Low temperature luminescence behaviours of 7-azatryptophan, 5-hydroxytryptophan and their chromophoric moieties

Excitation-wavelength dependent fluorescence of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.

Solvatochromic study of excited state dipole moments of some biologically active indoles and tryptamines

Absorption and fluorescence spectra of some biologically active indole and tryptamine derivatives have been recorded at room temperature in solvents of different polarities. The interest in the photophysical properties of these molecules arises mainly from their utility in medicinal chemistry as neurotransmitter and hallucination/hallucinic agents. Excited-state dipole moments of these molecules have been estimated from solvent-dependent Stokes shift data using a solvatochromic method based on a microscopic solvent polarity parameter (ETN). All indoles show a substantial increase in the dipole moment upon excitation to the emitting state. These results are generally consistent with the Parametric Method 3 (PM3) calculations, and are found to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts with the microscopic solvent polarity parameter (ETN) is superior to that obtained using bulk solvent polarity functions.

Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

Both the neurotransmitter serotonin and the unnatural amino acid 5-hydroxytryptophan (5HT), contain the 5-hydroxyindole chromophore. The photochemistry of 5HT is being investigated in relation to the multiphoton excitation of this chromophore to produce a characteristic photoproduct with green fluorescence ('hyperluminescence'). Laser flash photolysis (308 nm) of 5HT in aqueous solution at neutral pH produces both the neutral 5-indoloxyl radical (lambda(max) 400-420 nm) and another transient absorption with lambda(max) 480 nm and lifetime of 2 micros in deaerated solutions. Based on quenching by oxygen and beta-carotene, the species at 480 nm is identified as the triplet excited state of 5HT. In acidic solution a new oxygen-insensitive intermediate with lambda(max) 460 is assigned to the radical cation of 5HT. Time-resolved measurements of luminescence at 1270 nm have shown that the triplet state of 5HT is able to react with oxygen to form singlet excited oxygen (1O2*) with a quantum yield of approximately 0.1. However, 5HT has also been found to be an effective quencher of singlet oxygen with a second order rate constant of 1.3 x 10(8) dm3 mol(-1) s(-1). The results are discussed in the light of recent observations on the multiphoton-excited photochemistry of serotonin.