LOW-LYING EXCITED SINGLET-STATES AND S-2-]S-0 LUMINESCENCE OF DIPYRIDO[3,4-B-2,3-D]PHENAZINE

The absorption, fluorescence and excitation fluorescence spectra dipyr ido[3,4-b:2,3-d]-phenazine (DPPZ1) have been measured in non-polar and polar matrices at room temperature, and were taken into account to ex plain the origin of the relatively weak emission of this molecule in b oth type of environment. The electronic structure of DPPZ1 was calcula ted using a modified INDO CI method. The geometry optimization has bee n performed using the MNDO method. According to the spectra and the re sults of calculations, the lowest excited singlet state S-1 of DPPZ1 m olecule is of n,pi-type and the next one, S-2 state, is of pi,pi*-typ e. The energy gap Delta E-calc (1)(n, pi) - (1)(pi, pi*) is equal 477 0 cm(-1). The low efficiency of the emission observed in the hydroxyli c solvent can be interpreted in terms of thermal quenching of the pi,p i-type fluorescence. However, experimental results obtained suggest t hat in nonpolar solvents the emission of the molecule examined is an a nomalous S-2 --> S-0 fluorescence. (C) 1998 Elsevier Science B.V.