π-conjugation and charge polarization in fluorene-dibenzothiophene-S,S-dioxide co-oligomers by Raman spectroscopy and quantum chemistry

The Raman spectra of a series of fluorine (F)∕dibenzothiophene-S,S-dioxide co-oligomers (S) of different length and alternation sequences in their backbones (FSF, FFSFF, FSFSF, and FASAF; A is the -C≡C- bridge) have been recorded and simulated theoretically. It is shown that Raman spectroscopy is useful to probe π conjugation and ground state electron polarization in these molecules, phenomena directly related with the existence of intramolecular charge-transfer processes owing to the combination of electron donor (fluorene) and acceptor (dibenzothiophene-S,S-dioxide) groups. Their geometric, electronic, and vibrational properties have been studied by density functional theory B3LYP∕6-311G(2d,p) quantum chemical calculations and compared with those for fluorene homo-oligomers. Comparative studies in solution∕solid∕melted phase led to the conclusion that the Raman wavenumbers are also sensitive to intermolecular interactions.