Synthesis, electronic spectroscopy, and evaluation of photoconducting oligomers with pendant bichromophores. Part 1. Influence of donor–acceptor orbital overlap on aggregate emission and photoactivity

Abstract

The synthesis, electronic spectroscopy, and photoactivity of two polysebacate oligomers with pendant donor–acceptor bichromophores of the alkylcarbazole–dinitrofluorene type (P1) and the alkylcarbazole–trinitrofluorenone carboxylic acid ester type (P2) are reported. The crystal structure of the basic acceptor chromophore A2 and donor-acceptor bichromophore B2 of oligomer P2 are also reported. In bichromophore B2, the amount of overlap is evaluated at 50% while the distance between the donor and the acceptor is 3.42 Å. It is shown that while no ground state intramolecular charge-transfer interaction takes place in the bichromophore B1 of oligomer P1 itself (in solution or the solid state), aggregates (acceptor–acceptor) are present in 3MP and in the solid state. The spectrum of oligomer P1 in low temperature 3MP matrices indicates a small aggregation effect while aggregate fluorescence of bichromophore B1 and oligomer P1 is observed in the solid state. Absorption spectra show that bichromophores B2 and B2a and oligomer P2 give rise to charge-transfer bands with maxima at around 550 and 450 nm in CH3CN and 590 and 470 nm in the solid state. The fluorescence and the phosphorescence of the carbazolyl chromophore and the phosphorescence of the 2,4,7-trinitro-9-fluorenone chromophore are quenched for B2, B2a, and P2 in EPA but a structureless emission is observed between 500 and 680 nm in the solid state that can be assigned to aggregate fluorescence. The negative values of ΔGET indicate that intramolecular electron transfer in bichromophores B1, B2, and B2a should be highly probable on thermodynamic grounds if optimal orbital overlap is available. The spectroscopic properties of the bichromophoric compounds studied are discussed on the basis of conformational effects. The photoactivity evaluation of oligomers P1 and P2 shows that P2 is more photoactive than P1 due, mainly, to better orbital overlap between the donor and the acceptor moieties in oligomer P2 as compared to oligomer P1. Furthermore, a comparison of P1 and P2 to a standard PVK–TNF system reveals that the latter is much more photoactive due, again, mainly to better orbital overlap. Finally, the relative efficiencies of these systems as hole or electron conductors is discussed. Keywords: photoconducting polymers, donor–acceptor bichromophores, bichromophores, substituted carbazoles, poly-nitrofluorene derivatives.