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)]CC[(1,4-C₆H₄)NO₂] (1) and [(n-Hex)₂N(1,4-C₆H₄)]CC(1,4-C₆H₄)CC[(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.