Experimental and computational studies of 4H-cyclopenta[2,1-b:3,4-b′] dithiophen-4-one (CPDTO)-oligomers

4H-Cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO), CPDTO ketal (CPDTO-k) and their oligomers have been synthesized and their optical absorption, fluorescent, electrochemical properties are characterized. Structure optimization and time-dependent energy calculation have been carried out for (CPDTO-k)_1,2,3,6 and (CPDTO)_1,2,3,6 using the B3LYP functional and standard split valence plus polarization basis set 6-31G(d,p) in the DFT formalism. The pi conjugation between carbonyl oxygen and thiophene, which is orthogonal to the backbone conjugation, is evident from the DFT calculation and analysis, and is responsible for the observed slower narrowing of bandgaps (E_g) of CPDTO oligomers with increasing number of repeat units. The orthogonal conjugation also leads to different distribution of LUMO from that of HOMO and makes CPDTO oligomers weak in the lowest energy absorption. Enhanced intensity and red-shifted peak wavelength of the lowest energy UV-vis absorption of CPDTO film suggest the presence of strong intermolecular interactions among CPDTO molecules in the solid state, which may explain why the bandgap (1.1-1.2 eV) reported for the electrochemically deposited CPDTO homopolymer film is significantly lower than the bandgap (1.5-1.7 eV) estimated in this study for PCPDTO chloroform solution from the E_gs of CPDTO oligomers.