Seung Ki Park

Photophysical properties of some coumarin derivatives: 5,7-dimethoxycoumarin, 4′,5′-dihydropsoralen, 8-methoxypsoralen (8-MOP) and 8-MOPTHD C4-cyclomonoadduct (THD thymidine)

Abstract The photophysical properties of a 4t,5′-C 4 -cyclomonoadduct (F-2) of 8-methoxypsoralen (8-MOP) with thymidine were compared with those of 5,7-dimethoxycoumarin (DMC), 4t,5′-dihydropsoralen (DHP) and 8-MOP. The magnitudes of the fluorescence quantum yields are in the order DMC > DHP > F-2 > 8-MOP, which is exactly the opposite of the order for the temperature dependence of the fluorescence in ethanol. For DMC, DHP, 8-MOP and F-2 the ratios of the phosphorescence quantum yields to the fluorescence quantum yields are 0.056, less than 0.01, 0.68 and 5.6 and the phosphorescence lifetimes are 0.88 s, 0.88 s, 0.76 s and 1.2 s respectively, in ethanol at 77 K. The phosphorescence-to-fluorescence ratio increases sharply while the phosphorescence lifetime decreases by the external heavy-atom effect. At 77 K, the fluorescence maximum is very red shifted in isopentane compared with that in ethanol, and phosphorescence is not observed in isopentane. The solvent dependence of the fluorescence of the compounds is probably due to a change in the rate of internal conversion rather than to intersystem crossing.

Photoisomerization of 1,2-diarylethylenes: Photophysical properties of 5(E)-styryl-1,3-dimethyluracil

Abstract The photophysical properties of 5(E)-styryl-1,3-dimethyluracil (5(E)-SDU) were investigated. The fluorescence quantum yield and the lifetime are low and short relative to those of the parent aromatic hydrocarbon; this is probably because of the contribution of the n,π* state to the fast radiationless decay. However, the fluorescence of 5(E)-SDU at room temperature is not significantly affected by the solvent polarity, the presence of salts or the pH. This observation indicates that the 1(n,π*) state lies above the lowest 1(π,π*) state and that the vibronic mixing between the two states is not extensive. The highly positive fluorescence and the fluorescence excitation polarization confirm that the lowest excited singlet state has the π,π* configuration. The strong temperature dependence of the fluorescence intensity may be due to the operation of the activated process of twisting in the first excited singlet state competing with flurescence, and the rotation barrier is estimated to be 3.0 kcal mol−1. The strong quenching of fluorescence intensity by ethyl iodide at room temperature may be indicative of inefficient intersystem crossing. Moreover, only very weak phosphorescence is detected at 77 K and its quantum yield is too low to estimate in either isopentane-ethyl ether (1:1, v/v) or ethanol, supporting the idea of inefficient intersystem crossing suggested by laser flash photolysis results. Therefore, the major radiationless process appears to be internal conversion including rotatory radiationless decay. From the relatively long phosphorescence lifetime and its highly negative polarization the lowest triplet state is considered to be the 3(π,π*) state.

Photophysical properties of 1,4,5,8-tetraazaphenanthrene

Abstract 1,4,5,8-Tetraazaphenanthrene (TAP) has close-lying n,π* and π,π* singlet states and therefore shows very weak fluorescence (Φf = 0.001); laser flash photolysis indicated that it undergoes efficient intersystem crossing (Φisc = 0.95). The intensity of fluorescence of TAP increases on addition of sodium acetate because N-Na cation coordination elevates the lowest excited 1(n,π*) state to a higher energy. Because of mixing between the lowest excited 1(n,π*) state and 1(π,π*), TAP shows the heavy-atom effect, which is generally not observed in the pure 1(n,π*) state. Since the phosphorescence lifetime is relatively long (about 0.5 – 0.8 s) and the phosphorescence is negatively polarized, the lowest triplet state has the π,π* configuration. The photoreactivity of TAP is compared with the photophysical parameters of azaphenanthrenes.