Nitro End Groups: Remarkable Vibrational Reporters for Charge Transfer in the Excited States of Oligo(p‑phenyleneethynylene)-Bridged Donor–Acceptor Dyads

The D−π–A dyads [(n-Hex)2N­(1,4-C6H4)]­CC­[(1,4-C6H4)­NO2] (1) and [(n-Hex)2N­(1,4-C6H4)]­CC­(1,4-C6H4)­CC­[(1,4-C6H4)­NO2] (2) have been studied by ultrafast time-resolved infrared absorption spectroscopy. After excitation into their singlet charge-transfer (1CT) state, a fast decay (ca. 6 ps for 1 and 1 ps for 2) of the initially populated singlet state into a ground state (major pathway) and a longer lived excited state (possibly the triplet state) is observed. The nitro and alkyne groups were used as vibrational reporters to probe the changes induced by the charge-transfer process. For the first time, we confirm experimentally that these changes are consistent with expectations based on the traditional valence-bond representations of the CT states of these push–pull chromophores. An almost identical charge transfer takes place in the two dyads, despite π-bridges of different lengths between the donor and acceptor groups. Complementary DFT calculations support the experimental assignments and have helped clarify the photophysical behavior of 1 and 2.