Photocyclization Reactions of Diarylethenes via the Excited Triplet State

Article pubs.acs.org/JPCA

Photocyclization Reactions of Diarylethenes via the Excited Triplet State Ryutaro Murata, Tomoaki Yago, and Masanobu Wakasa* Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570, Japan ABSTRACT: Cyclization reactions of three diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), 1,2-bis(2-hexyl-3-benzothienyl)perfluorocyclopentene (BTHex), and 1,2-bis(2-isopropyl-3benzothienyl)perfluorocyclopentene (BTiPr), via their excited triplet states were studied by means of steady-state and nanosecond transient absorption spectroscopy. The excited triplet states of BT, BTHex, and BTiPr were generated by energy transfer from the photoexcited triplet states of sensitizers such as xanthone, phenanthrene, and pyrene. The single-step quantum yields of the cyclization reactions from the excited triplet states of BT, BTHex, and BTiPr were determined to be 0.34, 0.53, and 0.65, respectively. The triplet energies of these three BTs were estimated to be 190−200 kJ mol−1.

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INTRODUCTION Photochromic materials undergo fast light-induced changes in their properties and have potential applications in optoelectronic devices, memory devices, and switches.1−4 Diarylethene (DAE) derivatives, which are representative organic photochromic compounds, undergo efficient, thermally irreversible, and fatigue-resistant cyclization and cycloreversion reactions.1,5−10 Specifically, a closed-form isomer is transformed to an open-form isomer (cycloreversion reaction) by visible light irradiation, and the open form is transformed to the closed form (cyclization reaction) by UV light irradiation. These reactions, which are responsible for the photochromism of DAEs, induce large changes in their electronic structures. The primary photochemical processes of DAEs have been thoroughly investigated by femtosecond and picosecond timeresolved spectroscopy. Miyasaka et al. measured the femtosecond time-resolved absorption spectra of 1,2-bis(2-methyl-3benzothienyl)perfluorocyclopentene (BT) in nonpolar alkane solutions and reported that the cyclization reaction occurs within 0.45 ps via the excited singlet state of BT but does not occur via the triplet state.11 These investigations found that the fast cyclization reaction via the excited singlet state prevents the generation of the excited triplet state. Thus, the possibility of a cyclization reaction proceeding via the excited triplet state cannot be determined under direct-excitation conditions. Recently, several research groups, including ours, reported that photocyclization reactions of DAEs can also be initiated via their excited triplet states.12−15 Indelli et al. studied the reactions of dyads containing ruthenium(II) polypyridine (Ru(py)) and the DAE of 1,2-bis(2-methylbenzothiophene-3yl)maleimide (Ru(py)−DAE) in acetonitrile by a combination of stationary and time-resolved spectroscopy.12 These dyads can undergo photocyclization reactions not only via the excited singlet state of the DAE but also via the triplet state. Specifically, in the singlet pathway, energy transfer occurs from the excited singlet state of the DAE, 1DAE*, to the metal© 2015 American Chemical Society

to-ligand charge transfer (MLCT) state of the Ru(II) complex, Ru(py), generating the excited singlet state of the Ru(II) complex, 1Ru(py)*; then the excited triplet state, 3Ru(py)*, forms by means of fast intersystem crossing (ISC): Ru(py)−1DAE* → 1Ru(py)*−DAE → 3Ru(py)*−DAE (1) 3

In the triplet pathway, energy transfer occurs from Ru(py)* to DAE, generating the excited triplet state of DAE, 3DAE*: 3

Ru(py)*−DAE → Ru(py)−3DAE*

(2)

Both singlet and triplet energy transfer processes are expected to occur completely, generating 3Ru(py)*−DAE and Ru(py)−3DAE*, respectively. The lifetimes of the singlet and triplet exited states of the donors (on the order of nanoseconds for 1DAE* and microseconds for 3Ru(py)*, respectively) are much longer than the intrinsic times for the energy transfer processes (30 ps for eq 1 and 1.5 ns for eq 2, respectively). Thus, the cyclization reaction via the excited triplet state is more efficient than that of naked DAEs in these Ru(py)-DAEs dyads because of their internal triplet energy transfer. Jukes et al. also investigated the photochromic reactions of Ru(II)- or Os(II)-containing DAE derivatives upon excitation of the lowest singlet MLCT state. The Ru(II)-containing DAE derivative undergoes a photocyclization reaction from the triplet MLCT state generated by ISC. In contrast, the Os(II)containing DAE derivative does not undergo a photocyclization reaction.13 Fukaminato et al. reported the photochromic reactions of DAE−perylenebisimide dyads.14 Upon irradiation of the perylene unit alone (at 400−530 nm), the cyclization reaction Received: August 23, 2015 Revised: October 21, 2015 Published: October 22, 2015 11138

DOI: 10.1021/acs.jpca.5b08205 J. Phys. Chem. A 2015, 119, 11138−11145

Article

The Journal of Physical Chemistry A

Samples were deoxygenated by bubbling with argon gas for 10 min before measurements. All measurements were performed at 296 K. Nanosecond Laser Flash Photolysis. Nanosecond laser flash photolysis was carried out with an apparatus that was essentially the same as that described elsewhere.19 The third and fourth harmonics (355 and 266 nm) of a Nd:YAG laser (Quanta-Ray INDI) with a pulse width of 5−7 ns were used as the excitation light source. The pulsed probe light from a Xe short-arc lamp (PerkinElmer Optoelectronics Lx-300 or Ushio UXL-500D) with a custom-built pulse current generator was divided into two beams, creating a double-beam probe system. One beam passing through a lower wavelength light cutoff filter (cutoff wavelength < monitored wavelength − 20 nm), and a shutter was guided to a quartz sample cell. The other beam was detected directly. Both beams were guided to photomultipliers (Hamamatsu R636-10) through a monochromator (Oriel MS257 and Shimadzu SPG-120S, respectively). This doublebeam probe system was constructed to accurately observe transient absorption by maintaining a flat baseline signal. In the present study, the baseline was somewhat noisy, owing to Qswitching of the laser and the pulsed trigger of the Xe lamp. The signal voltage from the photomultiplier was terminated by a 50 Ω resistor and was recorded by a 2 GHz digitizing oscilloscope (LeCroy Wave Pro 960). A personal computer was used to control the apparatus and to record data. Samples were deoxygenated by bubbling with argon gas for 30 min before the measurements. A sample containing the open-form isomer (100%) of the DAE derivative was pumped through a quartz sample cell (20 mL/min). All measurements were performed at 296 K. In the present set up of the transient absorption measurements, the probe beam is weak enough compared with the excitation pulse and the contribution of the conversion of the close form of BTs to open form is negligibly small.

shows excitation wavelength dependence and is affected by the presence of oxygen, suggesting that the reaction occurs via the excited triplet state. The same research group recently synthesized a π-conjugated DAE compound consisting of Nbis(1-hexylheptyl)perylene-3,4-dicarboxylic monoimide, acetylene, and DAE and studied its photochromic reactions.15 The open form of the compound undergoes photocyclization upon irradiation with 560 nm light, whereas the closed form converts to the open form upon irradiation with 405 nm light; the reaction pathway was not discussed in the paper. To the best of our knowledge, the cyclization of naked DAE via its excited triplet state has not been reported. Moreover, basic data on the excited triplet states of DAEs are completely lacking because the triplet reaction pathway is generally masked by the fast cyclization reaction and relaxation of the excited singlet state. Recently, we carried out preliminary studies of the triplet-sensitized reaction of BT, a DAE derivative, by nanosecond laser flash photolysis and found that cyclization occurs via the excited triplet state of BT.16 However, this previous study was limited to the reaction of BT with xanthen9-one (xanthone), a triplet sensitizer. Here, we extended the study to include not only BT but also 1,2-bis(2-hexyl-3benzothienyl) perfluorocyclopentene (BTHex), and 1,2-bis(2isopropyl-3-benzothienyl) perfluorocyclopentene (BTiPr) (Scheme 1), as well as various sensitizers. In this paper, we fully describe the work, some of which was reported in a preliminary paper. Scheme 1. Molecular Structure of DAE Derivatives Used in the Present Study

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RESULTS AND DISCUSSION Reaction Scheme. We measured the UV−vis spectra of MeOH solutions of the open forms of BT, BTHex, and BTiPr, designated BTs(O), at a concentration of 1.0 × 10−5 mol dm−3 (Figure 1a) and then measured the spectra again 10 min after UV irradiation at 313 nm (Figure 1b). The spectra obtained after irradiation exhibited new absorption bands around 350 and 520 nm, which can safely be assigned to the corresponding closed forms of the BTs, designated BTs(C).17,20 The spectral properties of the BTs(O) and BTs(C) are listed in Table 1. Direct excitation of the BTs(O) by UV light generates the excited singlet states of the BTs(O), designated 1BTs(O)*, which cyclize to form the BTs(C).

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EXPERIMENTAL SECTION Materials. BT (Kanto Chemical (Cica)) was used as received. BTHex and BTiPr were synthesized and purified as described in the literatures.17,18 Xanthone (Xn, Tokyo Kasei (TCI)), 4-methoxybenzophenone (MBP, Cica), phenanthrene (Phen, Cica), coronene (Cor, TCI), pyrene (Py, Cica), benz[a]anthracene (BAnth, TCI), acrizine (Ac, Acros), and anthracene (Anth, TCI) were recrystallized from methanol or ethanol and used as triplet sensitizers. MeOH and toluene (Cica, HPCL grade) were used as reaction solvents. Steady-State UV−Vis Absorption Measurements. UV−vis absorption spectra were measured with a JASCO V7200 spectrometer. Quantum yields of the cyclization reactions were evaluated by monitoring the absorptions of the closed forms of BT, BTHex, and BTiPr at the wavelengths of the absorption peaks, λmax, at 522, 542, and 539 nm, respectively.

BTs(O) + hν(UV) → 1BTs(O)* → BTs(C)

(3)

BTs(O) did not absorb at wavelength higher than 370 nm (Figure 1a). We measured the stationary UV−vis spectra of MeOH solutions containing BTs [(0.8−1.1) × 10−4 mol dm−3] and Xn (4.0 × 10−2 mol dm−3) under an Ar atmosphere before and after UV irradiation at 370 nm (a wavelength that excites only Xn). A typical spectrum observed 180 s after irradiation of a MeOH solution of BTHex and Xn is shown in Figure 2, together with the spectrum observed after irradiation of solution containing BTHex only. The spectra clearly indicate that the closed form, BTHex(C), was generated. Similar results were also observed upon irradiation of MeOH solutions 11139


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Figure 1. UV−vis absorption spectra of BT (black), BTHex (blue), and BTiPr (red) observed in MeOH (1.0 × 10−5 mol dm−3), (a) before and (b) 10 min after irradiation with UV light at 313 nm.

where 1Xn* and 3Xn* represent the excited singlet and triplet states of Xn, respectively. Upon irradiation of Xn, 3Xn* is immediately generated from 1Xn* via the fast ISC (eq 4). Then energy transfer from 3Xn* to the BTs(O) to form the excited triplet state, 3BTs(O)*, occurs with an associated rate constant of kq (eq 5). Finally, the closed forms of the BTs, BTs(C), are generated from the 3BTs(O)* (eq 6). To study the triplet sensitized reactions of the BTs with Xn in detail, we carried out laser flash photolysis experiments. Upon irradiation of MeOH solutions of Xn (2.6 × 10−3 mol dm−3) and the BTs [(0.22−0.60) × 10−3 mol dm−3] with the third harmonic (355 nm) of Nd:YAG laser, the transient absorption spectra at delay times of 0.1, 0.5, and 5.0 μs after laser irradiation are observed (Figure 3). The strong triplet− triplet (T−T) absorption band at 600 nm due to 3Xn* disappeared quickly, and then new bands appeared around 520 nm. The intensities of latter bands did not decrease at a longer delay time of (15 μs) and the absorption peak wavelengths and shapes agreed well with those of the BTs(C).17,20 The time profiles of the transient absorption, A(t), observed at 600 nm for the reaction of BT with Xn, are shown in Figure 4 for three BT concentrations. The decay of 3Xn* was accelerated with increasing BT concentration of [(0−0.49) × 10−3 mol dm−3]. The relationship between BT concentration and the rate constant for 3Xn* decay at 600 nm, kdecay(600 nm), was linear (Figure 5). Similar results were observed for the reactions of BTHex and BTiPr with Xn. The quenching rate constants of 3Xn* by BTs, kq, were determined from the slopes of the plots of kdecay(600 nm) versus BTs concentrations and are listed in Table 2. These results indicate that energy transfer from 3Xn* to the BTs occurred efficiently (eq 5). The A(t) curves observed around 520 nm are shown in Figure 6. Each curve has a rise component (0 < t ≤ 1.0 μs), a decay component (1.0 < t ≤ 15 μs), and an almost constant component (15 μs < t). For the reactions of the BTs with Xn (1.0 × 10−3 mol dm−3), the rate constants of the rise component, krise(520 nm), obtained around 520 nm are listed in Table 3, together with the rate constants of the decay components observed at 600 nm, kdecay(600 nm), under the same conditions. For each reaction, the value of krise(520 nm) agreed well with the value of kdecay(600 nm) for 3Xn*. Thus, 3 Xn* and the species responsible for the rise component observed around 520 nm have a parent−child relationship.

Table 1. Absorption Peak Wavelengths (λmax) and Molar Extinction Coefficients (ε) of Open- and Closed-Form Isomers of BT, BTHex, and BTiPra open-form isomer (BTs(O)) BT BTHex BTiPr

closed-form isomer (BTs(C))

λmax/nm

ε/mol−1 dm3 cm−1

λmax/nm

ε/mol−1 dm3 cm−1 b

258 259 258

1.3 × 10 1.4 × 104 1.8 × 104

522 542 539

0.93 × 104 0.96 × 104 0.94 × 104

4

a BT, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene; BTHex, 1,2-bis(2-hexyl-3-benzothienyl)perfluorocyclopentene; BTiPr, 1,2-bis(2-isopropyl-3-benzothienyl)perfluorocyclopentene. bObtained from the conversion yield of the open-form isomer (measured by HPLC) and the observed absorbance of the closed-form isomer.

Figure 2. UV−vis spectra observed 180 s after UV irradiation at 370 nm for the reaction of (a) BTHex (8.4 × 10−3 mol dm−3) and Xn (4.0 × 10−2 mol dm−3) (red) and (b) of BTHex only in MeOH (black).

containing Xn and BT or BTiPr with Xn. These tripletsensitized reactions of BTs can be expressed to occur via the following pathway: Xn + hν → 1Xn* → 3 Xn* kq

BTs(O) + 3 Xn* → 3BTs(O)* + Xn 3

BTs(O)* → BTs(C)

(4) (5) (6) 11140


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Figure 4. Time profiles of the transient absorption, A(t), observed at 600 nm for the reaction of BT (0 (black), 0.15 (blue), and 0.49 (red) × 10−3 mol dm−3) with Xn (1.0 × 10−3 mol dm−3) in MeOH upon irradiation at 355 nm.

Figure 5. BT concentration dependence of the decay rate constant of 3 Xn* observed at 600 nm, kdecay(600 nm), for the reaction of BT [(0− 0.49) × 10−3 mol dm−3] with Xn (1.0 × 10−3 mol dm−3) in MeOH upon irradiation at 355 nm. 1

ap‐BTs(O)* → BTs(C)

1

(7)

p‐BTs(O)* → 3p‐BTs(O)* → BTs(O)

(8)

where 1ap-BTs(O)*, 1p-BTs(O)*, and 3p-BTs(O)* represent the excited singlet state of the ap-BTs(O), and the excited singlet and triplet states of the p-BTs(O), respectively. In previous work,16 we found that upon direct irradiation of BT, the T−T absorption of p-BT was observed at 450−600 nm, and the decay rate constant was 0.14 × 106 s−1. This value agrees well with the kdecay(520 nm) value observed for the present triplet-sensitized reaction of BT with Xn. The kdecay(520 nm) values observed for the triplet- sensitized reactions of the BTs with Xn are listed in Table 4, together with the values observed for the direct reactions of the BTs upon irradiation at 266 nm. Because the kdecay(520 nm) values observed for the triplet-sensitized reactions and the direct reactions were almost the same, we ascribed the decay component observed at 520 nm for the triplet-sensitized reactions to the decay of the 3pBTs(O)*.

Figure 3. Transient absorption spectra observed for the reactions of (a) BT (0.60 × 10−3 mol dm−3), (b) BTHex (0.22 × 10−3 mol dm−3), and (c) BTiPr (0.26 × 10−3 mol dm−3) with Xn (2.6 × 10−3 mol dm−3) in MeOH at delay times of 0.1 μs (black), 0.5 μs (blue), and 5.0 μs (red) after laser irradiation at 355 nm.

These results provide evidence that the reactions shown in eqs 4−6 did in fact occur in our system. We next investigated what was responsible for the decay component observed around 520 nm. Note that the open-form isomers of the BTs have a conformer in which the two aryl groups are parallel and a conformer in which the groups are anti-parallel, designated p-BTs(O) and ap-BTs(O), respectively.21 In the case of direct reaction of the BTs, the cyclization reactions from the excited singlet states occur only from the apBTs(O), whereas the p-BTs(O) convert to the excited triplet states, which then decay back to the ground states:16 11141


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Table 2. Triplet Energies (ET) of Sensitizers, Quenching Rate Constants (kq) of BT, BTHex, and BTiPr, and Generation of Closed-Form Isomers kq/109 dm3 mol−1 s−1 −1 a

ET/kJ mol

sensitizer xanthoneb 4-methoxybenzophenoneb phenanthrenec coronenec pyrenec benz[a]anthracenec acridine anthracene a

310 290 257 228 202 198 188 178

generation of closed-form isomer

BT

BTHex

BTiPr

BT

BTHex

BTiPr

5.1 1.2 0.13 0.06 0.04 NQd NQd NQd

4.9 1.8 0.10 0.02 0.02 0.03 NQd NQd

5.0 1.8 0.08 0.01 0.02 0.02 0.04 NQd

○ ○ ○ ○ ○ × × ×

○ ○ ○ ○ ○ ○ × ×

○ ○ ○ ○ ○ ○ ○ ×

Reference 19. bObserved in MeOH. cObserved in toluene because the sensitizer was insoluble in MeOH. dNot quenched.

On the basis of the laser flash photolysis experiments, the pathway for the triplet-sensitized reactions can be represented as follows: k1

p‐BTs(O) + 3 Xn* → 3p‐BTs(O)* + Xn k2

3

p‐BTs(O)* → BTs(O) k3

ap‐BTs(O) + 3 Xn* → 3ap‐BTs(O)* + Xn 3

k4

ap‐BTs(O)* → BTs(C)

cyclization reactions of the BTs upon irradiation in the presence of Xn (eqs 4−6) were evaluated by means of the standard procedures using the quantum yields of the cyclization reactions of the BTs from the excited singlet states (ϕS) for a reference.17,18,22 MeOH solutions of BT (typically concentration of 0.099 × 10−3 mol dm−3) and Xn (typically concentration of 0.47 × 10−3 mol dm−3) were irradiated for 30 s with steady-state UV light (355 nm, 0.28 mW cm−2). For reference, a MeOH solution containing only BT (0.83 × 10−3 mol dm−3) was also irradiated under the same conditions; in this case, the absorbance of the solution observed at 355 nm was same as that of the sample solution containing both BT and Xn. The determined ϕoverall values are listed in Table 6. In this T experiment, we assumed that the ϕS values obtained with the excitation wavelength of 355 nm were the same as the values obtained at a wavelength of 313 nm where the reference ϕS value was observed. Moreover, the contributions of direct excitation of the BTs during the triplet-sensitized reaction were subtracted from the raw data. Our results indicate that the cyclization reaction processes included triplet energy transfer from 3Xn* to the BTs. Thus, the determined values do not correspond exactly to the singlestep quantum yields for the cyclization reactions of the BTs from their triplet states. To obtain the single-step quantum yields (ϕT) for the cyclization reactions from the excited triplet states of the BTs (eq 6), we had to evaluate the triplet energy transfer efficiency (ϕET) from 3Xn* to the BTs. The value of ϕET can be calculated from the following relation,

(9) (10) (11) (12)

3

At 520 nm, p-BTs(O)* and BTs(C) have transient absorption. The 3ap-BTs(O)* may display the absorption at 520 nm, although there is no report of its absorption because of their fast cyclization reaction. However, the contribution of the 3apBTs(O)* is negligible because k4 is much larger than k3 (k4 is thought to be much larger than 109 s−1). The time profiles of concentrations of 3p-BTs(O)* and BTs(C) can be expressed as follows: [3p‐BTs(O)*] = a1

k1 (exp( −k1t ) − exp( −k 2t )) k 2 − k1 (13)

[BTs(C)] = a 2(1 − exp(−k 3t ))

(14)

where a1 and a2 are scaling factors. Thus, A(t) can be represented as follows: A(t ) = A1

k1 (exp( −k1t ) − exp(−k 2t )) k 2 − k1

+ A 2 (1 − exp(−k 3t ))

ϕET = (15)

kq[BTs] kq[BTs] + k 0

(16)

where kq and k0 denote the quenching rate constant of 3Xn* by the BTs and the decay rate constant of the excited triplet state of Xn in the absence of the BTs. These values can be measured by means of transient absorption analysis. In the presence of the same concentration of BTs determined the ϕoverall values, T the ϕET values were obtained by eq 16 and are listed in Table 6. The efficiency of ISC from the singlet state to the triplet state, ϕISC, should also be considered in the determination of the ϕT value. If ϕISC is considered, then the single-step quantum yields for the cyclization reactions from the excited triplet states of BTs, ϕT, can be represented by the following equation:

where A1 and A2 are the scaling factors. A(t) curves observed around 520 nm were analyzed by using eq 15. In Figure 4, red lines indicate the A(t) curves calculated by eq 15. The parameters used in the calculations are listed in Table 5. As is clearly shown in the figure, the observed A(t) curves were well reproduced by the proposed reaction pathway with a parameter set. The rate constants k1 and k3 correspond to the rate constant of triplet energy transfer from 3Xn* to the BTs. We found that the k1 and k3 values were almost the same as the kdecay(600 nm) values observed experimentally. The rate constant k2 corresponds to the rate constant for the decay of 3 p-BTs*, and the values of k2 were the almost the same as the values of kdecay(520 nm). Quantum Yields of the Cyclization Reactions from the Triplet States. The overall quantum yields (ϕoverall ) of the T

ϕT = 11142

ϕToverall ϕETϕISC

(17) DOI: 10.1021/acs.jpca.5b08205 J. Phys. Chem. A 2015, 119, 11138−11145

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Table 3. Rate Constants of Rise Component Observed around 520 nm and Decay Rate Constants Observed at 600 nm for the Reactions of BT, BTHex, and BTiPr with Xn (1.0 × 10−3 mol dm−3) upon Irradiation at 355 nm

BT BTHex BTiPr

concentration/ mol dm−3

krise(520 nm)a/ s−1

kdecay(600 nm)/ s−1

0.33 × 10−3 0.42 × 10−3 0.27 × 10−3

2.5 × 106 3.6 × 106 2.5 × 106

2.7 × 106 3.3 × 106 2.4 × 106

a

Observed at 522 nm for BT, 542 nm for BTHex, and 539 nm for BTiPr.

clearly blocked because of the lower triplet energy and the difference in spin multiplicity from that of the singlet ground state. Such factors may contribute to the higher ϕT values observed in the present study. As mentioned above, because the p-BTs(O) do not undergo the cyclization reaction, the quantum yield of the cyclization reaction (ϕS and ϕT) should not exceed the proportion of the ap-BTs(O) in the BTs(O). Thus, consideration of the proportion of the ap-BTs(O) is important for controlling the efficiency of the cyclization reaction. We measured the proportions of the ap-BTs(O) in the BTs(O) by NMR (Table 6) and found that the proportions increased in the order BT < BTHex < BTiPr. In the case of BT, ϕT and ϕS were almost the same (0.34 and 0.35, respectively); because the proportion of ap-BT(O) was 0.56, 60% of the ap-BT(O) was transformed to BT(C). In contrast, for BTiPr, ϕT (0.65) was much larger than ϕS (0.52). Because the proportion of apBTiPr(O) is 0.93, 70% of the ap-BTiPr(O) could react to form BTiPr(C). These results indicate that the efficiency of the cyclization reaction of BTs can be controlled by using the triplet-sensitized reaction and increasing the proportion of the ap-isomer. Triplet Energies of BTs. The triplet energy level of the DAE segments was proposed qualitatively from the timeresolved studies on Ru(II) and Os(II) complexes with DAE appendant.12,13 The triplet energy was also estimated from the density functional theory.12 To the best of our knowledge, triplet energies of naked DAE derivatives have not yet been determined quantitatively from the experimental observations. Because the triplet energy of a sensitizer strongly influences the efficiency of energy transfer and thus the yields of the cyclization reaction, the triplet energies of BTs can be determined by carrying out sensitization reactions using various triplet sensitizers. In this study, we used xanthone (Xn), 4methoxybenzophenone (MBP), phenanthrene (Phen), coronene (Cor), pyrene (Py), benz[a]anthracene (BAnth), acrizine (Ac), and anthracene (Anth); their triplet energies, ET, employed in the preset study are summarized in Table 2.23 First, energy transfer from the excited triplet state of each sensitizer to the BTs was examined by quenching the T−T absorption of each triplet sensitizer excited at 355 nm. In the presence of various concentrations of BTs, quenching rate constants, kq, were observed (Table 2). When Xn, MBP, Phen, Cor, Py or BAnth was used as a sensitizer, the T−T absorption of the sensitizer was clearly quenched by BT, BTHex, and BTiPr; the only exception was the combination of BAnth and BT. Next, upon irradiation of each triplet sensitizer in solution with the BTs, steady-state absorption spectra were measured to confirm generation of the BTs(C). The results of the

Figure 6. Time profiles of the transient absorption, A(t), observed around 520 nm for the reaction of (a) BT (0.33 × 10−3 mol dm−3), (b) BTHex (0.42 × 10−3 mol dm−3), and (c) BTiPr (0.27 × 10−3 mol dm−3) with Xn (1.0 × 10−3 mol dm−3) in MeOH upon irradiation at 355 nm (black), together with the A(t) curves observed at 600 nm (blue). Red lines indicate the A(t) curves calculated with eq 15 and the parameters listed in Table 4.

The ϕISC value of Xn is 1.0,23 and the resulting ϕT values for BT, BTHex, and BTiPr are listed in Table 6. Surprisingly, the ϕT values for BT, BTHex, and BTiPr (0.34, 0.53, and 0.65, respectively) obtained were either almost equal to or much higher than the corresponding ϕS values (0.35, 0.49, and 0.52, respectively). Ishibashi et al. suggested that an ultrafast relaxation from the photoexcited singlet state to the charge transfer state (time constant of 450 fs) competes with the cyclization reaction.11 In the triplet state, this fast relaxation is 11143


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Table 4. Decay Rate Constants Observed at 520 nm for the Triplet-Sensitized Reactions of BT, BTHex, and BTiPr with Xn (1.0 × 10−3 mol dm−3) upon Irradiation at 355 nm and for the Direct Reaction upon Irradiation at 266 nm kdecay(520 nm)/s−1 a (concentration) BT BTHex BTiPr a

triplet-sensitized reaction

direct reaction

0.14 × 106 (0.33 × 10−3 mol dm−3) 0.17 × 106 (0.42 × 10−3 mol dm−3) 0.15 × 106 (0.27 × 10−3 mol dm−3)

0.14 × 106 (0.83 × 10−3 mol dm−3) 0.19 × 106 (0.80 × 10−3 mol dm−3) 0.13 × 106 (0.48 × 10−3 mol dm−3)

Observed at 522 nm for BT, 542 nm for BTHex, and 539 nm for BTiPr.

are either almost equal to or much higher than the corresponding quantum yields from the excited singlet state. Our results indicate that the efficiency of the cyclization reactions of diarylethene derivatives can be controlled by using the corresponding triplet-sensitized reactions. These alternatives to cyclization reactions initiated by visible light irradiation provide a new method for fabricating various functional devices with diarylethene derivatives.

Table 5. Parameters Used for Fitting of Time Profiles Observed around 520 nm for the Reaction of BT, BTHex, BTiPr, and Xn (1.0 × 10−3 mol dm−3) upon Irradiation at 355 nm BT

k1 and k3/s−1

k2/s−1

BT BTHex BTiPr

2.70 × 10 3.50 × 106 2.40 × 106

0.135 × 10 0.170 × 106 0.150 × 106

6

6

k4/s−1

A1

A2

>109 >109 >109

0.020 0.010 0.008

0.011 0.015 0.015

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cyclization reactions are also shown in Table 2. The trend observed for the sensitizers in the quenching experiments of T−T absorption was similar to the trend for the steady-state absorption spectra. The BTs(C) were generated when Xn, MBP, Phen, Cor, Py, or BAnth was used as the sensitizer (designated ○ in the table); the only exception was the combination of BAnth and BT. On the basis of the results of these experiments, we estimated the triplet energies of BT, BTHex, and BTiPr to be 200, 200, and 190 kJ mol−1, respectively. The determined triplet energies of BTs are consistent with the triplet energy (225 kJ mol−1) estimated from the density functional theory.12 Although not all of the measurements were carried out in the same solvent, owing to the poor solubility of some of the sensitizers, the triplet energy of BTiPr may be smaller than that of BT. This difference may have contributed to the higher ϕT value observed for BTiPr in the present study.

AUTHOR INFORMATION

Corresponding Author

*M. Wakasa. Tel: +81-48-858-3909. Fax: +81-48-858-3909. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research on the Innovative Area of “All Nippon Artificial Photosynthesis for Living Earth” (Area No. 2406) (No. 25107509) and “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” (Area No. 2408) (No. 15H00917) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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REFERENCES

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CONCLUSIONS Cyclization reactions of naked diarylethene via its excited triplet state have not been reported, and basic data for the excited triplet state of diarylethene derivatives are completely lacking. Here, we studied the cyclization of three diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene, 1,2-bis(2-hexyl-3-benzothienyl)perfluorocyclopent ene, and 1,2-bis(2-isopropyl-3benzothienyl)perfluorocyclopentene with various triplet sensitizers. Upon irradiation of a sensitizer with a triplet energy larger than ca. 200 kJ mol−1, cyclization reactions of three derivatives occurred via the excited triplet states. Single-step quantum yields for the cyclization reactions of three derivatives were as large as 0.34, 0.53, and 0.65, respectively. These values

Table 6. Quantum Yields of Overall Cyclization Reaction of BT, BTHex, and BTiPr via the Excited Triplet States (ϕoverall ), T Quantum Yields via the Excited Singlet State (ϕS), Triplet Energy Transfer Efficiencies (ϕET) from 3Xn*, Single-Step Quantum Yields via the Excited Triplet States (ϕT), and Proportions of ap-BTs(O) in BTs(O) BT b

BT BTHexd BTiPre a

concentration of BTs/mol dm−3 −3

0.099 × 10 0.096 × 10−3 0.054 × 10−3

ϕoverall T 0.24 0.32 0.46

ϕS c

0.35 0.49c 0.52f

ϕET

ϕT

proportion of ap-BTs(O)a

0.73 0.61 0.70

0.34 ± 0.01 0.53 ± 0.03 0.65 ± 0.04

0.56 0.83 0.93

In MeOD. bObserved at 522 nm. cReference 18. dObserved at 542 nm. eObserved at 539 nm. fReference 17. 11144


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Photocyclization Reactions of Diarylethenes via the Excited Triplet State

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