Photophysical Properties of a Terarylene Photoswitch with a Donor-Acceptor Conjugated Bridging Unit

Article abstract of DOI:10.1021/acs.jpca.7b00296

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.

Full text of DOI:10.1021/acs.jpca.7b00296

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Article
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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Photophysical Properties of a Terarylene
Photoswitch with a DonorꢀAcceptor Conjugated
Bridging Unit
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Rui Kanazawa,† Takuya Nakashima*,†, and Tsuyoshi Kawai*,†,‡
† Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST,
Ikoma, Nara, 630ꢀ0192, Japan
‡
NAISTꢀCEMES 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)743ꢀ72ꢀ6179
* Eꢀmail: tkawai@ms.naist.jp, ntaku@ms.naist.jp
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ABSTRACT
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A terarylene photoswitch composed of an electronꢀdonating thienothiophene unit which is
connected to the central bridging imidazole was designed. The electronic and geometrical
structures of the central πꢀconjugation 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 closelyꢀlying 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 openꢀring forms, which is explained by the formation of different pushꢀpull πꢀconjugation
systems.
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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 sideꢀchain 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 6πꢀframework.
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
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Meanwhile, imidazole ring play an important role as a key building unit in various pushꢀpull
πꢀconjugated architectures due to their electronic nature, robustness and convenience in synthesis
and chemical modifications.43 We developed πꢀconjugated fluorophores composed of Nꢀ
methylbenzimidazole and thieno[3,2ꢀb]thiophene moiety, forming acidꢀresponsive pushꢀpull
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 thienothiopheneꢀimidazole fluorescent unit into a terarylene photoswitching
system. Given the ordinary photophysical processes follow Kasha’s rule in principle,46,47 the
introduction of pushꢀpull 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 thienothiopheneꢀimidazole(imidazolium)
pushꢀpull system. The state of chemical modifications on the imidazole ring successfully
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modulated the excited state interaction and led to the wavelengthꢀselective
photophysicochemical processes.
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Scheme 1. The quinoidal resonance structure in an imidazoliumꢀthienothiophene fluorophore.
We constructed a new fluorescent terarylene photoswitch having an imidazole ring as a central
bridging unit, which extends the πꢀconjugation system from the C2ꢀposition of imidazole ring
with an electronꢀdonating thienothiophene moiety. The C4ꢀ and C5ꢀpositions of imidazole ring
carry electronꢀwithdrawing phenylthiazole units to build a terarylene backbone (TTIM, Scheme
2). 2ꢀPhenylꢀimidazole derivative (IM) was also prepared as a reference molecule. The central
Nꢀmethylimidazole ring is readily converted into a cationic imidazolium by chemical
modifications such as protonation and further Nꢀmethylation, enhancing the electronꢀaccepting
capability in the central bridged unit. The N,Nꢀdimethylation also introduces a twist between
imidazolium and thienothiophene moieties due to the steric interaction, leading to the suspension
of πꢀconjugation 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. Analyticalꢀgrade solvents purchased from Wako Pure
Chemicals were used for the optical measurements.
1H NMR and 13C NMR spectra were recorded by using a JEOL JNMꢀAL300 (1H: 300 MHz,
13C: 75 MHz) spectrometer. Mass spectra were measured by using a JEOL AccuTOF JMSꢀ
T100LC (ESI) instrument and a Bruker Daltonics Autoflex II (MALDIꢀTOF) spectrometer. UVꢀ
vis absorption spectra were recorded by using a JASCO Vꢀ670 spectrophotometer. Fluorescence
spectra were measured with a JASCO FPꢀ6500 fluorescence spectrophotometer. The absolute
fluorescence quantum yields were evaluated by using a Hamamatsu Photonics C9920ꢀ02
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 QYMꢀ01 photoreaction quantum yield measurement system.
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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 exchangeꢀcorrelation functional51,52 with 6ꢀ31G+(d,p) basis set
(DGDZVP basis set53,54 was used for the iodine atom). Excited state properties were computed
with timeꢀdependent 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 BeckeꢀJohnson damping
(GD3BJ).55–57
Synthetic procedures. Terarylenes with an Nꢀmethylimidazole bridge unit were synthesized
according to Scheme 3. The imidazole ring was prepared by the condensation cyclization
reaction between the βꢀdiketone (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) nꢀBuLi, DMF, dry
THF, −78°C to r.t.; ii) 3ꢀbenzylꢀ5ꢀ(2ꢀhydroxyethyl)ꢀ4ꢀmethylthiazolium chloride,
diisopropylamine, dry ethanol, reflux; iii) 5ꢀphenylthieno[3,2ꢀb]thiopheneꢀ2ꢀcarbaldehyde (for
TTIMoꢀNH) or benzaldehyde (for IMoꢀNH), 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
closedꢀring form (TTIMc) and recovery of the openꢀring form with the reversible 6πꢀ
electrocyclization 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+Iꢀ, 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 openꢀring and closedꢀring forms was improved from18% (TTIM)
to 31% (TTIM+Iꢀ).
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Figure 1. UVꢀvis absorption changes upon irradiation with 250 W highꢀpressure Hg lamp at 313
nm in CH2Cl2 (1.0 × 10−6 M): a) TTIM, b) TTIM·H+, and c) TTIM+Iꢀ; openꢀring 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.
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compd.a
TTIMo
λ
max / nm (ε / 104 L mol−1cm−1)
ΦOCb (313 nm)
ΦCO c (600 nm)
—
295 (3.7), 355 (4.3)
0.0083
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—
321 (3.1), 423 (4.9), 690 (1.8)
286, 365
0.016
TTIMc
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TTIMo·H+
TTIMc·H+
TTIMo+Iꢀ
TTIMc+Iꢀ
—
n.d.
—
360, 435, 625
n.d.
307 (4.5), 341 (3.2)d
0.1
—
—
325 (3.7), 419 (3.0), 670 (1.3)
0.03
aε values and photoreaction quantum yields were not determined for TTIMo·H+.
bPhotocyclization quantum yields (λirr = 313 nm). cPhotocycloreversion quantum yields (λirr =
600 nm). dShoulder of the absorption spectra.
DFT calculations for geometrical investigations. DFT calculations provided the optimized
ground state structures of the openꢀring forms in neutral state and its Nꢀsubstituted 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.58ꢀ60 The nearly planar structure was also found in the protonated form
with a NH···S hydrogen bonding interaction (TTIMo·H+, Figure 2b), whereas TTIMo+Iꢀ
possesses 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 openꢀring 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 lowestꢀlying excited state consists mainly of HOMO
→ LUMO transition with a weak oscillator strength. The HOMO mostly lies on the electronꢀ
donating thienothiophene unit, and the LUMO is localized on the electronꢀaccepting
phenylthiazole unit (Figure 4a). Because of the spatially separated MOs and a weak oscillator
strength, the lowestꢀlying 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
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(Table 2). The LUMO+1 of TTIMo expands to the imidazoleꢀthienothiophene moieties with a
planar geometry. It should be noted that both LUMO and LUMO+1 had little contribution to the
photoreactive 6πꢀsystem.
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TTIMo·H+ showed a redꢀshift 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 electronꢀaccepting 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+Iꢀ, the HOMO, HOMOꢀ1 and HOMOꢀ2 orbitals are mainly distributed on the
iodine ion, which are involved in the electronic transitions with negligible oscillator strengths.
The lowestꢀlying excited state of TTIMo+Iꢀ is mainly described by a mixing of HOMOꢀ4 →
LUMO and HOMOꢀ3 → 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 HOMOꢀ3 and LUMO suggested the CT nature between the electronꢀ
donating thienothiophene and the electronꢀaccepting imidazoliumꢀphenythiazole moieties. The
maximum absorption peak was computed to be 316 nm which was assigned to the HOMOꢀ3 →
LUMO+1 transition with the largest oscillator strength. Because of the twisted geometries in
TTIMo+Iꢀ, 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).
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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 HOMOꢀ1 →
LUMO+1 and HOMOꢀ2 → 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+Iꢀ).
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 πꢀconjugated 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+Iꢀ
is consistent with this consideration, which has a disconnection in the πꢀconjugation by the
twisting between thienothiophene and imidazolium units. The lower photoreactivity of
TTIMo+Iꢀ in comparison with that of IMo+Iꢀ could be partly attributed to the competing
emission process (see below).
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Table 2. Calculated electronic transition properties.
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compd.
fc
assignmentd
H → L (96%)
λexpa / nm
λcalcdb / nm
394
0.11
1.25
0.64
TTIMo
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H → L+1 (96%)
Hꢀ2 → L (47%)
H → L+3 (22%)
H → L (98%)
301
TTIMo·H+
365
286
341e
307
403
356
297
1.17
0.14
0.87
Hꢀ1 → L (96%)
Hꢀ1 → L+1 (45%)
H → L+3 (33%)
Hꢀ4 → L (40%)
Hꢀ3 → L (40%)
Hꢀ3 → L+1 (53%)
Hꢀ4 → L+1 (44%)
Hꢀ3 → L+1 (12%)
Hꢀ3 → L+2 (36%)
TTIMo+Iꢀ
320
0.28
316
312
0.81
0.28
aExperimental maximum of the absorption wavelengths. bCalculated maximum of the absorption
wavelengths. cOscillator strength. dH and L represent the HOMO and LUMO, respectively.
eShoulder of the absorption spectra.
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Figure 3. Experimental absorption spectra of the openꢀring forms (dotted lines) in CH2Cl2
together with TDDFT excitation energies and oscillator strength: a) TTIMo, b) TTIMo·H+, and
c) TTIMo+Iꢀ.
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Figure 4. Calculated MOs distributions.
Fluorescence property. TTIMo and its derivatives exhibited clear fluorescence in their openꢀ
ring forms, which was quenched by irradiation with UV light, converting the fluorescent openꢀ
ring forms to the nonꢀfluorescent closedꢀring forms (Figure 5). No emission was observed in
both of isomers for the reference compounds (IM and IM+Iꢀ). 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 nonꢀtwisting intramolecular CT emission characteristic to the thienothiopheneꢀ
imidazole fluorophores.44,45
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The structured emission in less polar solvents changed into a broad structureꢀless shape after
the protonation in TTIMo·H+ (Figure 6b). Furthermore, the emission maximum shifted to longer
wavelength, except for in 1,4ꢀdioxane and DMF solutions (Table 3). To elucidate the origin of
emission in TTIMo·H+, timeꢀresolved emission measurements were performed in solutions. As
shown in Table 3, the emission lifetime (τem) increased in response to acid, especially in nꢀ
hexane, cyclohexane and CH2Cl2. The decrease in nonradiative rate constants (knr) was also
observed in nꢀhexane and CH2Cl2 after protonation. The enhancement of electron accepting
character in the protonated imidazolium should increase the CT nature (nonꢀtwisting 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+Iꢀ 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+Iꢀ because of the steric hindrance. This twisting in TTIMo+Iꢀ should modify the
emissive excited state character. The emission origin of TTIMo+Iꢀ can thus be explained by the
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twisted CT state between the electronꢀdonating thienothiophene and the electronꢀaccepting
imidazolium moieties. The characteristic twisted structure in TTIMo+Iꢀ 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.
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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+Iꢀ.
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Table 3. Fluorescence properties in various solvents.a
3
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8
compd.
solventb
nꢀhexane
cyclohexane
toluene
kr / ns−1
0.53
0.69
0.57
0.51
0.49
0.45
0.46
0.36
0.33
0.34
0.48
0.46
0.52
0.45
0.04
0.11
0.24
0.48
0.52
knr / ns−1
λ
em / nm
ꢀνST / cm−1×103
5.55
Φem
0.25
0.35
0.36
0.35
0.35
0.33
0.39
0.22
0.20
0.22
0.33
0.35
0.45
0.38
0.027
0.064
0.19
0.41
0.39
τ
em / ns
0.48
0.51
0.63
0.69
0.72
0.73
0.84
0.62
0.60
0.64
0.69
0.76
0.87
0.84
0.63
0.58
0.79
0.85
0.74
414, 439
416, 441
427, 447
425, 446
447
1.58
1.29
1.01
0.94
0.91
0.92
0.72
1.26
1.33
1.21
0.97
0.86
0.63
0.73
1.54
1.61
1.02
0.70
0.82
TTIMo
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5.49
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5.48
1,4ꢀdioxane
THF
5.51
5.56
CH2Cl2
DMF
448
5.84
451
5.76
TTIMo·H+
nꢀhexane
cyclohexane
toluene
445
5.38
448
5.61
443
4.98
1,4ꢀdioxane
THF
424, 446
451
5.43
6.00
CH2Cl2
DMF
457
5.52
451
5.92
TTIMo+Iꢀ
toluene
443
9.79
1,4ꢀdioxane
THF
443
9.89
447
9.89
CH2Cl2
DMF
448
10.25
9.63
448
aParameters: maximum of the emission wavelength (λem), Stokes shift (ꢀνST), fluorescence
quantum yields (Φem), emission lifetime (τem), radiative (kr) and nonradiative (knr) rate constants.
bTTIMo+Iꢀ is insolved in nꢀhexane 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 shorterꢀwavelength 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
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Figure 7 shows the excitation wavelength dependence of the photocyclization reaction
quantum yields in CH2Cl2 for TTIMo and TTIMo+Iꢀ. 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 6πꢀelectron 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+Iꢀ 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+Iꢀ, 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 πꢀconjugation
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system, which critically affects the distribution of MOs on the reactive 6πꢀsystem as well as the
interplay between the two closelyꢀlying 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+Iꢀ. Experimental absorption spectra are also shown.
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Scheme 4. Schematic energy diagram of terarylene photoswitches; a) TTIMo and b) TTIMo+Iꢀ.
CONCLUSION
In summary, we introduced a thienothiopheneꢀimidazole 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 6πꢀsystem, exhibiting the less prominent dependence of
the excitation wavelength. In contrast, TTIMo+Iꢀ gave more efficient photocyclization reactivity
because of the rather localized MOs on the 6πꢀsystem, 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 closelyꢀ
lying excited states. The less reactivity at longerꢀwavelength 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 lowestꢀ
lying 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 wavelengthꢀselective 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.66ꢀ71 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
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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.
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AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail: ntaku@ms.naist.jp, tkawai@ms.naist.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This research was partly supported by JSPS KAKENꢀHI 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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