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. In the ground and first excited singlet states, the trans-trans structure of C symmetry is the most 2h stable structure in the gas phase and in DMSO, which agrees with the experimental findings. Upon excitation, the flexibility of the molecule decreases, leading to a subsequent decrease in the radiationless deactivation pathway and this increases the fluorescence 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 Ž and experiment is quite good. An estimated absorption band at 377 nm expt . 380 nm is assigned to the S ª S transition. The excited state absorption band 0 1 Ž . at 457 nm expt 460 nm is assigned to the S ª S transition. The emission 1 1 2 Ž .
X X band at 458 nm expt 460 nm is assigned to the S ª S transition. The overlap 1 0 between the emission and the excited-state absorption spectra is presumably the main reason behind the reduced laser activity of the investigated dye.