral shapes, displaying wide excited state absorption bands after 440 nm and an isolated excited state absorption peak at 400 nm. There is also an isolated excited absorption peak of 3b at 330 nm, but the excited absorption peak of 3e is suppressed at this position, which scarcely shows a positive sig nal. The attenuation curves of 3b and 3e at the absorption peak of 480 nm are shown in Figure S20, and the attenuation lives of their triple excited states for 3d and 3e were 334.4 ns and 263.2 ns, respectively. On the whole, there was little difference between the two compounds, even though the excited state absorption peak of 3e at 480 nm is slightly stronger than that of 3b. However, according to the Z-scan test results, 3e has a lower Tmin value than 3b which may be related to the different excitation pathways of the two compounds. Gener ally, molecular excitation is believed to result from the absorption of the band gap and the absorption of the defect level near the band gap for ns laser pulse irradiation. The UV visible absorption spectra show that the ground state absorption (GSA) of 3b and 3e at the wavelength of 532 nm is relatively weak (Figure 2a), which is not conducive to the gener ation of excited molecules. In this case, the GSA of 3e at 532 nm is slightly stronger than that of 3b, which is beneficial to generating more excited molecules under laser irradia tion, and may lead to a stronger RSA signal. In addition, the TPA excitation pathway o However, according to the Z-scan test results, 3e has a lower T min value than 3b, which may be related to the different excitation pathways of the two compounds. Generally, molecular excitation is believed to result from the absorption of the band gap and the absorption of the defect level near the band gap for ns laser pulse irradiation. The UVvisible absorption spectra show that the ground state absorption (GSA) of 3b and 3e at the wavelength of 532 nm is relatively weak (Figure 2a), which is not conducive to the generation of excited molecules. In this case, the GSA of 3e at 532 nm is slightly stronger than that of 3b, which is beneficial to generating more excited molecules under laser irradiation, and may lead to a stronger RSA signal. In addition, the TPA excitation pathway of excited molecules cannot be excluded. The two-photon fluorescence (TPF) spectra at the excitation wavelength of 800 nm were tested and the logarithmic power-dependent TPF intensity curve displayed the linear-fitted slopes of 2.01 and 2.04 ( Figure S21), which indicated that the TPF intensity exhibited a quadratic curve relationship with the excitation power, proving the existence of TPA [45]. Therefore, we reasonably speculate that the nonlinear absorption signals of 3b and 3e should be caused by TPA/GSA and ESA.

Materials and Methods
1 H NMR and 13 C NMR spectra were measured on a WNMR 400 spectrometer at 400 MHz for 1 H and 100 MHz for 13 C without any internal standard. The chemical shifts are labelled in ppm with δ of CDCl 3 (7.26 ppm in 1 H NMR and 77.16 in 13 C NMR). MALDI-TOF mass spectra were performed on a Bruker Biflex III MALDI-TOF. UV-visible absorption and fluorescence spectra were carried out by using a 10 mm quartz cell on an Analytic Jena SPECORD 210 PLUS and Hitachi F-7000 spectrometers, respectively. Cyclic voltammetry investigations were performed on a CHI 630A electrochemical analyzer using a standard three-electrode cell containing a Pt working electrode, a Pt wire counter electrode and an Ag/AgNO 3 reference electrode under a nitrogen atmosphere. Tetrabutylammonium hexafluorophosphate solution (0.1 M, anhydrous dichloromethane) was used as an electrolyte. The scan rate was 0.1 V s −1 and the redox potentials were labelled against the Fc + /Fc couple (a standard). The photoswitching behaviors were performed through an electrochemical workstation (Modulab XM, Solartron Analytical, UK) and the voltage was 0.5 V.

Synthesis of 3a
TfOH (0.3 mL) was slowly dropped into a mixture of compound 2a (30 mg, 0.04 mmol) and DDQ (39 mg, 0.17 mmol) in anhydrous dichloromethane (15 mL) at −30 • C under an argon atmosphere. After 7 min, methanol was added to quench the reaction. The mixture solution was partitioned between Na 2 CO 3 solution/brine and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with petroleum ether (PE) as an eluent to produce a yellow solid (3a, 10 mg, 40%). 1

Synthesis of 2b
A mixture of 1 (510 mg, 0.98 mmol), 2-amino-3-chlorobenzoic acid (204 mg, 1.19 mmol), isoamyl nitrate (0.2 mL) was stirred in anhydrous tetrachloroethane (TCE, 15 mL) at 150 • C under argon. After 24 h, the TCE was removed at a reduced pressure. The mixture was then partitioned between brine and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with PE as an eluent to give a light green solid (2b, 235 mg, 40%). 1

Synthesis of 3b
TfOH (0.3 mL) was slowly dropped into a mixture of compound 2b (20 mg, 0.03 mmol) and DDQ (22 mg, 0.1 mmol) in anhydrous dichloromethane (15 mL) at −30 • C under an argon atmosphere. After 5 min, methanol was added to quench the reaction. The mixture solution was partitioned between Na 2 CO 3 solution and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with PE as an eluent to produce a yellow solid (3b, 13 mg, 66%). 1 13

Synthesis of 2d
A mixture of 1 (1.5 g, 2.89 mmol), 2-amino-3-iodobenzoic acid (913 mg, 3.47 mmol), isoamyl nitrate (1.0 mL) was stirred in anhydrous tetrachloroethane (TCE, 15 mL) at 150 • C under argon. After 24 h, TCE was removed at a reduced pressure. The mixture was then partitioned between brine and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with PE as an eluent to produce a light green solid (2d, 1.13 g, 56%). 1

Synthesis of 3d
TfOH (0.3 mL) was slowly dropped into a mixture of compound 2b (20 mg, 0.03 mmol) and DDQ (20 mg, 0.09mmol) in anhydrous dichloromethane (15 mL) at −30 • C under an argon atmosphere. After 5 min, methanol was added to quench the reaction. The mixture solution was partitioned between Na 2 CO 3 solution/brine and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with PE as an eluent to produce a light yellow solid (3d, 18 mg, 90%). 1

Synthesis of 3e
A mixture of 3c (100 mg, 0.16 mmol) and CuCN (28 mg, 0.31 mmol) was stirred in anhydrous NMP (6 mL) at 180 • C under argon. After 3 d, ammonium ferrous sulfate solution was added when the mixture solution was cooled to 60 • C for 2 h. The solution was then cooled down to room temperature and was partitioned between brine and methylene chloride. The organic layer was dried over Na 2 SO 4 and evaporated in vacuo. The crude product was purified over silica gel column chromatography with PE and dichloromethane (v/v, 8:1) as an eluent to produce a light yellow solid (3e, 22 mg, 24%). 1

Conclusions
In summary, we have designed and synthesized five novel coronene-containing πsystems bearing different substituents. Such an investigation highlights a significant effect of the substituents on the absorption, emission and redox properties of 3a-3e. Molecule 3d has the lowest quantum yield owing to the strong heavy atom effect of iodine. The photocurrent response of 3e-C 60 is superior to that of 3b-C 60 , which is assigned to the higher quantum yield of 3e, leading to a highly efficient photo-induced charge transfer in the donor and acceptor system. The expanded applications suggest that the synthesized compounds 3b and 3e have a positive optical limiting performance resulting from GSA and ESA phenomena. Further examination of the post-functionalization of such key building blocks for approaching large curved PAHs with attractive optoelectronic properties are currently being undertaken in our laboratory.

Data Availability Statement:
The data presented in this study are available in the paper. The CCDC 2232268 contains supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033).