Triplet state studies of beta-carbolines

Low temperature phosphorescence spectra, quantum yields and lifetimes, and room temperature triplet-triplet absorption spectra and lifetimes have been obtained for the neutral and cationic forms of the three beta -carbolines: harmine (Hi), harmane (Ha) and norharmane (NorH). Based on the phosphoresc ence spectra, the energies of the lowest triplet states are 25 250 (Hi), 24 750 (Ha) and 24 400 cm(-1) (NorH) for the neutral forms and 22 900 (Hi), 2 1 750 (Ha) and 21 850 cm(-1) (NorH) for the corresponding cations. Phosphor escence quantum yields are in the range 0.4 and 0.6 for the neutral forms, and are found to decrease on protonation, while phosphorescence lifetimes v ary from 3.15 s for the neutral form of Hi to 6.89 s for the cationic form of NorH. In all the cases, the lowest excited triplet state is suggested to have (3)(pi, pi*) character. For several of the systems, the sum of emissi on yields is less than 1, indicating that internal conversion may also be a n important deactivation route for the lowest excited singlet state. Micros econd flash photolysis studies show that the triplet states of all three ca rbolines undergo acid-base equilibria in water with pK(a) values in the reg ion 4.4-5.0 at room temperature. In water and ethanolic solutions, the cati onic forms show two absorption maxima at ca. 450 and 580 nm, while the neut ral forms have only one absorption maximum in the visible spectral region a t ca. 510 mu. In nitrogen purged solutions, triplet state lifetimes are 530 (Hi), 3 80 (Ha) and 600 mus (NorH), and the triplet states are quenched by beta -carotene with rate constants k (av) = 3.3 (+/-0.9) x 10(9) M-1 s(-1) close to but slightly lower than those expected for a diffusion controlled process. (C) 2001 Published by Elsevier Science B.V.