Photophysical Properties of a Terarylene Photoswitch with a Donor

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Photophysical Properties of a Terarylene Photoswitch with a Donor-Acceptor Conjugated Bridging Unit Rui Kanazawa, Takuya Nakashima, and Tsuyoshi Kawai J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00296 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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The Journal of Physical Chemistry

Photophysical Properties of a Terarylene Photoswitch with a Donor-Acceptor Conjugated Bridging Unit Rui Kanazawa,† Takuya Nakashima*,†, and Tsuyoshi Kawai*,†,‡ †

Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST,

Ikoma, Nara, 630-0192, Japan ‡

NAIST-CEMES International Collaborative Laboratory for Supraphotoactive System, Cebtre

d’Élaboration de Matériaux et d’Etudes Structurales, CEMES, 29, rue Jeanne Marvig, BP 94347, Toulouse 31055, France Corresponding Author Fax: (+81)743-72-6179 * E-mail: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT

A terarylene photoswitch composed of an electron-donating thienothiophene unit which is connected to the central bridging imidazole was designed. The electronic and geometrical structures of the central π-conjugation unit was controlled by chemical modifications on the imidazole unit, in which the distribution of frontier molecular orbitals was modulated between the neutral state and its protonated or quaternized cationic forms. These electronic and geometrical changes modulated the mixing of the closely-lying excited state potential energy surfaces, resulting in the excitation wavelength dependent photocyclization performance. The chemical modifications on the imidazole ring also had an effect on the fluorescence property in the open-ring forms, which is explained by the formation of different push-pull π-conjugation systems.

INTRODUCTION

Molecular photoswitches are attracting much interest as a key component in advanced functional materials for molecular devices.1–5 Among a variety of molecular photoswitches, diarylethenes (DAEs) and their derivatives have been extensively investigated for several decades due to their high fatigue resistance, thermal stability and sensitivity.6,7 While this class of photoswitches have demonstrated various types of switching capabilities, fluorescence modulation is a promising output in applications for optical data storage, biological sensors, and super resolution fluorescence microscopy.8–12 Reasonable designs of fluorescent DAEs often employ the covalent attachment of fluorophores to the DAEs backbone as a side-chain unit.13–17 Recent development


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in the design of DAE derivatives have enabled the flexible choice of central bridging unit.18,19 In this context, we introduced a heteroaryl ring as the central bridging unit of DAEs to construct a series of photochromic “terarylenes”.20–27 Terarylenes demonstrated highly efficient photocyclization28–32 and various photoswitching capabilities33–36 in response to the molecular design in terms of the aryl unit combinations constituting the photoresponsive 6π-framework. For example, terarylenes containing an imidazole or imidazolium ring as a bridging unit showed versatile photoswitching performances including the modulations in electrophilicity,37,38 coordination to metal ions,39,40 and fluorescence.41,42 Meanwhile, imidazole ring play an important role as a key building unit in various push-pull π-conjugated architectures due to their electronic nature, robustness and convenience in synthesis and chemical modifications.43 We developed π-conjugated fluorophores composed of Nmethylbenzimidazole and thieno[3,2-b]thiophene moiety, forming acid-responsive push-pull systems.44,45 The photophysical investigation revealed that the protonated forms possessed a planar quinoidal structure in their excited states (Scheme 1), leading to the intense charge transfer (CT) emission with a small extent of Stokes shift.44,45 These findings motivated us to incorporate the thienothiophene-imidazole fluorescent unit into a terarylene photoswitching system. Given the ordinary photophysical processes follow Kasha’s rule in principle,46,47 the introduction of push-pull structures in DAEs often lead to the suppression of photocyclization due to the competing CT states formation. In the present study, we carefully investigated the interplay of excited state potential energy surfaces responsible for the CT and photochemical reactive states for the photoswitches containing the thienothiophene-imidazole(imidazolium) push-pull system. The state of chemical modifications on the imidazole ring successfully


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modulated the excited state interaction and led to the wavelength-selective photophysicochemical processes.

Scheme 1. The quinoidal resonance structure in an imidazolium-thienothiophene fluorophore.

We constructed a new fluorescent terarylene photoswitch having an imidazole ring as a central bridging unit, which extends the π-conjugation system from the C2-position of imidazole ring with an electron-donating thienothiophene moiety. The C4- and C5-positions of imidazole ring carry electron-withdrawing phenylthiazole units to build a terarylene backbone (TTIM, Scheme 2). 2-Phenyl-imidazole derivative (IM) was also prepared as a reference molecule. The central N-methylimidazole ring is readily converted into a cationic imidazolium by chemical modifications such as protonation and further N-methylation, enhancing the electron-accepting capability in the central bridged unit. The N,N-dimethylation also introduces a twist between imidazolium and thienothiophene moieties due to the steric interaction, leading to the suspension of π-conjugation extension.48,49


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Scheme 2. a) Photochromic reaction and b) chemical modification of terarylene photoswitches. Reagents/conditions: i) trifluoroacetic acid (TFA); ii) CH3I in neat condition.

EXPERIMENTAL SECTION General. All reagents and solvents for the synthesis were purchased from commercial sources and used without further purification. Analytical-grade solvents purchased from Wako Pure Chemicals were used for the optical measurements. 1

H NMR and 13C NMR spectra were recorded by using a JEOL JNM-AL300 (1H: 300 MHz,

13C: 75 MHz) spectrometer. Mass spectra were measured by using a JEOL AccuTOF JMST100LC (ESI) instrument and a Bruker Daltonics Autoflex II (MALDI-TOF) spectrometer. UVvis absorption spectra were recorded by using a JASCO V-670 spectrophotometer. Fluorescence spectra were measured with a JASCO FP-6500 fluorescence spectrophotometer. The absolute fluorescence quantum yields were evaluated by using a Hamamatsu Photonics C9920-02 instrument with an integrating sphere. Fluorescence lifetime was measured with a Hamamatsu Photonics C4780 picosecond fluorescence lifetime measuring system equipped with a streak


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scope and a Coherent Mira 900F Ti:Sapphire laser as an excitation source. The double frequency of 760 nm obtained after passing a LiB3O5 (LBO) crystal was used as the incident light (λirr = 380 nm). Quantum yields of photocyclization and cycloreversion were measured by using a Shimadzu QYM-01 photoreaction quantum yield measurement system. Computational approaches. Quantum chemical calculations were performed by using Gaussian 09 program package.50 Ground state geometry was optimized with density functional theory (DFT) at the PBE0 hybrid exchange-correlation functional51,52 with 6-31G+(d,p) basis set (DGDZVP basis set53,54 was used for the iodine atom). Excited state properties were computed with time-dependent density functional theory (TDDFT) method at their optimized ground state geometries using the same functional and basis set. In our all calculations, dispersion corrections were also treated with the D3 version of Grimme’s dispersion with Becke-Johnson damping (GD3BJ).55–57 Synthetic procedures. Terarylenes with an N-methylimidazole bridge unit were synthesized according to Scheme 3. The imidazole ring was prepared by the condensation cyclization reaction between the β-diketone (3) with corresponding aldehydes. Details of the synthetic procedures and compound characterizations are given in the Supporting Information. To obtain the protonated compounds, an excess amount of trifluoroacetic acid (TFA, 0.01 mL) was added to the solutions (1.0 × 10−6 M, 3 mL) that was excessive enough to protonate all the molecules of compounds in the solutions.44


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Scheme 3. Preparation of terarylene photoswitches. Reagents/conditions: i) n-BuLi, DMF, dry THF, −78°C to r.t.; ii) 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride, diisopropylamine, dry ethanol, reflux; iii) 5-phenylthieno[3,2-b]thiophene-2-carbaldehyde (for TTIMo-NH) or benzaldehyde (for IMo-NH), NH4OAc, acetic acid, reflux; iv) CH3I, DMF, r.t.

RESULTS AND DISCUSSION Photochromic behavior. Figure 1a shows UV/vis absorption spectral change of TTIMo solution in CH2Cl2 upon UV irradiation at 313 nm. While only a single absorption peak was observed in the solution of reference IMo (Figure S15 in the Supporting information), the introduction of thienothiophene unit to the C2 position of central imidazole ring resulted in a bimodal absorption band with peak maxima at 355 and 295 nm in TTIMo. Irradiation to the colorless solution of TTIMo with UV light resulted in the coloration of solution into green, with an emergence of new absorption band in visible region around 690 nm. The initial colorless solution was recovered by irradiation with visible light, leading to an absorption spectrum identical to that of TTIMo. These absorption changes are characteristic to the formation of closed-ring form (TTIMc) and recovery of the open-ring form with the reversible 6πelectrocyclization reactions. In the photocyclization reaction of TTIMo, the growth of visible


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band was distinctly small and was completed in a few hundreds of seconds to reach to a photostationary state (PSS). TTIMo·H+ also showed the suppressed photocyclization reactivity (Figure 1b). In contrast, the reference compound IMo showed relatively efficient photocoloration compared to TTIMo (Figure S15 in the Supporting Information). The quantum yield for the photocyclization is 65% for IMo, whereas the value for TTIMo dropped to less than 1% (λirr = 313 nm). The photocoloration performance was improved by the quaternization to TTIMo+I-, which showed 10% of photocyclization quantum yield (Table 1 and Table S2 in the Supporting Information). Along with the increase in the photocyclization quantum yield, the conversion ratio between the open-ring and closed-ring forms was improved from18% (TTIM) to 31% (TTIM+I-).


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Figure 1. UV-vis absorption changes upon irradiation with 250 W high-pressure Hg lamp at 313 nm in CH2Cl2 (1.0 × 10−6 M): a) TTIM, b) TTIM·H+, and c) TTIM+I-; open-ring form (dotted lines) and PSS (solid thick lines); the insets represent the evolutions of absorbance at the absorption band in the visible range (> 500 nm).


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Table 1. Photochromic properties in CH2Cl2. compd.a

λmax / nm (ε / 104 L mol−1cm−1)

ΦOCb (313 nm)

ΦCO c (600 nm)

TTIMo

295 (3.7), 355 (4.3)

0.0083

—

TTIMc

321 (3.1), 423 (4.9), 690 (1.8)

—

0.016

TTIMo·H+

286, 365

n.d.

—

TTIMc·H+

360, 435, 625

—

n.d.

TTIMo+I-

307 (4.5), 341 (3.2)d

0.1

—

TTIMc+I-

325 (3.7), 419 (3.0), 670 (1.3)

—

0.03

a

ε values and photoreaction quantum yields were not determined for TTIMo·H+. Photocyclization quantum yields (λirr = 313 nm). cPhotocycloreversion quantum yields (λirr = 600 nm). dShoulder of the absorption spectra.

b

DFT calculations for geometrical investigations. DFT calculations provided the optimized ground state structures of the open-ring forms in neutral state and its N-substituted cationic derivatives (Figure 2). TTIMo possesses almost planar configuration with distortion of 20.5° between the thienothiophene and imidazole moieties, which seems to be supported by a S···N heteroatom interaction.58-60 The nearly planar structure was also found in the protonated form with a NH···S hydrogen bonding interaction (TTIMo·H+, Figure 2b), whereas TTIMo+Ipossesses an orthogonal geometry between the thienothiophene and imidazolium moieties (Figure 2c). The difference in the ground state geometries should give an impact on the excited state properties.


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Figure 2. Optimized ground state structures of the open-ring forms. Torsion angles between thienothiophene and central imidazole/imidazolium units, α, are also shown. The sum values of van der Waals radii are 3.35 Å (S, N) and 3.00 Å (S, H).

TDDFT calculations and electronic transition property. To gain an insight into the excited state properties, TDDFT calculations were performed for the designed photoswitches. Table 2, Figures 3 and 4 summarize the electronic transition properties and the corresponding frontier molecular orbitals (MOs), respectively. The calculated excitation energies well reproduced the experimental absorption spectra (Figures 1 and 3). The calculations with PBE0 method showed the most reasonable reproducibility among the functionals investigated (Figure S11 in the Supporting Information). For TTIMo, the lowest-lying excited state consists mainly of HOMO → LUMO transition with a weak oscillator strength. The HOMO mostly lies on the electrondonating thienothiophene unit, and the LUMO is localized on the electron-accepting phenylthiazole unit (Figure 4a). Because of the spatially separated MOs and a weak oscillator strength, the lowest-lying excited state can be assigned to a CT state. The calculated wavelength of maximum absorption of TTIMo is 373 nm, which corresponds to the experimental value of


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355 nm. The HOMO → LUMO+1 transition dominantly contributes to this absorption band (Table 2). The LUMO+1 of TTIMo expands to the imidazole-thienothiophene moieties with a planar geometry. It should be noted that both LUMO and LUMO+1 had little contribution to the photoreactive 6π-system. TTIMo· ·H+ showed a red-shift in the calculated maximum absorption peak compared to the neutral state, which is in good accordance with the experimental result. This electronic transition corresponds to the lowest lying excited state with a HOMO → LUMO transition (Table 2). In contrast to TTIMo, the LUMO of TTIMo·H+ is rather localized on the imidazolium moiety. The enhancement of electron-accepting ability in the imidazolium unit appears to be responsible for the difference in the distribution of the LUMO (Figure 4b). In a similar manner to TTIMo, the LUMO had small distribution on the photoreactive carbon atoms in TTIMo·H+, leading to the suppressed photocyclization reactivity. For TTIMo+I-, the HOMO, HOMO-1 and HOMO-2 orbitals are mainly distributed on the iodine ion, which are involved in the electronic transitions with negligible oscillator strengths. The lowest-lying excited state of TTIMo+I- is mainly described by a mixing of HOMO-4 → LUMO and HOMO-3 → LUMO transitions, which corresponds to a shoulder absorption band around 341 nm in the observed UV/vis absorption spectrum(Figure 4c, Table 2). The spatially separated MOs between HOMO-3 and LUMO suggested the CT nature between the electrondonating thienothiophene and the electron-accepting imidazolium-phenythiazole moieties. The maximum absorption peak was computed to be 316 nm which was assigned to the HOMO-3 → LUMO+1 transition with the largest oscillator strength. Because of the twisted geometries in TTIMo+I-, the LUMO+1 distribution was suspended between thienothiophene and imidazolium


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moieties. The increased distribution of LUMO and LUMO+1 on the photoreactive carbon atoms was prominent compared to those of TTIMo and TTIMo·H+, supporting the higher photocyclization quantum yield (Table 1). DFT and TDDFT calculations were also performed for IMo and its derivatives (Figures S12– S14 and Table S1 in the Supporting Information). Only a single absorption peak was observed for each reference compound both in the theoretical and experimental studies. The calculated absorption maximum of IMo (λcalcd = 290 nm) was assigned to a mixing of HOMO-1 → LUMO+1 and HOMO-2 → LUMO transitions. The LUMO+1 and LUMO are placed on a part of phenylthiazole moiety with an apparent component on one of the photoreactive carbon atoms. The distinct contribution of LUMO densities on the photoreactive carbon atoms was also observed in the electronic transition of its quaternized forms (IMo+I-). On the basis of the above theoretical findings, the differences in photocyclization reactivity can be explained partly by the degree of distribution of LUMO and/or LUMO+1 on the photoreactive carbon atoms.61 The suppression of photocyclization reactivity in TTIMo is in good accordance with our previous report, in which the extension of π-conjugated system in the central bridging unit of terarylens led to a decrease in the LUMOs distributions at the photoreactive carbon atoms.24 The enhancement of phtocyclization quantum yield in TTIMo+Iis consistent with this consideration, which has a disconnection in the π-conjugation by the twisting between thienothiophene and imidazolium units. The lower photoreactivity of TTIMo+I- in comparison with that of IMo+I- could be partly attributed to the competing emission process (see below).


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Table 2. Calculated electronic transition properties. compd.

λcalcdb / nm

fc

assignmentd

394

0.11

H → L (96%)

355

373

1.25

H → L+1 (96%)

295

301

0.64

H-2 → L (47%)

λexpa / nm

TTIMo

H → L+3 (22%) TTIMo·H+

365

286

403

1.17

H → L (98%)

356

0.14

H-1 → L (96%)

297

0.87

H-1 → L+1 (45%) H → L+3 (33%)

TTIMo+I-

341e

320

0.28

H-4 → L (40%) H-3 → L (40%)

307

316

0.81

H-3 → L+1 (53%)

312

0.28

H-4 → L+1 (44%) H-3 → L+1 (12%) H-3 → L+2 (36%)

a

Experimental maximum of the absorption wavelengths. bCalculated maximum of the absorption wavelengths. cOscillator strength. dH and L represent the HOMO and LUMO, respectively. e Shoulder of the absorption spectra.


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Figure 3. Experimental absorption spectra of the open-ring forms (dotted lines) in CH2Cl2 together with TDDFT excitation energies and oscillator strength: a) TTIMo, b) TTIMo·H+, and c) TTIMo+I-.


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Figure 4. Calculated MOs distributions.

Fluorescence property. TTIMo and its derivatives exhibited clear fluorescence in their openring forms, which was quenched by irradiation with UV light, converting the fluorescent openring forms to the non-fluorescent closed-ring forms (Figure 5). No emission was observed in both of isomers for the reference compounds (IM and IM+I-). To investigate the CT character,


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fluorescence spectra were measured in various polarity of solvents (Figure 6). While all compounds showed fluorescence solvatochromism in varying degrees as a result of the CT processes, TTIMo exhibited the most prominent solvent effect, wherein both the structured locally excited (LE) emission and the broad CT emission were observed in response to solvent polarity (Figure 6a). The relatively small Stokes shift (~5000 cm−1) even in DMF and CH2Cl2 suggested the non-twisting intramolecular CT emission characteristic to the thienothiopheneimidazole fluorophores.44,45 The structured emission in less polar solvents changed into a broad structure-less shape after the protonation in TTIMo·H+ (Figure 6b). Furthermore, the emission maximum shifted to longer wavelength, except for in 1,4-dioxane and DMF solutions (Table 3). To elucidate the origin of emission in TTIMo·H+, time-resolved emission measurements were performed in solutions. As shown in Table 3, the emission lifetime (τem) increased in response to acid, especially in nhexane, cyclohexane and CH2Cl2. The decrease in nonradiative rate constants (knr) was also observed in n-hexane and CH2Cl2 after protonation. The enhancement of electron accepting character in the protonated imidazolium should increase the CT nature (non-twisting CT emission) in the excited state.44,45 The enhanced emission quantum yield (τem) in CH2Cl2 is also consistent with the contribution of stiff quinoidal resonance structures, which may suppress the deactivation processes in the excited states.44,45 TTIMo+I- gave a broad structureless emission profile in all solvents (Figure 6c). In contrast to TTIMo and TTIMo·H+, the theoretical calculations suggested the nearly orthogonal geometry for TTIMo+I- because of the steric hindrance. This twisting in TTIMo+I- should modify the emissive excited state character. The emission origin of TTIMo+I- can thus be explained by the


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twisted CT state between the electron-donating thienothiophene and the electron-accepting imidazolium moieties. The characteristic twisted structure in TTIMo+I- should also be responsible for the larger Stokes shift (∆νST) in comparison with those of TTIMo and TTIMo·H+. The relatively small fluorescence solvatochromism may be attributed to the ionic structure both in the ground state and in the excited state.

Figure 5. Fluorescence change of TTIM upon UV irradiation at 365 nm in CH2Cl2.


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Figure 6. Fluorescence spectra in solvents with different polarity: a) TTIMo, b) TTIMo·H+, and c) TTIMo+I-.


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Table 3. Fluorescence properties in various solvents.a compd.

solventb

λem / nm

∆νST / cm−1×103

Φem

τem / ns

kr / ns−1

knr / ns−1

TTIMo

n-hexane

414, 439

5.55

0.25

0.48

0.53

1.58

cyclohexane

416, 441

5.49

0.35

0.51

0.69

1.29

toluene

427, 447

5.48

0.36

0.63

0.57

1.01

1,4-dioxane

425, 446

5.51

0.35

0.69

0.51

0.94

THF

447

5.56

0.35

0.72

0.49

0.91

CH2Cl2

448

5.84

0.33

0.73

0.45

0.92

DMF

451

5.76

0.39

0.84

0.46

0.72

n-hexane

445

5.38

0.22

0.62

0.36

1.26

cyclohexane

448

5.61

0.20

0.60

0.33

1.33

toluene

443

4.98

0.22

0.64

0.34

1.21

1,4-dioxane

424, 446

5.43

0.33

0.69

0.48

0.97

THF

451

6.00

0.35

0.76

0.46

0.86

CH2Cl2

457

5.52

0.45

0.87

0.52

0.63

DMF

451

5.92

0.38

0.84

0.45

0.73

toluene

443

9.79

0.027

0.63

0.04

1.54

1,4-dioxane

443

9.89

0.064

0.58

0.11

1.61

THF

447

9.89

0.19

0.79

0.24

1.02

CH2Cl2

448

10.25

0.41

0.85

0.48

0.70

DMF

448

9.63

0.39

0.74

0.52

0.82

TTIMo·H+

TTIMo+I-

a

Parameters: maximum of the emission wavelength (λem), Stokes shift (∆νST), fluorescence quantum yields (Φem), emission lifetime (τem), radiative (kr) and nonradiative (knr) rate constants. b TTIMo+I- is insolved in n-hexane and cyclohexane.

Excitation wavelength dependent photocyclization reaction. Several DAEs have been reported to show the excitation wavelength dependent photocycloreversion reactivity,62,63 which


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decreased with the red shift of irradiation wavelength. This exceptional behavior was ascribed by the existence of an activation energy barrier on the excited state potential energy surfaces (PESs) for the photocycloreversion reaction. Namely, the shorter-wavelength excitation gives the molecules excess vibrational energy to go over the energy barrier, resulting relatively efficient photocycloreversion quantum yields. Although rarely reported, the excitation wavelength dependent photocyclization reaction has also been found in a few kinds of DAEs having two different photocyclization pathways on the excited states.64 Figure 7 shows the excitation wavelength dependence of the photocyclization reaction quantum yields in CH2Cl2 for TTIMo and TTIMo+I-. Both compounds showed a distinct decrease in the photocyclization quantum yields with the red shift of excitation wavelength. Similar wavelength dependence was also found in the photocyclization reaction of TTIMo·H+ (Figure S17 in the Supporting Information). The shorter wavelength excitation seems to cause electronic transition involving the 6π-electron system for both compounds resulting in higher photocyclization sensitivity. Excitation at longer transition band should fall into the CT states, leading to the suppressed photocylclization induction. Concurrently, the fluorescence quantum yields increase with the red shift of excitation wavelength (Table S4 in the Supporting information). The wavelength dependence of the quantum yields is more significant in TTIMo+I- compared to TTIMo. This difference could be explained from the viewpoint of the interaction in the excited state PESs between two main transition states (Scheme 4). One is responsible for the photocyclization reaction (ESII1) and the other one predominantly has a CT character (ESI1). In the case of TTIMo+I-, a sharp dependence of photocyclization quantum yield on the excitation wavelength suggested that the mixing of these two excited state PESs is considerably weak. In contrast, PESs of the two excited states are considered to be more mixed


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in TTIMo, resulting in the gradual wavelength dependence. Based on the theoretical and spectroscopic studies, we could conclude that the photocyclization and CT processes in the excited states of terarylene photoswitches was controlled by the modulation of π-conjugation system, which critically affects the distribution of MOs on the reactive 6π-system as well as the interplay between the two closely-lying excited states. Since such the adiabatic excited state reaction is often accompanied by an activation barrier,44 the photochromic performance might be also dependent on temperature.

Figure 7. Excitation dependent photocyclization quantum yields (ΦOC) in CH2Cl2; a) TTIMo and b) TTIMo+I-. Experimental absorption spectra are also shown.


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Scheme 4. Schematic energy diagram of terarylene photoswitches; a) TTIMo and b) TTIMo+I-.

CONCLUSION

In summary, we introduced a thienothiophene-imidazole conjugated system into a terarylene structure to investigate the excited state interaction between the CT state and photocyclization


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reaction. TTIMo showed much suppressed photocyclization reactivity due to the less distribution of MOs on the photoreactive 6π-system, exhibiting the less prominent dependence of the excitation wavelength. In contrast, TTIMo+I- gave more efficient photocyclization reactivity because of the rather localized MOs on the 6π-system, which drastically decreased with the red shift of irradiation wavelength. The differences in the dependence of irradiation wavelength on the photocyclization reactivity can be explained by the interplay between the PESs of closelylying excited states. The less reactivity at longer-wavelength excitation is hence ascribed to the competing CT emission processes. According to the Kasha’s rule,46,47 which is an empirical principle in photophysics and photochemistry, fluorescence emission undergoes from the lowestlying excited state. The concept has been extended to the excited state adiabatic reaction and the quantum yields of photoreactions are mostly independent of the excitation wavelength.65 In the present study, we demonstrated the wavelength-selective photoprocesses by changing the excited state electronic structure through slight chemical modifications, modulating the interplay between the excited state PESs. Meanwhile, Kasaha’s rule breaking has been recently reported for several photochemical reaction systems.66-71 These observations possess great potential to carve out the novel photochemical molecular systems. The present study would provide a promising approach for the design of a new generation of fluorescent photoswitches, which can be demanding in many applications.


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ASSOCIATED CONTENT Supporting Information. Synthetic details, 1H and 13C NMR spectra, optical properties, and quantum chemical calculations. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was partly supported by JSPS KAKEN-HI Grant Numbers JP15K13709, JP15H03858 and JP26107006. The authors are grateful to Prof. Keitaro Nakatani and Dr. Rémi Métivier for fruitful discussions.

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TOC GRAPHICS


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Photophysical Properties of a Terarylene Photoswitch with a Donor

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