Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: Conformation-specific effects in the weak coupling limit
Authors: Buchanan Evan G. , Walsh Patrick S. , Plusquellic David F. , Zwier Timothy S. 
Published: May 2013

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5–O–CH2–CH2–O–C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm−1 of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0–S1 and S0–S2 states (36 400–36 750 cm−1). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C–C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]10.1021/jp400691a. Comparison of 2C-R2PI spectra recorded in the m/z 214 (all 12C) and m/z 215 (one 13C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm−1 between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36 422.91 and 36 423.93 cm-1, with transition dipole moments perpendicular to one another. These are assigned to the S0–S1 and S0–S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm−1. The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36 508.77 cm−1. Here, the asymmetry induced by a single 13C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm−1of both conformers. In the tgt conformer, both “a” and “b” symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm−1 above the Bu origin is assigned to a bu fundamental (labeled $\bar R$R¯) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag($\bar R^1$R¯1) level contains strong transitions to v″($\bar R$R¯) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v″ = 1 arises from collisions of DPOE while in the excited state Ag(vb′ = 1) level with He, producing Bu($\bar R$R¯ = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x/D is ascribed to emission from the 13C isotopomer, for which this emission is dipole-allowed.