Photophysics of photoconducting polymers with pendant bichromophores I: Intramolecular photoprocesses in 9-[γ-(N-carbazolyl)propyl]-9-methyl-2,7-dinitrofluorene

Abstract

Electronic absorption and luminescence spectroscopies were used to study the intramolecular photoprocesses in the bichromophoric D—A compound 9-[γ-(N-carbazolyl)propyl]-9-methyl-2,7-dinitrofluorene (II) together with the monochromophoric model molecules N-ethylcarbazole (NEC) and 9-γ-hydroxypropyl-9-methyl-2,7-dinitrofluorene (I). It was shown that no intramolecular charge transfer interaction exists between the carbazolyl and the 2,7-dinitrofluorene chromophores in the electronic ground state of the bichromophoric compound II in EPA at room temperature and at 77 K. In contrast, the strong quenching effect of the fluorescence and phosphorescence emission of the carbazolyl chromophore in the bichromophoric molecule II was explained in terms of intramolecular energy transfer and the possible electron transfer interactions between the carbazolyl and the 2,7-dinitrofluorene groups. These two intramolecular photoprocesses in II were characterized well by the Förster critical transfer distance (R₀ = 2.6 Å at 296 K and R₀ = 30.4 Å at 77 K) and by the Rehm-Weller free energy change of the electron transfer step (ΔG_ET < 0) respectively. Electron transfer processes are usually facilitated by good orbital overlap, but this is not favored by the spatial conformation of the chromophores in II. This is in agreement with the fact that the phosphorescence lifetime (τ_P = 0.12 ± 0.01) and the phosphorescence quantum yield (φ_P = 0.060 ± 0.006) of the monochromophoric molecule I remain the same in II and in the polymer (III) (polysebacate containing II as a pendant group). We concluded that long-range energy transfer (Förster type) seems to be the dominant path in these systems.