Fujio Tanaka

Natural radiative lifetimes of anthracene derivatives and their dependence on refractive index

Abstract Fluorescence lifetimes of the electronic origins of S 1 are measured in supersonic free jets for nine anthracene derivatives. For most of the meso di-substituted derivatives, these lifetimes are virtually equal to the natural radiative lifetimes in vacuum and they are satisfactorily related to the radiative lifetimes determined in n -hexane through a factor of n 2 ( n is the refractive index). This unambiguous relationship found for the anthracene derivatives will help to bring a long-standing debate on the refractive-index dependence of the radiative lifetime to an end. It will also provide a basis for the discussion of the dependence of the radiative lifetime on single vibronic levels (SVL).

Inverse correlation between efficiency of singlet oxygen production and rate constant for oxygen quenching in the S1 state of anthracene derivatives

The quantum yield of singlet oxygen (1Δg) production (ΦΔ) in the quenching by oxygen of photoexcited anthracene derivatives was measured with a time-resolved thermal lensing (TRTL), and the efficiency of singlet oxygen production (fSΔ) in the quenching of the lowest excited singlet state (S1) was estimated in several solvents. In methylcyclohexane, fSΔ was approximately zero for anthracene and its derivatives possessing electron-donating groups of methyl and methoxy, and their rate constants of oxygen quenching (kSqu2006=u2006(26–31)u2006×u2006109 M−1 s−1) are nearly in a diffusion-controlled limit. On the other hand, values of fSΔ for 9-cyanoanthracene (CNA) and 9,10-dichloroanthracene (DCLA) possessing electron-withdrawing groups were 0.42 and 0.45, respectively, and values of kSq were (9.0 and 11)u2006×u2006109 M−1 s−1, respectively, which are definitely smaller than the diffusion-controlled rate constant. The values of fSΔ for CNA and DCLA depended significantly on the solvent, and these values were 0.50, 0.46, 0.45, 0.16 and 0.03u2006∼u20060.18 for CNA and 0.68, 0.54, 0.56, 0.12 and 0.23 for DCLA in hexane, dodecane, cyclohexane, benzene and acetonitrile, respectively. The corresponding values of kSq n (M−1 s−1) are (7.7, 8.3, 8.6, 12 and 15)u2006×u2006109 for CNA and (13, 13, 11, 21, 31)u2006×u2006109 for DCLA, respectively. These results on the solvent effect indicate the decrease of fSΔ and the increase of kSq in benzene and acetonitrile when compared to those values in nonpolar solvents except benzene. The results obtained from both substituent and solvent effects represent the inverse correlation between fSΔ and kSq, suggesting the involvement of charge transfer interaction in the oxygen quenching.

Effects of clustering of rare-gas atoms on the rate of S1T2 intersystem crossing for 9-methylanthracene

Abstract Laser induced fluorescence excitation spectra for 9-methylanthracene (MEA) in a supersonic free jet were measured as a function of the stagnation pressure of Ar, Kr and Xe, used as carrier gas. The 0-0 excitation bands for complexes of type MEA · Ar n were observed successively corresponding to an increase in coordination number n with increasing stagnation pressure. On the other hand, excitation bands associated with small clusters of Kr and Xe were scarcely observed, although strong excitation bands associated with larger clusters were observed at largely red-shifted wavelengths. These observations combined with the measurement of fluorescence lifetimes were interpreted in terms of two effects of the clustering of rare-gas atoms on the intersystem crossing rate; energy shift of S 1 relative to T 2 and heavy atom effect.

Contribution of local density augmentation in the vicinity of 9-cyanoanthracene and dynamic fluorescence quenching by oxygen in liquid and supercritical carbon dioxide

The local density augmentation in the vicinity of 9-cyanoanthracene (CNA) and the dynamic fluorescence quenching of CNA by oxygen is described in order to study the contribution of local composition enhancement near the fluorophore molecule to the quenching in supercritical fluid (SCF) CO2 at 35°C. For comparison, the fluorescence quenching of CNA by oxygen in liquid CO2 and n-hexane at 25°C was also investigated. Local density augmentation determined by conventional spectral shifts in absorption and fluorescence spectra was observed at pressures below about 10 MPa in SCF CO2 at 35°C, in good agreement with the results reported by other workers. The pressure dependence on the quenching rate constant, kq, showed a monotonic increase, a maximum and a minimum in liquid CO2, n-hexane and SCF CO2, respectively. The apparent activation volumes for kq were evaluated to be −220±10 (6.65–8.15 MPa) and −28±2 (9.17–35.24 MPa) in liquid CO2, −6.7±1.4 (0.1–75 MPa) and 4.5±0.2 cm3 mol−1 n(100–500 MPa) in n-hexane and 820±50 (7.11–8.25 MPa) and −27±2 (8.53–35.17 MPa) in SCF CO2. For liquid CO2 and n-hexane, the pressure dependence of kq was successfully interpreted by using the quenching model we reported previously. However, for SCF CO2, this model failed at pressures below about 10 MPa, but held in the higher-pressure region where the local density augmentation was not observed. From these results, the contribution of the local composition enhancement in the vicinity of the fluorophore molecule to the quenching was discussed.

Fluorescence quenching of 9,10-dicyanoanthracene by oxygen in liquid and supercritical carbon dioxide

Abstract The fluorescence quenching of 9,10-dicyanoanthracene (DCNA) by oxygen in liquid at 25°C and supercritical carbon dioxide (SCF CO 2 ) at 35°C was investigated at pressures from 8.0 to 60.0 MPa. The quenching rate constant, k q , increased significantly with increasing pressure in both the systems. The pressure dependence of k q in SCF CO 2 was satisfactorily interpreted in the same framework as the quenching mechanism in liquid phase that involves solvent cage.

Comparative study on the photodynamics of chloromethylanthracenes in a supersonic free jet and solution

Fluorescence excitation spectra and fluorescence decays nof 9-chloromethylanthracene (CMA) and n9-methyl-10-chloromethylanthracene (MCMA) were measured in a nsupersonic free jet. Excitation bands associated with the nbare molecules were observed only at excess vibrational nenergies at less than 530 cm n -1 n for CMA and 400 ncm n -1 n for MCMA. All the bands of the bare CMA nmolecule yielded fluorescence lifetimes (τ n f n) nshorter than 3 ns. The τ n f n of the bare MCMA nmolecule was 16.6 ns at the electronic origin and it was nshortened by increasing the excess energy. The excitation nbands attributed to van der Waals (vdW) complexes of MCMA nwith rare-gas atoms were observed with fluorescence nlifetimes longer than those of the bare molecule, whereas nfluorescence from vdW complexes of CMA was observed only for nlarge complexes of Kr and Xe atoms. Activation energies for nthe radiationless transition obtained from the temperature ndependence of the fluorescence lifetime in solution were n610–650 cm n -1 n for CMA and 520–610 ncm n -1 n for MCMA, and large values exceeding n10 n 11 n s n -1 n were obtained as the nfrequency factor. These results obtained in jets and nsolution are discussed from the viewpoint of the effects of nmedium on the photodissociation and intersystem crossing n(ISC) processes.

Radiationless deactivation of singlet oxygen (1Δg) sensitized by 9-acetylanthracene in liquid and supercritical ethane: local density augmentation in the vicinity of the singlet oxygen and sensitizer molecules

The lifetime of singlet oxygen (1Δg) sensitized by apparently non-fluorescent 9-acetylanthracene (ACA) was determined at pressures up to about 35 MPa in supercritical ethane (SCF ethane; 35, 45 and 55u2006°C) as well as in liquid ethane (25u2006°C) from the phosphorescence decay measurements. For all the systems examined, the decay rate constant (τΔ−1) increased significantly at the lower pressure region and monotonically as pressure increased further. It was also found that the pressure dependence of the bimolecular rate constant for quenching, kD, is similar to that of τΔ−1. The apparent activation volume for kD varied from −120 to −32 cm3 mol−1 at the lower pressure region and from −24 to −15 cm3 nmol−1 at the higher pressure region. In liquid solution over the pressure range examined, the pressure dependence of kD was interpreted well by a mechanism of the encounter complex formed between the singlet oxygen and the solvent molecules, together with that of the radial distribution function at contact with hard spheres, whereas it was not for the SCF systems in the low pressure region. In SCF solution, the deviation from this model was attributed to the contribution of the local density augmentation around the solute molecule. The degree of the local density augmentation evaluated is compared with that determined from the peak shift in absorption spectra of ACA measured in this work.