QUANTUM-CHEMICAL STUDIES ON STRUCTURES AND SPECTRA OF 2,5-DISTYRYLPYRAZINE (DSP) LASER-DYE

Semiempirical (MNDO and PM3) molecular orbital calculations have been undertaken to study the structures of the ground and excited states of 2,5-distrylpyrazine dye to assess its activity as a laser dye. Ln the ground and first excited singlet states, the trans-trans structure of C-2h symmetry is the most stable structure in the gas phase and in DM SO, which agrees with the experimental findings. Upon excitation, the flexibility of the molecule decreases, leading to a subsequent decreas e in the radiationless deactivation pathway and this increases the flu orescence efficiency of DSP. The absorption, excitation, and emission spectra have been calculated at the MNDO level using the PM3 optimized geometries in DMSO. At this level the agreement between theory and ex periment is quite good. An estimated absorption band at 377 nm (expt 3 80 nm) is assigned to the S-0 --> S-1 transition. The excited state ab sorption band at 457 nm (expt 460 nm) is assigned to the S-1 --> S-12 transition. The emission band at 458 nm (expt 460 nm) is assigned to t he S-1' --> S-0' transition. The overlap between the emission and the excited-state absorption spectra is presumably the main reason behind the reduced laser activity of the investigated dye. (C) 1998 John Wile y & Sons, Inc.