Picosecond-microsecond dynamics of photoinduced electron-transfer processes in amorphous solid films of dimeric carbazolyl compounds doped with 1,2,4,5-tetracyanobenzene

Picosecond and nanosecond transient absorption spectroscopy and picosecond transient dichroism measurements were applied for elucidating photoinduced charge-separation, charge-recombination (CR), and hole-transfer (HT) processes in amorphous solid films of carbazolyl dimeric compounds (1,3-di(N-carbazolyl)propane and 1,2-trans-di(N-carbazolyl)cyclobutane) doped with 1,2,4,5-tetracyanobenzene (TCNB). Time-resolved fluorescence spectra were also measured for elucidating fluorescent states responsible for recombination emission. These results were compared with those for poly(N-vinylcarbazole) (PVCz), where the concentration of carbazolyl (Cz) chromophores is the same as the above dimeric compounds. The ground-state charge-transfer (CT) absorption and the steady-state CT fluorescence spectra were different between the present dimeric and the PVCz films. In both the films, the CR process was in competition with the HT process in the initial charge-separated state produced via the photoexcitation of the ground-state CT complex. The time constants of these processes in the amorphous dimeric compounds films were close to the values obtained for the PVCz ones. The holes (Cz(+)) that escaped from the initial CR process and the immobile TCNB anion disappeared via the CR process with a t(-0.5) time dependence. The CR process was interpreted as the hole diffusion-controlled geminate recombination process, as is the same as that of PVCz films. The behavior of time-resolved fluorescence attributable to the geminate CR process was different between the amorphous dimeric compounds and the PVCz films. These differences and similarities between the amorphous dimeric compounds and the PVCz films were discussed.