Photophysical Properties of The DCM and DFSBO Styryl Dyes Consequence for Their Laser Properties

The two styryl dyes, 4-dicyanomethylene-2-methyl-6-P-dimethylaminostyryl-4H-pyran (DCM) and 7- dimethylamino-3-(p-formylstyryl)-l, 4-benzoxazin-2-one (DFSBO) exhibit similar solvent-induced shifts of their absorption and emission spectra related to a large intramolecular charge transfer (ICT) in the first singlet excited state. From the Stokes shift values (vA−vF) and a vectorial analysis of their ground state dipole moment (μg= 6.1 D for DCM and 5.8 D for DFSBO), and using the Lippert-Mataga theory, we have estimated the dipole moments of their fluorescent excited states S1 (μe = 26.3 D for DCM and 27.6 D for DFSBO). Intersystem crossing to the triplet state is totally inefficient in DCM but significant in DFSBO. Moreover the absorption of the DFSBO triplet is quite large in the emission band (600–650 nm), which makes of DFSBO a poor laser dye. Although DCM trans-cis photoisomerization can be quite efficient in non polar solvents (chloroform, tetrahydrofuran), DFSBO does not photoisomerize probably due to steric hindrance and to the S1 character which should be more "benzoxazinone" than ethylenic. DFSBO is also shown to exhibit rotamerism.