Kazuyuki Takehira

Fluorescence on–off switching mechanism of benzofurazans

Many fluorescent reagents with a benzofurazan (2,1,3-benzoxadiazole) skeleton have been developed and widely used in bio-analyses. In this study, we try to elucidate the fluorescence on-off switching mechanism of three fluorogenic reagents and their derivatives. Ten 4,7-disubstituted benzofurazans were used for this purpose and the measurements of their fluorescence, phosphorescence, photolysis, and time-resolved thermal lensing signal in acetonitrile were obtained in order to understand the relaxation processes of these compounds. These results indicate that the competition of fluorescence with a fast intersystem crossing or fast photoreaction plays a key role in the fluorescence on-off switching. Semi-empirical molecular orbital calculations show that the existence of the triplet n pi* state is responsible for the fast intersystem crossing while the proximity of the reactive second single pi pi* state to the first singlet pi pi* state contributes to the fast photoreaction in the excited states.

Photoluminescence study of electron–hole recombination dynamics in the vacuum-deposited SiO2/TiO2 multilayer film with photo-catalytic activity

Abstract Photoluminescences were observed from vacuum-deposited TiO 2 (240 nm set-thick) and SiO 2 /TiO 2 multilayer films with photo-catalytic activities. More intense luminescence was observed from the SiO 2 (20 nm set-thick)/TiO 2 multilayer film than from the bare TiO 2 film, the former film being known to have higher photo-catalytic activity and photocurrent. The time-resolved photoluminescence upon excitation at 266 nm using a femtosecond pulse laser system showed bi-exponential decays. The two decay lifetimes of the emission from the multilayer film were both found to be longer than those from the bare TiO 2 film. This finding indicates the presence of longer-lived photo-generated charge carriers at the SiO 2 /TiO 2 interface, where the nanometer-scale SiO 2 overlayer suppresses electron–hole recombination.

Photophysical study of 5-substituted benzofurazan compounds as fluorogenic probes

Nine 5-substituted benzofurazans including the non-substituted benzofurazan were synthesized, and measurements of the fluorescence, photolysis, transient absorption, and time-resolved thermal lensing signal were performed in order to understand the relaxation processes of these compounds. The results indicated that the main relaxation process was a photoreaction from an excited singlet state and the rate of the primary bond cleavage in the excited state tended to increase in the compound in which the S2 ← S0 absorption band is located close to the S1 ← S0 absorption band. These results suggest that the reactive state might be the S2 state, and that the interaction between the S1 and S2 states promotes the photoreaction. The separation of the S1 state from the S2 state decreases the rate of photoreaction, resulting in an increase in the Φf values. The Φf values of the 5-substituted benzofurazans in cyclohexane and acetonitrile were compared with their ΔE(S1, S2) (energy gap between the S1 and S2 states) values calculated by using a combination of AM1(EXCITED) and AM1-CAS/CI(CI = 6) for geometric optimization and calculation of the energy levels, respectively. The ΔE(S1, S2) values correlated well with the Φf values in each solvent, thus enabling us to predict the fluorescence properties of the 5-substituted benzofurazans based on their chemical structures.

Photophysical and photochemical properties of 1,4-tetracenequinone in solution

Photophysical and photochemical properties of 1,4-tetracenequinone (1,4-TQ), whose lowest singlet (S1) and triplet states (T1) are both of π,π* type, have been investigated in solution by means of emission measurements, a time-resolved thermal lensing technique and nanosecond laser flash photolysis at 295 K. The fluorescence spectrum of 1,4-TQ is shown to shift to the red with increasing solvent polarity. Based on the Lippert–Mataga equation, the change in the dipole moment between S1 and the ground state (S0) is estimated to be as large as 10.4 D, which is considered to originate from the aromatic subsystem of 1,4-TQ. The fluorescence quantum yield and lifetime are obtained to be 0.03 and 1.4 ns, respectively, and the quantum yields for the S1→T1 intersystem crossing (ISC) and the S1→S0 internal conversion are determined to be 0.78 and 0.19, respectively, in CCl4 at 295 K. The relatively slow ISC rate indicates that the energy level of the T2(n,π*) state lies slightly higher than that of the S1(π,π*) state. The transient absorption spectrum of triplet 1,4-TQ is measured in CCl4 at 295 K and its molar absorption coefficient is determined to be 25 300 dm3 mol−1 cm−1 at 440 nm. In CCl4 , triplet 1,4-TQ is quenched by the ground state 1,4-TQ with a self-quenching rate constant of 5.3 × 107 dm3 mol−1 s−1. Absence of H-atom abstraction from phenol by triplet 1,4-TQ confirms that the T1 state is π,π* in nature.