FLUORESCENCE OF ANTIAROMATIC SYSTEMS - AN EXPERIMENTAL AND THEORETICAL-STUDY OF 1,3,5-TRI-TERT-BUTYLPENTALENE

The fluorescence spectrum of 1,3,5-tri-tert-butylpentalene (TTBP), the stable derivative of antiaromatic pentalene, has been measured in sol ution at room and low temperature in condensed phase. A good correspon dence between the first allowed absorption band and the fluorescence s pectrum is observed. The fluorescence quantum yield of TTBP 2 x 10(-4) M in cyclohexane is estimated to be approximate to 22 x 10(-3) (lambd a(ex) = 313 nm) at room temperature. At low temperature and exciting a t 355 nm, the fluorescence spectrum of TTBP shows appreciable vibronic structure. The fluorescence data have been related to the electronic structure of the parent molecule. MCSCF/CAS calculations for the groun d and lowest excited states of pentalene, up to S-4, have been perform ed in D-2h symmetry with 3-21G and 6-31G basis sets including polariza tion and diffuse orbitals. As a result of the strong interaction betwe en the pairs of states S-0/S-1 and S-2/S-3, the lowest component of ea ch pair distorts from D-2h to lower symmetry, giving rise to new equil ibrium geometries. According to ab initio calculations, the observed f luorescence is assigned to upper stale, S-3-->S-0, emission.