Femtosecond electronic realignment in a free base naphthalocyanine observed by pump-probe polarization anisotropy

Ultrafast electronic motion can occur in systems where molecular vibrations are coupled to electronic motion. Femtosecond polarized pump-probe measurements have been used to explore electronic relaxation in molecular aggregates and degenerate states of square symmetric molecules. This thesis begins with a derivation of analytical optical response functions that treat the electronic dephasing of a "quasi-degenerate" electronic state coupled to an asymmetric normal coordinate within the Brownian oscillator formalism. The theory is then used to calculate the anisotropy for model systems that may be observed when all asymmetric vibrations and distortions have the same symmetry. Time-resolved density matrix evolution calculations of a vibronically-coupled electronic state are presented in order to understand the population transfer and electronic dephasing expected for a molecule with a static splitting and a single asymmetric mode of different symmetry. The results of polarized femtosecond pump-probe anisotropy experiments on 2,11,20,29-tetra-tert-butyl 2,3-naphthalocyanine performed at two center wavelengths are presented and analyzed. The electronic relaxation is complete for pulse spectra centered on the absorption spectrum but incomplete at lower photon energy. The anisotropy of the most prominent vibrational beat observed in the data is consistent with an asymmetric vibration on the ground electronic state. Based on density matrix evolution calculations and Brownian oscillator models of the system, a single asymmetric vibration and a static perturbation of different symmetry can explain the ∼100 fs anisotropy decay and the wavelength dependence of the degree of electronic equilibration on the excited state. The splitting of the naphthalocyanine Q-band is estimated to be 100 +/- 50 cm -1.