One-Dimensionally Conjugated Carbocyanine Dyes Isolated under Cold Gas-Phase Conditions: Electronic Spectra and Photochemistry

One-dimensionally conjugated carbocyanine dyes are of significant research interest, particularly for their electronic photoexcitation, owing to a wide variety of characteristics, including a good analogy to "free electrons in a one-dimensional box" model and trans-cis photoisomerization along the conjugated chain. Despite these important aspects, their electronic spectra remain ambiguous in terms of their assignment owing to the significant effects of their surrounding environment. In this study, we present the electronic spectra of two cyanine dyes, 1,1'-diethyl-2,2'-carbocyanine (pinacyanol, 1) and 1,1'-diethyl-4,4'-carbocyanine (cryptocyanine, 2), measured under cold (∼10 K) gas-phase conditions, to determine the intrinsic electronic transition energy and provide clear assignments for the spectra. The obtained visible photodissociation spectra demonstrate (1) spectral shifts in response to both solvent and temperature, (2) the contribution from the vibrational excitation in the excited state (Franck-Condon (FC) activity), and (3) the coexistence of conformers caused by the orientation of the side ethyl groups. These factors affect the electronic transition energy up to ∼1000 cm-1 in total for both 1 and 2, which corresponds to an effective length of 0.5 Å in terms of the "one-dimensional box" model. Furthermore, a difference was observed in the effective bandwidth of the spectra between 1 and 2 based on a comparison with the simulated FC patterns around the origin band; the bandwidth was substantially larger for 2 than that of 1, implying the shorter lifetime of 2 in the photoexcited S1 state. With the aid of density functional theory (DFT) calculations of the relaxed potential energy curves, we partly ascribed this to the fast trans-cis photoisomerization via C═C bond twisting on the S1 surface, followed by S1-S0 internal conversion.