Substituent effect on the photoinduced structural change of Cu(I) complexes observed by femtosecond emission spectroscopy.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 4143

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Substituent effect on the photoinduced structural change of Cu(I) complexes observed by femtosecond emission spectroscopy† Munetaka Iwamura,‡a Satoshi Takeuchiab and Tahei Tahara*ab The Cu(I) complexes having phenanthroline derivatives as ligands are known to exhibit photo-induced ‘flattening’ structural change in the metal-to-ligand charge transfer (MLCT) excited state. Our recent ultrafast spectroscopic studies of [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline) showed that the photo-induced structural change predominantly occurs in the S1 state on a subpicosecond time scale, with the appearance of the ‘perpendicular’ S1 state before the structural change. In this work, we carried out femto/picosecond time-resolved emission spectroscopy of [Cu(phen)2]+ (phen = 1,10phenanthroline) and [Cu(dpphen)2]+ (dpphen = 2,9-diphenyl-1,10-phenanthroline) in dichloromethane with the S2 ’ S0 photo-excitation to examine the substituent effect on the ultrafast structural change. The femtosecond time-resolved emission spectra of the two complexes exhibit ultrafast fluorescence changes that are attributed to the structural change in the S1 state after fast (50–100 fs) S2 - S1 internal conversion. By comparing with the dynamics of [Cu(dmphen)2]+, it was found that the time constant of the structural change increases as the substituents at 2- and 9- positions of the ligand become bulkier, i.e., [Cu(phen)2]+ (200 fs) o [Cu(dmphen)2]+ (660 fs) o [Cu(dpphen)2]+ (920 fs). This implies that the

Received 12th October 2013, Accepted 9th December 2013

complex needs a longer time to flatten with the bulkier substituent. This demonstrates that the dynamics

DOI: 10.1039/c3cp54322f

of the photo-induced structural change of Cu(I) complexes is substantially affected by the substituent of the ligand. The dynamics of the ultrafast structural change and the substituent effect are discussed with

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the multidimensional S1 potential energy surface of Cu(I) complexes.

1. Introduction In the last four decades, metal complexes have been one of the most attractive classes of molecules in application fields such as solar energy conversion and photo-devices, and their photochemistry and photophysics have received much attention.1–5 In fact, photochemical properties of metal complexes meet the a

Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. E-mail: [email protected]; Fax: +81-48-467-4539; Tel: +81-48-467-4592 b Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan † Electronic supplementary information (ESI) available: Following representations are shown: (1) viscosity dependencies of the emission time profile. (2) Analytical expressions for time-dependent population of excited states and the formula that connects Ai’s and ai’s in the quantitative analysis of femtosecond fluorescence up-conversion data. (3) Procedure for obtaining the absolute oscillator strengths using the phosphorescence intensity as a reference and estimations of f1 and f2. (4) Procedure for obtaining the oscillator strength from absorption spectra. (5) Full-set of femtosecond time-resolved fluorescence data for [Cu(phen)2]+ and [Cu(dpphen)2]+ in dichloromethane obtained by upconversion. See DOI: 10.1039/c3cp54322f ‡ Current address: Department of Chemistry, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan.

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requirements for the sensitizer for solar energy conversion, i.e., strong absorption in the visible region and long-lived excited states to hold the excitation energy. From this viewpoint, copper(I) complexes of 1,10-phenanthroline derivatives (Cu(I) phenanthroline complexes) have been studied intensively because they exhibit intense metal to ligand charge transfer (MLCT) transitions in the visible region and have long-lived 3 MLCT states. These characteristics of the copper(I) complexes are comparable to much more expensive [Ru(bipyridine)3]2+ which is the most well-known metal complex used as a sensitizer for solar energy conversion.3–6 Besides its importance in applications, the Cu(I) phenanthroline complex also provides an important prototype for fundamental photophysical/photochemical studies of metal complexes because it exhibits a characteristic structural change in the MLCT state. For example, [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline) in the ground state has a tetrahedral-like D2d structure, where two ligands are attached to the copper(I) ion perpendicularly to each other.3 With the MLCT excitation, the central metal is formally oxidized from Cu(I) to Cu(II), and it induces a large conformational change from the initial tetrahedral-like structure toward the square

Phys. Chem. Chem. Phys., 2014, 16, 4143--4154 | 4143

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Fig. 2

Fig. 1

Photoinduced flattening distortion of [Cu(dmphen)2]+.

planar-like structure which is expected for the Cu(II) complexes. The dihedral angle between the two ligands becomes smaller than 90 degrees by this ‘‘flattening’’ motion, and the molecular symmetry changes from D2d to D2 accordingly (Fig. 1). This flattening was experimentally proved in the 3MLCT state by a recent transient XAFS study,7–11 and it was also theoretically reproduced by quantum chemical calculations.12–14 Although the flattening structural change has been revealed as the predominant structural change in the excited state of the Cu(I) phenanthroline complexes, relevant dynamics had been controversial until recently.8,12,15,16 Chen and coworkers reported time-resolved absorption spectra of [Cu(dmphen)2]+ in 2003, which showed two dynamics proceeding on the time scales of several hundreds of fs and 10–20 ps. They attributed the faster process to the intersystem crossing and the slower to the structural change.8 However, Nozaki and coworkers reported that the fluorescence lifetime of [Cu(dmphen)2]+ is B10 ps using the time-correlated single photon counting technique, and argued that the intersystem crossing occurs with this time constant.12 In 2007,16 we reported a femtosecond fluorescence upconversion study in which we clarified that the structural change occurs in the lowest excited singlet (S1) state with a time constant of B660 fs, followed by the intersystem crossing to the T1 state with the time constant of B7.4 ps. Chen and coworkers also reported femtosecond fluorescence upconversion data independently, and they reassigned the B10 ps dynamics to the intersystem crossing.15 More importantly, our femtosecond fluorescence data indicated that the S1 state remains at the perpendicular configuration for a finite lifetime (B660 fs) before the structural change (Fig. 1), which contradicted the expectation from a simple mechanism due to the pseudo-Jahn–Teller effect. This finding was further confirmed by our recent ultrafast time-resolved absorption study carried out with a time resolution as high as 30 fs.17,18 We observed clear oscillation in transient absorption which reflects coherent nuclear wavepacket motions of the perpendicular S1 state generated by direct S1 ’ S0 photoexcitation. This revealed that the perpendicular S1 state has a well-defined vibrational

4144 | Phys. Chem. Chem. Phys., 2014, 16, 4143--4154

2,9-Substituted phenanthroline ligands of copper(I) complexes.

structure and can vibrate in a short but finite lifetime. These femtosecond time-resolved emission/absorption studies have provided a new insight into the realistic potential energy surface relevant to the flattening structural change in the S1 state, and it changed our understanding about the mechanism and dynamics of photo-induced structural change of the Cu(I) complexes. The flattening structural change is an essential component in the excited-state property of Cu(I) phenanthroline complex, and hence the photophysical/photochemical properties of the complex are strongly correlated with the flattening motion. Because it is expected that the flattening motion and structure in the excited state are affected by the substituents introduced at the 2- and 9- positions of the 1,10-phenanthroline ligand, the substituent effect on steady-state spectroscopic properties of the Cu(I) complexes has been studied.3–6,19–24 These reports showed that, when bulky substituents are absent, the steadystate emission is not observed even in dichloromethane, which does not quench the triplet state of Cu complexes by a ligation of itself.25–27 This suggests that the steric prevention of the flattening motion substantially alters the photophysical properties of the Cu(I) complexes. Although this substituent effect on the steady-state emission is widely known, the studies on the excited-state dynamics have been limited.15,28,29 Therefore, it is highly desirable to obtain sufficient knowledge about the substituent effect on excited-state dynamics to get an overall picture of the excited-state properties of the Cu(I) complexes. In this paper, we report our femto/picosecond time-resolved emission study of the substituent effect on the excited-state dynamics of Cu(I) phenanthroline complexes in solution. We performed fluorescence upconversion measurements for [Cu(phen)2]+ (phen = 1,10-phenanthroline) and [Cu(dpphen)2]+ (dpphen = 2,9-diphenyl-1,10-phenanthroline) in dichloromethane (Fig. 2) and fully clarified their ultrafast dynamics. By comparison with the emission dynamics of [Cu(dmphen)2]+, it was found that the bulkiness of the substituent is clearly correlated to the dynamics of the structural change and emission properties. The obtained results provide a coherent view of how substituents affect excited-state dynamics and relevant potential energy surfaces of the Cu(I) complex.

2. Experimental section 2.1.

Measurements

The experimental setup for the femtosecond fluorescence up-conversion measurement is essentially the same as that described previously.16,30 The light source was a femtosecond


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mode-locked Ti:sapphire laser (Coherent, MIRA) that produced 76 MHz pulse train with a typical pulse duration of 100 fs. The oscillator laser was tuned to 840 nm or 900 nm for measurements with two different excitation wavelengths. This fundamental output was converted to the second harmonic pulse (420 nm or 450 nm) by using a b-BaB2O4 crystal (0.2 mm thickness), and it was focused into the sample solution for photoexcitation. The residual fundamental pulse after the second harmonic generation was used as the gate pulse for the up-conversion process. The fluorescence from the sample was collected and focused onto a b-BaB2O4 mixing crystal with use of an aluminum-coated elliptic mirror. A cutoff filter was placed between the mirror and the mixing crystal to block the excitation light. The fluorescence was up-converted by type-I sum-frequency generation with the gate pulse in the mixing crystal. The up-converted signal was separated from other lights by an iris, band-pass filters, and a monochromator (HR-320, Jobin Yvon), and then it was detected by a photoncounting system. The fluorescence measurement was performed under the magic angle condition by rotating the polarization of the excitation pulse with respect to that of the gate pulse. We circulated sample solution and irradiated laser pulses on a thin-film-like jet stream that was generated using a nozzle. For measuring emission at 750 nm in dichloromethane (as well as those at 625 nm in acetonitrile and butyronitrile shown in ESI†), the sample solution contained in a 1 mm-thick static cell was used for the experiments. This is because these measurements require a much longer data accumulation period due to the low signal intensity, but the liquid jet of dichloromethane is not very stable for a very long exposure time. Note that we performed measurements of [Cu(dpphen)2]+ in dichloromethane using both the jet and the cell at several wavelengths to confirm that there is no difference except for the instrumental response. The instrumental response was evaluated from the up-converted signal of Raman scattering of the solvent, and its full width at half maximum was 160 fs (solution jet) and 200 fs (static cell). Picosecond time-resolved emission measurements were carried out using a streak camera system (C4334, Hamamatsu). To generate excitation pulses at 400 nm, 420 nm, and 550 nm, we used a Ti:sapphire regenerative amplifier (Spitfire, SpectraPhysics) seeded by a femtosecond mode-locked Ti:sapphire laser (Tsunami, Spectra-Physics). The 400 nm pulse was generated by frequency doubling in a b-BaB2O4 crystal. For the 420 nm pulse, the amplified pulse was converted to a nearinfrared pulse (1680 nm) in an optical parametric amplifier (TOPAS, Quantronix), and it was frequency-doubled twice by using two b-BaB2O4 crystals. The 550 nm pulse was generated by sum-frequency mixing between the 800 nm output of the Ti:sapphire regenerative amplifier and a near-infrared pulse at 1760 nm from the optical parametric amplifier. The sample solution was contained in a 1 mm-thick static cell, and the emission was collected through a polarizer with a backscattering geometry. The relative polarization of the excitation and detection was set at the magic angle. Time-resolved traces were measured in the sweep ranges of 1 ns and 100 ns with


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instrumental response times (FWHM) of 20 ps and 1 ns, respectively. All measurements were performed at room temperature (299 K). Steady-state absorption and emission spectra were recorded by commercial spectrometers (U-3310, Hitachi and Fluorolog 3, Spex). Quinine in 1 N H2SO4 aqueous solution and 4-dimethylamino40 -nitrostilbene in o-dichlorobenzene were used as the reference for correction of the wavelength-dependent instrumental sensitivity in the steady-state and time-resolved emission measurements.31 2.2.

Sample

[Cu(phen)2]PF6 and [Cu(dpphen)2]PF6 were prepared by the method reported in the literature.32,33 The purity of the sample was checked by absorption, NMR and elemental analysis. [Cu(dpphen)2]PF6 C: 66.01 H: 3.69 N: 6.42 (calcd) C: 65.83 H: 3.87 N: 6.30 (found). [Cu(phen)2]PF6 C: 50.66 H: 2.81 N: 9.84 (calcd) C: 49.96 H: 2.91 N: 9.86 (found). Dichloromethane (Wako, HPLC grade), acetonitrile (Wako, HPLC grade) and butyronitrile (Wako, first grade) were used without further purification. We prepared a fresh sample solution for each time-resolved measurement. All sample solutions were air saturated. Stability of the sample was checked by comparing absorption spectra before and after the measurements.

3. Results and discussion 3.1.

Molecular structure and steady-state spectra

The steady-state absorption spectra of [Cu(phen)2]+, [Cu(dmphen)2]+ and [Cu(dpphen)2]+ in dichloromethane are shown in Fig. 3 (gray). Each spectrum shows two absorption bands in the visible region due to the 1MLCT transitions.3 The intense band around 450 nm and weaker band around 550 nm are assigned to the S2 ’ S0 and S1 ’ S0 transitions, respectively, for all the three complexes. The relative intensity of the S1 absorption to the S2 absorption is similarly small in [Cu(phen)2]+ and [Cu(dmphen)2]+, whereas it is significantly larger in [Cu(dpphen)2]+. This difference reflects the structural difference in the ground state. In solution, [Cu(phen)2]+ and [Cu(dmphen)2]+ in the ground state have a D2d structure, in which the two ligands are perpendicular to each other.22 With this symmetry, the S2 state is attributed to the optically allowed B2 state, while the S1 state is attributed to the optically forbidden A2 state. This assignment has been supported by an experiment which showed that the S1 ’ S0 transitions have higher intensities in the copper complexes having more distorted structures.34 DFT calculations also gave consistent results.12–14 It has been considered that the weak absorption of the ‘optically forbidden’ S1 ’ S0 transition of D2d [Cu(phen)2]+ and [Cu(dmphen)2]+ arises from the vibronic coupling with the strong S2 ’ S0 transition and/or dynamic symmetry lowering due to the flat potential energy curve along the flattening coordinate of the ligands.22 The S1 ’ S0 transition becomes optically allowed as the equilibrium structure in the S0 state is distorted from D2d. For [Cu(dpphen)2]+, it is said that the intramolecular p interaction between the phenyl substituent of one ligand and the phenanthroline moiety of the other makes the structure flattened already in the S0 state.5,33,35


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Fig. 3 Absorption, steady-state and picosecond time-resolved emission spectra of (a) [Cu(phen)2]+, (b) [Cu(dmphen)2]+ and (c) [Cu(dpphen)2]+ in dichloromethane at room temperature. Gray areas are absorption spectra. Blue lines are steady-state emission spectra (lex = 420 nm). Green dots and red dots are the time-resolved emission spectra in the 0.05–0.05 ns and the 2–100 ns region, respectively. (lex = 420 nm for [Cu(dmphen)2]+; lex = 400 nm for [Cu(phen)2]+ and [Cu(dpphen)2]+). (d) Picosecond timeresolved emission signal of [Cu(dpphen)2]+ in dichloromethane. The trace was obtained by the integration of the signal measured by a streak camera in the wavelength region from 650 to 750 nm. The sample concentration for the steady-state emission measurements was 3 103 mol dm3 for the all samples, and those for the time-resolved emission measurement were 1 102 mol dm3 for [Cu(dmphen)2]+ and [Cu(dpphen)2]+, and 3 103 mol dm3 for [Cu(phen)2]+. Re-absorption corrections have been made for all the emission spectra. Intensities of steady-state and time-resolved spectra are normalized for comparison.


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In fact, the distorted structure has been directly observed in crystals of [Cu(dpphen)2]X (X = CuCl2, PF6), and the S1 absorption of the crystal has high intensity. Because the absorption spectra of [Cu(dpphen)2]+ in the crystal and solution are very similar, the high S1 ’ S0 transition intensity of [Cu(dpphen)2]+ in solution is also attributed to the distorted S0 structure, in which the dihedral angle between the two ligands is less than 90 degrees.36 The steady-state emission spectra of [Cu(dmphen)2]+ and [Cu(dpphen)2]+ in dichloromethane are also shown in Fig. 3 (blue line). The steady-state emission of [Cu(dmphen)2]+ and [Cu(dpphen)2]+ around 750 nm is the phosphorescence from the 3MLCT state that is formed after intersystem crossing from the photogenerated 1MLCT state. Although this emission has a contribution of delayed fluorescence (i.e., the emission from the closely lying S1 state that is thermally populated),37 we just call it phosphorescence in this paper for simplicity. As shown in the figure, [Cu(dmphen)2]+ and [Cu(dpphen)2]+ exhibit strong phosphorescence peaked around 750 nm. For [Cu(dmphen)2]+, we have shown that the structural change occurs with a time constant of B660 fs in the S1 state before the intersystem crossing,16 and recent time-resolved X-ray studies also demonstrated that the 3MLCT state is distorted.8,11 Because [Cu(dmphen)2]+ and [Cu(dpphen)2]+ exhibit very similar emission spectra, the steadystate emission of both [Cu(dmphen)2]+ and [Cu(dpphen)2]+ is safely assigned to the phosphorescence from the 3MLCT state that has a distorted structure. We note that although [Cu(dpphen)2]+ is already distorted in the S0 state, Miller et al. suggested that ‘further flattening’ occurs in the excited state based on a crystallographic analysis.33 They examined crystal structures with different counter anions, and found that the dihedral angle between the two ligands is 1001 in the [Cu(dpphen)2]PF6 crystal with the Cu(I) ion whereas the angle becomes 1181 in the [Cu(dpphen)2](ClO4)2 crystal with the Cu(II) ion. Thus, they argued that the dihedral angle of [Cu(dpphen)2]+ is further increased in the MLCT excited state because the copper ion is formally oxidized from Cu(I) to Cu(II) with MLCT excitation. The further flattening in the excited state of [Cu(dpphen)2]+ was also indicated by a recent time-resolved X-ray absorption study.10 In contrast to the strong phosphorescence of [Cu(dmphen)2]+ and [Cu(dpphen)2]+, no detectable phosphorescence is observed for [Cu(phen)2]+. This indicates that the excited-state dynamics of [Cu(phen)2]+ is drastically different from those of the other two complexes. The absence of the steady-state emission of [Cu(phen)2]+ is consistent with a general tendency that Cu(I) phenanthroline complexes without bulky substituents at the 2 and 9- positions exhibit no detectable phosphorescence.5 Because it is natural to think that the largest structural change occurs in the MLCT state of [Cu(phen)2]+ because of much less steric hindrance, the substituent effect on the steady-state emission indicates that the flattening motion of the two ligands plays an essential role in quenching of the emission. 3.2.

Emission dynamics in the nano- to picosecond time region

Time-resolved emission data for [Cu(phen)2]+ and [Cu(dpphen)2]+ in the nano- to the picosecond time region were measured with


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S2 ’ S0 excitation using a streak camera, and they are shown in Fig. 3 together with the data for [Cu(dmphen)2]+. The nanosecond emission spectra were obtained by integration over the 2–100 ns region, and they are shown as red curves in Fig. 3 for [Cu(dmphen)2]+ and [Cu(dpphen)2]+. ([Cu(phen)2]+ did not show any detectable emission in this time region.) The nanosecond spectra are almost identical to the steady-state spectra, indicating that they are attributed to phosphorescence from the 3MLCT state. The phosphorescence decayed mono-exponentially, giving the 3MLCT lifetime of 41 ns for [Cu(dmphen)2]+ 16 and 135 ns for [Cu(dpphen)2]+. (The timeresolved emission trace of [Cu(dpphen)2]+ is shown in Fig. 3(d).) Picosecond emission spectra were also obtained by integrating the streak-camera data from 50 ps to 50 ps, and they are shown as green curves in Fig. 3. In the picosecond region, all the three complexes exhibit two emission bands although relative intensities are very different. This dual emission feature is most readily seen for [Cu(dmphen)2]+ that exhibits two comparable bands peaked around 480 nm and 700 nm. (Note that the band peaked around 700 nm is significantly shifted from the phosphorescence spectra.) [Cu(phen)2]+ exhibits a strong blue band (B480 nm) and a very weak red band (B700 nm) whereas [Cu(dpphen)2]+ exhibits a weak blue band (B480 nm) and a strong red band (B730 nm). The picosecond emission band of [Cu(dpphen)2]+ in the red region is also shifted from the steady-state emission although the shift is smaller than the case of [Cu(dmphen)2]+. The time-resolved intensities of these picosecond emissions are much higher than that of the phosphorescence that is observed in the nanosecond time region (e.g. Fig. 3(d)). This implies that the emission observed in the picosecond time region, including the emission around 750 nm, is not phosphorescence but is the fluorescence from the 1MLCT state before the intersystem crossing. The observed complex spectral feature reflect ultrafast dynamics occurring in the excited singlet states, as described in the next section.

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3.3.

Femtosecond time-resolved emission data

To investigate the early-time dynamics of the fluorescence emission, we carried out femtosecond fluorescence up-conversion measurements for [Cu(phen)2]+ and [Cu(dpphen)2]+ in dichloromethane with S2 ’ S0 excitation. The temporal fluorescence traces observed at typical wavelengths are depicted in Fig. 4 for the two complexes, together with the data of [Cu(dmphen)2]+ that were reported previously.16 The observed fluorescence behavior varies significantly, depending on the detection wavelength. However, the global fitting analysis showed that they were successfully fit with bi- or tri-exponential functions: the fluorescence decays of [Cu(phen)2]+ were reproduced well by bi-exponential functions with time constants of 47 fs and 200 fs. Tri-exponential functions were needed for the other two complexes, and the time constants were determined to be 45 fs, 660 fs and 7.4 ps for [Cu(dmphen)2]+ and 125 fs, 920 fs and 9.4 ps for [Cu(dpphen)2]+. To properly interpret the fluorescence up-conversion data, it is very important to reconstruct time-resolved fluorescence spectra because the spectral information is essential for the assignment of the observed dynamics.16,30,38 To do this, we normalized the fluorescence trace measured at each wavelength so that the time-integration of the trace is proportional to the intensity of the picosecond emission spectrum (Fig. 3) at the corresponding wavelength. This normalization corrects the wavelength-dependent up-conversion efficiency. Using the parameters obtained from the global fitting, time-resolved fluorescence spectra of the three complexes were reconstructed, and these are compared in Fig. 5. As already reported in detail,16 the time-resolved emission spectra of [Cu(dmphen)2]+ depicted in Fig. 5(b) show a conspicuous spectral change that reflects the fast internal converIS FS sion and following structural change, i.e. SIS 2 - S1 - S1 , IS FS where Sn and Sn denote the Sn states having the initial and flattened structures, respectively. The strong fluorescence band observed around 500 nm in the 0–0.3 ps time region exhibits a mirror-image of the S2 ’ S0 absorption band (Fig. 5(b-1)) so

Fig. 4 Femtosecond time-resolved emission signals in dichloromethane measured by the up-conversion method, (a) [Cu(phen)2]+, (b) [Cu(dmphen)2]+ and (c) [Cu(dpphen)2]+. (420 nm or 450 nm excitation; 1.0 102 mol dm3) The temporal profile of the Raman scattering of dichloromethane is also given to show the instrumental response. The data of [Cu(dmphen)2]+ have been already reported in ref. 16.



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Fig. 5 The femtosecond time-resolved emission spectra of [Cu(phen)2]+ (a-1) 0.0–0.4 ps, (a-2) 0.4–1.0 ps, [Cu(dmphen)2]+ (b-1) 0.0–0.3 ps, (b-2) 0.3–1.0 ps, (b-3) 1.0–10.0 ps and [Cu(dpphen)2]+ (c-1) 0.0–0.5 ps, (c-2) 0.5–1.0 ps, (c-3) 1.0–10.0 p in dichloromethane. (420 nm or 450 nm excitation; 1.0 102 mol dm3).

that it is assigned to the fluorescence from the initiallypopulated S2 state that has the perpendicular configuration (SIS 2 ). Following the 45 fs decay of this S2 - S0 fluorescence, a fluorescence band extending from 500 nm to 700 nm appears, which was assignable to the fluorescence from the S1 state having the same perpendicular configuration (SIS 1 ) which is generated with the S2 - S1 internal conversion. Then, the S1 - S0 fluorescence shows a characteristic spectral change showing a clear isoemissive point at 675 nm, keeping its integrated emission intensity (i.e., oscillator strength) almost constant (Fig. 5(b-2)). Since the time constant of this spectral change (660 fs) is substantially changed by the solvent viscosity,16 it is attributed to the flattening structural change occurring in the S1 state. The resultant S1 state having the flattened structure (SFS 1 ) emits a fluorescence of around 720 nm, and it decays with a time constant of 7.4 ps to the 3MLCT state through the intersystem crossing (Fig. 5(b-3)). Femtosecond time-resolved fluorescence spectra of [Cu(dpphen)2]+ (Fig. 5(c)) show a spectral change that well corresponds to the change observed for [Cu(dmphen)2]+: a strong fluorescence band that exhibits a mirror image of the S2 ’ S0 absorption band is first observed around 500 nm. As this band vanishes, a new fluorescence band appears around 700 nm, and this band exhibits further spectral change while showing an isoemissive point at 730 nm (Fig. 5(c-2)). A solvent viscosity dependence of the time constant of this change was also found (see ESI†), which indicates that this spectral change is attributed to the structural change in the S1 state. The 730 nm band vanishes within a few tens of picoseconds, which leaves phosphorescence in almost the same wavelength region.


Because of the similarity of the spectral changes and their time scale, it is safely concluded that the observed time-resolved fluorescence change arises from a similar relaxation process, IS FS i.e. SIS 2 - S1 - S1 , which takes place in the excited singlet state of [Cu(dpphen)2]+ before the intersystem crossing. The small fluorescence shift accompanying the structural change accord well with that the S0 state (and hence the SIS 1 state) is already substantially distorted because of the steric hindrance due to the substituents. In other words, a small ‘further’ flattening structural change occurs in the excited state of [Cu(dpphen)2]+.33 In sharp contrast, the fluorescence dynamics of [Cu(phen)2]+ is almost complete within 1 ps, as shown in Fig. 5(a). Actually, the first fluorescence band around 480 nm, which is assignable to the S2 - S0 fluorescence, decays rapidly with a time constant of 47 fs (Fig. 5(a-1)). Then a weak fluorescence appears around 670 nm, and it almost completely vanishes within 1 ps. Unlike the other two complexes, no fluorescence was recognized after the decay of this 670 nm band (Fig. 5(a-2)). We note that the decay time constant of the 670 nm band (200 fs) also depends on a solvent viscosity, indicating that the relaxation process is also attributable to the flattening structural change (see ESI†). 3.4.

Fitting analysis and properties of emissive excited states

The femtosecond time-resolved emission spectra of the Cu(I) complexes can be consistently interpreted by considering the IS contributions of the three emissive excited states, SIS 2 , S1 and FS S1 . The quantitative analysis of the emission spectrum gives decisive information such as the radiative rate constant (oscillator strength) and excitation energy of the emissive excited


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state. We analyzed the time-resolved emission data using a procedure employed for [Cu(dmphen)2]+ in the previous work16 and obtained quantitative information about each excited state. In this analysis, we assume that the following sequential relaxation process occurs after S2 ’ S0 excitation,

Scheme 1

Here, ki = 1/ti is the decay rate of each excited state, and fi represents the quantum yield of the relevant relaxation process. Note that we introduce f factors to take into account the other relaxation paths from the Sn state such as intersystem crossing FS that occurs before the SFS 1 - T1 intersystem crossing. By solving the rate equations corresponding to this scheme with an initial condition that only the SIS 2 state is populated at t = 0, we obtain the following expressions for the time dependent population of each excited state: IS [SIS 2 (t)] = [S2 ]0 exp(k1t)

IS SIS 1 ðtÞ ¼ f1 S2 0

k1 ðexpðk2 tÞ expðk1 tÞÞ k1 k2

FS S1 ðtÞ ¼ f1 f2 SIS 2 0 k1 k2 þ

(1)

(2)

expðk1 tÞ ðk2 k1 Þðk3 k1 Þ

expðk2 tÞ expðk3 tÞ þ ð k 1 k 2 Þ ð k3 k 2 Þ ð k 1 k 3 Þ ð k 2 k 3 Þ

(3)

The time-resolved emission intensity observed at each wavelength is the sum of these three contributions, and it can be written as: IS FS I(t,l) = a1(l)[SIS 2 (t)] + a2(l)[S1 (t)] + a3(l)[S1 (t)]

= A1(l)exp(k1t) + A2(l)exp(k2t) + A3(l)exp(k3t) (4) The coefficient ai(l) denotes the intrinsic radiative transition probability of the excited state i at wavelength l, and it is related to the amplitudes of the three exponential components (Ai’s) that can be obtained by the fitting analysis of the observed time-resolved emission signals: a1(l)C0 = A1 + A2 + A3 a2 ðlÞ f1 C0 ¼

1 ½ðk1 k2 ÞA2 þ ðk1 k3 ÞA3 ; k1

a3 ðlÞ f1 f2 C0 ¼

1 ½ðk1 k3 Þðk2 k3 ÞA3 : k 1 k2

(5) (6)

(7)

Here, C0 is the population (concentration) of the excited state species at t = 0 (i.e., C0 = [SIS 2 ]0 in the case of S2 ’ S0 excitation). The relative values of a1, a2f1 and a3f1f2 are evaluated at each wavelength by using eqn (5)–(7), and their spectra are shown in Fig. 6(a) and (c) for [Cu(phen)2]+ and [Cu(dpphen)2]+, respectively. (The corresponding spectra of [Cu(dmphen)2]+ are also shown in Fig. 6(b) for comparison.) We set a3(l) = 0 for


Fig. 6 Fluorescence spectra of the three components observed in the up-conversion experiments of (a) [Cu(phen)2]+, (b) [Cu(dmphen)2]+ and (c) [Cu(dpphen)2]+ in dichloromethane. Blue: the SIS 2 component (a1), FS green: SIS component (a3f1f2). Solid1 component (a2f1), red: S1 dashed curves represent fits to the fluorescence spectra using single or multiple Gaussian line shape functions represented in the wavenumber. The absorption spectra are also shown for comparison (gray area).

[Cu(phen)2]+ because the SFS 1 emission was not observed in the present measurements. For all the three copper complexes studied, the a1, a2, and a3, spectra exhibit sequential red shifts, which is consistent with the relaxation processes depicted in Scheme 1. The integration of the ai value over the whole spectral region yields a quantity proportional to the number of photons emitted within a unit time so that it is proportional to the radiative rate of the relevant excited state. With a procedure detailed in ESI,† we obtained the ratios of the radiative rates as kr1 : kr2f1 = 1.0 : 0.028 for [Cu(phen)2]+, kr1 : kr2f1 : kr3f1f2 = 1.0 : 0.026 : 0.026 for [Cu(dmphen)2]+, and kr1 : kr2f1 : kr3f1f2 = 1.0 : 0.22 : 0.18 for [Cu(dpphen)2]+ by integrating Gaussian line


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shape functions fitted to the ai values. Then, the quantum yield f1 was evaluated from the ratio of the S1 emission intensities measured with the S2 ’ S0 and S1 ’ S0 excitations,16 and it was determined to be 0.90 and 0.75 for [Cu(phen)2]+ and [Cu(dpphen)2]+, respectively (see ESI† for the details). The f1 value of [Cu(dmphen)2]+ has been determined to be 0.7 in our previous work.16 As for f2, it has been shown that the f2 value was safely considered to be unity for [Cu(dmphen)2]+ on the basis of the viscosity dependence of the S1 emission dynamics as well as femtosecond time-resolved absorption data.16,18 Because the S1 emission dynamics of [Cu(phen)2]+ and [Cu(dmphen)2]+ showed similar viscosity dependence to [Cu(dmphen)2]+ (see ESI†), we set f2 at unity also for the two complexes. Consequently, the ratio of the radiative rates was determined as kr1 : kr2 = 1.0 : 0.031, kr1 : kr2 : kr3 = 1.0 : 0.038 : 0.038, and kr1 : kr2 : kr3 = 1.0 : 0.30 : 0.25 for [Cu(phen)2]+, [Cu(dmphen)2]+ and [Cu(dpphen)2]+, respectively. Using these ratios, the absolute values for kr were determined by comparison with the references. For [Cu(dpphen)2]+ as well as [Cu(dmphen)2]+,16 the timeresolved intensities of the S1 emissions were compared with that of phosphorescence for which the kr value has been reported.32 For [Cu(phen)2]+, we measured the phosphorescence of [Ru(bipyridine)3]2+ as the reference sample3 and compared the time-resolved intensity. Then, we obtain the absolute radiative rate as follows: kr1 = 2.4 107 s1, kr2 = 7.5 105 s1 for [Cu(phen)2]+, kr1 = 5.0 107 s1, kr2 = 1.9 106 s1 and kr3 = 1.9 106 s1 for [Cu(dmphen)2]+, kr1 = 2.4 107 s1, kr2 = 4.0 106 s1 and kr3 = 3.4 106 s1 for [Cu(dpphen)2]+. The corresponding oscillator strengths ( f ) were also evaluated by using a relation f = 1.5 kr/nmax2. The properties of each emissive excited state of the three complexes are summarized in Table 1. As shown in Table 1, the oscillator strengths of the SIS 2 and IS S1 states evaluated from time-resolved fluorescence data are in fair agreement with the values estimated by integration of the corresponding absorption bands. This ensures our assignment of the first two short-lived emission components to the excited IS singlet states retaining the initial structure, i.e., SIS 2 and S1 . The IS FS emission oscillator strengths of the S1 and S1 states are very Table 1

similar to each other in [Cu(dmphen)2]+ and [Cu(dpphen)2]+, which confirms that these two states are both S1 states and the flattening distortion occurs in the S1 manifold. The quantitative information obtained by fitting analysis of the time-resolved emission data provides solid support for the assignment of the emission components observed in the fluorescence up-conversion measurements. 3.5.

Substituent effect on ultrafast relaxation dynamics

Ultrafast relaxation dynamics of the three copper(I) complexes are summarized in Fig. 7 using schematic potential energy diagrams. The three complexes exhibit comparable relaxation processes: The photoexcitation at B400 nm excites the molecule to the MLCT optically allowed SIS 2 state that emits strong fluorescence around 500 nm. The SIS 2 state has a structure similar to the initial S0 state, and it relaxes to the SIS 1 state keeping its structure by internal conversion with a time constant of B50 fs (in [Cu(dmphen)2]+ and [Cu(phen)2]+) or 125 fs (in [Cu(dpphen)2]+). Then, the flattening structural change occurs in the S1 state on the sub-picosecond time scale, and the angle between the two ligands becomes smaller. For [Cu(dmphen)2]+ and [Cu(dpphen)2]+, the isoemissive point is observed in the spectral change due to the structural change, which confirms that the SIS 1 state having the initial structure is a well-defined state that has a finite lifetime. The SFS state 1 appears in accordance with the decay of the SIS 1 state as the flattening structural change occurs. Then, the intersystem crossing proceeds from the flattened SFS 1 with time constants of B10 ps. The time scale of the structural change (o1 ps) and the intersystem crossing (B10 ps) is well separated. In the case of [Cu(phen)2]+, the time-resolved fluorescence attributable to IS the SFS 1 is not observed after the fluorescence from the S1 state decays, in the wavelength region monitored in the present experiment. However, it is considered that the major relaxation pathway from the SIS 1 state is also the structural change because the SIS lifetime changes with the change of the solvent viscosity 1 as in the case of the [Cu(dmphen)2]+ and [Cu(dpphen)2]+ (see ESI†). Because the steric hindrance due to the substituents at

Properties of low-lying excited states of the copper(I) complexes in dichloromethane at room temperature

Assignment

lmax/nm

t/ps

6 1 (oscillator strength f ) K obs r /10 s

[Cu(phen)2]+

SIS 2 SIS 1

480 670

0.047 0.20

24a 0.75a

(0.082) (0.0052)

39b 2.1b

(0.14) (0.014)

[Cu(dmphen)2]+

SIS 2 SIS 1 SFS 1 T FS 1

500 600 720 770

0.045 0.66 7.4 4.1 104

50a 1.9a 1.9a 0.0023c

(0.20) (0.010) (0.012)

43b 2.2b —

(0.16) (0.012)

[Cu(dpphen)2]+

SIS 2 SIS 1 SFS 1 T FS 1

480 690 730 750

0.13 0.92 9.4 1.35 105

24a 4.0a 3.4a 0.0044c

(0.11) (0.030) (0.027)

26b 3.1b —

(0.094) (0.022)

Radiative rate constant (kr) and oscillator strength ( f ) in parenthesis are related with each other by an equation: f = 1.50 kr/nmax2. a kr value evaluated from the time-resolved emission intensity. b kr value evaluated from the steady-state absorption coefficient. Details of separation of each spectral component in absorption spectra are given in the ESI. c Emission quantum yield and lifetime in ref. 25. Note that the values of [Cu(dmphen)2]+ are slightly altered from those values in ref. 16 because we have done the re-absorption correction of the fluorescence spectra in the present analysis.



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Fig. 7 The scheme of the electronic relaxation and structural distortion after S2 ’ S0 excitation for (a) [Cu(phen)2]+, (b) [Cu(dmphen)2]+ and + (c) [Cu(dpphen)2]+ in dichloromethane. a The lifetime of SFS 1 state of [Cu(phen)2] is given in ref. 39.

2- and 9- positions of the ligands is lacking in [Cu(phen)2]+, it is FS expected that the SIS 1 - S1 structural change is much larger than those of the other two complexes and that the energy gap between the S1 and the S0 state is much smaller at the flattened geometry after the structural change (Fig. 7(a)). Thus, it is highly likely that the fluorescence from the SFS 1 state appears in the near infrared region that is outside of the detection range of the present experiment. We note that our femtosecond timeresolved absorption experiments showed a spectral change FS attributable to the SIS 1 - S1 structural change that proceeds with a time constant of B0.2 ps.39 The femtosecond timeresolved absorption also showed that the main relaxation path+ way of the SFS 1 state of [Cu(phen)2] is not intersystem crossing but internal conversion to the S0 state (B2 ps) in dichloromethane.39 This is consistent with that no phosphorescence of [Cu(phen)2]+ is observed in dichloromethane in the present experiment. The very efficient S1 - S0 internal conversion is also attributable to the small S1–S0 energy gap in the flattened geometry of [Cu(phen)2]+. The femtosecond fluorescence data clearly showed that the S IS state, which has a similar structure to the initial S0 state, 1 appears in the S1 manifold of all the three complexes and it retains its structure for a finite lifetime. Importantly, a substantial substituent effect is observed for the lifetime of the IS SIS 1 state: The S1 lifetime becomes longer as the substituents at 2- and 9- positions become bulkier: 200 fs ([Cu(phen)2]+) o 660 fs ([Cu(dmphen)2]+) o 920 fs ([Cu(dpphen)2]+). This means that the flattening distortion in the S1 state requires a longer time as the substituents become bulkier. It is natural that the structural change (i.e. the change of the dihedral angle) becomes smaller as the substituent becomes bulkier because of the steric hindrance. The present experiments clearly show that the bulky substituent not only makes the amplitude of the structural change small but also makes its dynamics slow. We discuss this substituent effect on the dynamics of the structural change in the next section. Here, we need to mention that the conclusion of the present time-resolved emission study is markedly different from the arguments in recent time-resolved absorption studies of Chen and co-workers.28,29 They measured femtosecond time-resolved absorption spectra of several Cu bis-phenanthroline complexes


with the S2 ’ S0 excitation by changing the substituent of the phenanthroline moiety. (Note that the S2 state in the present paper corresponds to the state called ‘S3’ in their papers.) They reported the time constants of the structural change as 0.2–0.8 ps, 0.3 ps and 0.45 ps for [Cu(dmphen)2]+, [Cu(phen)2]+ and [Cu(3,8-di(ethynyltrityl)-phen)2]+ in dichloromethane, respectively, and claimed that the dynamics of the structural change is not affected by the substituent. We note that their conclusion obtained from the femtosecond time-resolved absorption measured with the S2 ’ S0 excitation (lex B 400 nm) is questionable. In fact, our previous time-resolved absorption study of [Cu(dmphen)2]+ carried out with direct S1 ’ S0 excitation (lex = 550 nm) showed that the transient absorption change due to the flattening structural change is very small, compared with the spectral change due to the intersystem crossing.18 Therefore, in their time-resolved absorption obtained with S2 ’ S0 excitation, the spectral change due to the structural change is buried in the change due to other competing processes such as triplet formation and hence it is impossible to properly extract information about the dynamics of the structural change. (Note that the yield of the competing triplet formation has been evaluated as 1 f1 in the present study.)16,18 In contrast, femtosecond time-resolved fluorescence spectroscopy only observes ultrafast dynamics of the excited singlet states that emit fluorescence, and hence we can clearly obtain information about the dynamics of the structural change occurring in the S1 state. We also note that we have carried out femtosecond timeresolved absorption experiments not only for [Cu(dmphen)2]+ 18 but also for [Cu(phen)2]+ and [Cu(dpphen)2]+ 39 with direct S1 ’ S0 excitation, and obtained results are consistent with the conclusion of the present time-resolved emission study. 3.6. Mechanism of the substituent effect on the photo-induced structural change Previously, the mechanism of the photo-induced structural change of the Cu(I) complexes was explained using a simple mechanism based on the pseudo-Jahn–Teller effect.6,26 In this mechanism, a strong vibronic interaction makes a symmetric structure unstable, and the energy of the excited state falls down as the structure distorts along the relevant nuclear coordinate. Thus, the structural change immediately occurs due to the


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spontaneous instability of the initial structure of the excited state. However, our femtosecond time-resolved fluorescence study of [Cu(dmphen)2]+ revealed that the perpendicular S1 state retains its structure for a finite lifetime (660 fs) before the structural change.16,18 These results indicated that the mechanism of the photoinduced structural change of the Cu(I) complexes is not so simple as had been believed. The substituent effect on the dynamics of the structural change found in the present study enables us to present a deeper discussion about the mechanism of the structural change occurring in the S1 manifold of the Cu(I) complexes. The femtosecond fluorescence data obtained in this work show that the lifetime of the SIS 1 state before the structural change becomes longer as the substituent at the 2- and 9- positions of the ligand becomes bulkier. In other words, the S1 state needs more time to become flattened as it has bulkier substituent on the ligands ([Cu(phen)2]+ o [Cu(dmphen)2]+ o [Cu(dpphen)2]+). The simplest idea explaining this substituent effect may be that the friction between solvent and ligands changes the dynamics because it is expected that the friction is larger for a bulkier substituent. Actually, the rate of the structural change of the three complexes shows viscosity dependence and the change becomes slower in a viscous solution where the friction is large. However, this explanation is not enough to fully explain the substituent effect observed in this work. For example, the time constant of [Cu(phen)2]+ (200 fs) is much shorter than the other two whereas the time constants of [Cu(dmphen)2]+ (660 fs) and [Cu(dpphen)2]+ (920 fs) are not very different, although the difference in size between the dmphen and the phen ligands is not so large while difference between dpphen and dmphen is substantial. (The solvent excluded volumes of phen, dmphen and dpphen are ca. 139, 173 and 268 Å3, respectively.) More importantly, the explanation based on friction does not explain why the initial S1 states retain initial structure for a finite lifetime before the structural change. We note that it is not appropriate to discuss the ultrafast dynamics occurring on the femtosecond time scale in a traditional way based on the transition state theory. In the transition state theory, it is assumed that the reactant state is thermally equilibrated and is in equilibrium with the transition state, and then the reaction rate is connected to the energy barrier of the reaction along a relevant reaction coordinate. In ultrafast reactions taking place within B1 ps, however, the excited state generated by photoexcitation (i.e., the reactant) is highly un-equilibrated and hence its dynamics should be treated using trajectories of the nuclear wavepacket on the multidimensional excited-state potential energy surface. In this view, it is very natural that the molecule needs a finite time to escape from the initial region where the excited state is generated. Furthermore, the difference in the lifetime of the initial S1 state can be considered as the difference in the trajectories leading to the flattened geometry on the S1 potential energy surface. Obviously, as the complex is flattened, the dihedral angle of the two ligands becomes smaller and a pair of substituents at 2- and 9-positions get close to each other. Because of the repulsion between the two substituents, bulky substituents need to rotate


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on the C–C axis to achieve the flattened structure having the minimum energy. In fact, our TD-DFT calculations showed that the four substituents are rotated in the flattened SFS 1 state of [Cu(dmphen)2]+ to reduce steric hindrance.18 This implies that, in the case of [Cu(dmphen)2]+ and [Cu(dpphen)2]+, the fattening motion needs to be coupled with the rotational motion of the substituent during the structural change of the S1 state. The S1 state cannot be flattened without rotation of the substituents. On the other hand, [Cu(phen)2]+ has no degree of freedom about the substituent rotation. Therefore, the relevant coordinate is merely the dihedral angle change of the two ligand planes. This difference makes trajectories on the multidimensional S1 potential surface substantially different. In the case of [Cu(dmphen)2]+ and [Cu(dpphen)2]+, the trajectory needs to run along the rotational coordinate of the substituent to find a path toward the flattened structure in the S1 manifold. This makes the trajectories of [Cu(dmphen)2]+ and [Cu(dpphen)2]+ long and complicated, compared to the trajectories of [Cu(phen)2]+. We think that this is the reason why the dynamics of flattening structural change in the S1 state becomes substantially slow when they have bulky substituents at 2- and 9-positions of the substituent. The flattening structural change occurring with bulky substituents is sketched in Fig. 8. In this figure, the S1 potential energy surface is schematically drawn using two coordinates, the flattening and the substituent rotation coordinates. On this potential energy surface, the S1 state is generated in a flat region (or in a shallow potential minimum) corresponding to a large dihedral angle. From this structure, the S1 state cannot change its structure directly along the flattening coordinate because of the steric effects of the substituent. Meanwhile, rotation of the substituents occurs randomly (it is represented as a walk along the substituent rotation coordinate in Fig. 8),

Fig. 8 Mechanism of the flattening relaxation process of copper bisphenanthroline complexes with substituents at 2,9 position. The case of [Cu(dmphen)2]+ is taken as an example.


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and then, once the substituents take the configuration that reduces the steric hindrance, the dihedral angle of the ligands changes and the S1 state is relaxed toward the flattened structure that is the global minimum of the S1 potential energy surface. Although Fig. 8 is depicted for [Cu(dmphen)2]+, the same scenario is also valid for [Cu(dpphen)2]+ although its initial SIS 1 state is already distorted substantially. In the case of [Cu(phen)2]+, there is no degree of freedom for rotation of the substituent. Therefore, the short lifetime of the SIS 1 state (200 fs) can be considered as the intrinsic time for the S1 state to leave the initial flat region of the potential energy surface along the flattening coordinate. We consider that this is the mechanism that causes the structural change in [Cu(dmphen)2]+ and [Cu(dpphen)2]+ comparable and much slower than [Cu(phen)2]+. This mechanism also explains well the reason why the SIS 1 states of the Cu(I) complexes have finite lifetimes.

4. Conclusion The present study clarified the emission dynamics that directly manifest the ultrafast structural relaxation of [Cu(phen)2]+ and [Cu(dpphen)2]+ in the femtosecond time region. The ultrafast dynamics of the three Cu(I) diimine complexes (including [Cu(dmphen)2]+ studied in our previous studies) are substantially different, and it is well explained by the substituent effect on the structural change occurring in the S1 state. We discuss the observed substituent effect in terms of trajectories on the multidimensional S1 potential energy surface, which also explains why the initial S1 state exhibits a finite lifetime before the flattening structural change. The obtained results clearly showed that the time constant of the structural change can be controlled by the substituents introduced at 2- and 9-positions in phenanthroline ligands. The present study also confirms that the sub-picosecond dynamics of the S1 state of the Cu(I) complexes is dominated by the structural change, not by the intersystem crossing, although Chen and coworkers argued that the substantial intersystem crossing occurs on this time scale at different structures appearing during the flattening motion.10,15,28 Lastly, we mention the relevance of the obtained results to applications such as a photo-energy conversion process. Recent ultrafast spectroscopic studies have suggested that not only slow processes from the long-lived emission state (i.e., triplet state) but also ultrafast electron and energy transfer processes are important for photo-energy conversion in dye-sensitized solar cells.40–44 For example, it was reported that electron injection from RuN3 photo-sensitizer to the TiO2 semiconductor proceeds within 150 fs.40,41 This implies that the yield of the electron injection process is highly affected by other competing ultrafast processes such as intersystem crossing and structural change. For instance, in photo-energy conversion using a Cu(I) phenanthroline complex as sensitizer, the electron injection yield is likely increased when the process is faster than the structural change of the complex. Because the excited-state lifetime before the structural change becomes longer with the


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bulkier substituents, the Cu(I) phenanthroline complex having bulky substituents would provide higher yield of photo-energy conversion.

Acknowledgements This research was partly supported by Grant-in-Aid for Scientific Research on Innovative Areas (No. 25104005) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and by Grant-in-Aid for Scientific Research (A) (No. 25248009) from Japan Society for the Promotion of Science (JSPS).

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