Absorption-emission symmetry breaking and the different origins of vibrational structures of the 1Qy and 1Qx electronic transitions of pheophytin a

The vibrational structure of the optical absorption and fluorescence spectra of the two lowest-energy singlet electronic states (Q(y) and Q(x)) of pheophytin a were carefully studied by combining low-resolution and high-resolution spectroscopy with quantum chemical analysis and spectral modeling. Large asymmetry was revealed between the vibrational structures of the Qy absorption and fluorescence spectra, integrally characterized by the total Huang-Rhys factor and reorganization energy in absorption of S-vib(A) = 0.43 +/- 0.06, lambda(A) = 395 cm(-1) and in emission of S-vib(E) = 0.35 +/- 0.06, lambda(E) = 317 cm(-1). Time-dependent density-functional theory using the CAM-B3LYP, omega B97XD, and MN15 functionals could predict and interpret this asymmetry, with the exception of one vibrational mode per model, which was badly misrepresented in predicted absorption spectra; for CAM-B3LYP and omega B97XD, this mode was a Kekule-type mode depicting aromaticity. Other computational methods were also considered but performed very poorly. The Q(x) absorption spectrum is broad and could not be interpreted in terms of a single set of Huang-Rhys factors depicting Franck-Condon allowed absorption, with Herzberg-Teller contributions to the intensity being critical. For it, CAM-B3LYP calculations predict that S-vib(A) (for modes >100 cm(-1)) = 0.87 and lambda(A) = 780 cm(-1), with effective x and y polarized Herzberg-Teller reorganization energies of 460 cm(-1) and 210 cm(-1), respectively, delivering 15% y-polarized intensity. However, no method was found to quantitatively determine the observed y-polarized contribution, with contributions of up to 50% being feasible. Published under license by AIP Publishing.