Probing the Role of the Intermediate State in Cyanine Dyes through Classical and Entangled Two-Photon Absorption Spectra

Abstract The one-photon, classical two-photon, and entangled two-photon absorption (1PA, TPA, and ETPA) excitation wavelength-dependent spectra in a series of structurally related cyanine dyes are obtained to probe the role of the intermediate state on their nonlinear and quantum optical properties. The cyanine dye systems are 3,3′-diethyloxacarbocyanine iodide (DOCI) with an oxygen substituent, 1,1′-diethyl-3,3,3′,3′-tetramethylindocarbocyanine iodide (DiIC2) with a dimethyl substituent, and 3,3′-diethylthiacarbocyanine iodide (DTCI) with a sulfur substituent. Combined experimental measurements and electronic structure calculations reveal an unexpected reordering of absorption spectral trends across the series. For DOCI, the absorption spectral ordering follows TPA, ETPA, 1PA, whereas for DiIC2 and DTCI, the ordering shifts to ETPA, TPA, 1PA. The calculations indicate that the classical TPA process is driven primarily by the two-photon transition strength of the excited state. The ETPA process is more strongly influenced by the two-photon excited state with a longer radiative lifetime. The role of the intermediate state is uniquely revealed by the excitation of the entangled photons. The heteroatom substituent-dependence reveals the influence of greater multiconfigurational character and smaller charge transfer for the excited states of DOCI compared to the other two molecules, with the longer-lived state associated with single configurations and greater charge transfer. These results establish ETPA as a sensitive probe of electronic structure, and we demonstrate how intentional organic molecular design can systematically tune quantum and nonlinear optical responses.