Photophysical properties of 3-[2-(N-phenylcarbazolyl)benzoxazol-5-yl]alanine derivatives--experimental and theoretical studies.

Solvatochromic probes are often used in biophysical studies to obtain information about polarity of the microenvironment. As there is not much natural fluorophores with such properties, there is still need for new synthetic compounds such as 3-(2-benzoxazol-5-yl)alanine derivatives. Among this group of non-proteinogenic fluorescent amino acids especially interesting are 3-[2-(4-aminophenyl)benzoxazol-5-yl]alanine derivatives whose solvatochromism depends on the substituents on the nitrogen atom, as revealed by our recent studies. To expand them we synthesized two new derivatives with an N-phenylcarbazole moiety in position 2 of the benzoxazole ring and studied their photophysical properties in solvents of different polarity and ability to form hydrogen bonds using absorption and steady-state and time-resolved fluorescence spectroscopy. Applying single parameter and multi-linear correlations with different solvent parameters, the excited state dipole moments were determined as well as the influence of solvent parameters on each photophysical property was estimated. Moreover, the geometry of compounds and vertical absorption transition were theoretically calculated (DFT and TD DFT methods). It was found that the place of substitution of the N-phenylcarbazole part by the benzoxazole unit determines the character of the electron transition (π-π* or ICT) and thereby the spectral and photophysical properties of the compounds studied.

Time-resolved emission spectra of 4-dimethylamino-4'-cyano-stilbene and resveratrol in high viscosity solvents and silica matrices.

Time-resolved emission spectra of 4-dimethylamino-4'-cyano-stilbene (DMACS) and 3,5,4'-trihydroxy-stilbene (resveratrol, RSV) in propylene glycol and in rigid silica xerogel matrix at 23°C were studied. For the polar molecule DMACS in propylene glycol, a 66nm shift of maximum wavelength of emission spectra was observed within 1ns after excitation, and most of the shift occurred during the first 200ps. For resveratrol in propylene glycol no such a shift was observed. The rigid silica environment eliminates some deactivation pathways and stabilizes spectroscopic properties of both molecules. Spectral properties of nonpolar and high dipole moment molecules in viscous liquids and rigid environments are compared. Results are explained on the basis of intramolecular processes and solute-solvent relaxation, as well.