Two-photon absorption spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene: Theory and experiment
Two-photon absorption (2PA) spectroscopy provides complementary, and sometimes more detailed, information about the electronic structure of a molecule relative to one-photon absorption (1PA) spectroscopy. However, our understanding of the 2PA processes is rather limited due to technical difficulties in measuring experimental 2PA spectra and theoretical challenges in computing higher-order molecular properties. This paper examines the 2PA spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene by a combined experimental and theoretical approach. The experimental 2PA spectra are measured under identical conditions for all three compounds. The theoretical 2PA cross-sections are computed using the equation-of-motion coupled-cluster method with single and double substitutions (EOM-CCSD). Simulated 2PA spectra based on the calculations reproduce the main features of the experimental spectra in solution, although the quantitative comparison is complicated by a number of uncertainties, including limitations of the theoretical model, vibronic structure, broadening of the experimental spectra, and solvent effects. The systematic comparison of experimental and theoretical spectra for this series of structurally similar compounds provides valuable insight into the nature of 2PA transitions in conjugated molecules.