Triplet state studies of β-carbolines

Abstract

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 β-carbolines: harmine (Hi), harmane (Ha) and norharmane (NorH). Based on the phosphorescence 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), 21 750 (Ha) and 21 850 cm−1 (NorH) for the corresponding cations. Phosphorescence 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 vary 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(π, π∗) character. For several of the systems, the sum of emission yields is less than 1, indicating that internal conversion may also be an important deactivation route for the lowest excited singlet state. Microsecond flash photolysis studies show that the triplet states of all three carbolines undergo acid–base equilibria in water with pKa values in the region 4.4–5.0 at room temperature. In water and ethanolic solutions, the cationic forms show two absorption maxima at ca. 450 and 580 nm, while the neutral forms have only one absorption maximum in the visible spectral region at ca. 510 nm. In nitrogen purged solutions, triplet state lifetimes are 530 (Hi), 380 (Ha) and 600 μs (NorH), and the triplet states are quenched by β-carotene with rate constants k(av)=3.3(±0.9)×109M−1s−1 close to but slightly lower than those expected for a diffusion controlled process.