Analysis of the vibrational structure in the electronic spectra of molecules: Phenylacetylene as a prototype molecule

Abstract

Experimental studies of phenylacetylene (PA) in the gas phase previously revealed an unusual new set of photophysical properties following the S1←S0 excitation. The decay of the initially excited level showed the expected short (75ns) lifetime component assigned to the S1 state and a second very long lifetime (>100 microsec) component that were both created during the excitation laser pulse [Hofstein J., Xu H., Sears T. J., Johnson P. M., J. Phys. Chem. A 112 (2008)]. To investigate the identity and formation mechanism of the long-lived species, a good understanding of the vibrational structure in the S1←S0 spectrum of PA is required. The interaction of the vibrational and electronic (vibronic) motions was calculated in order to predict the S1 and ←S0 electronic spectrum of PA using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second order vibronic model developed previously [Johnson P. M., Xu H. F., Sears T. J., J. Chem. Phys. 125 (2006)]. The sensitivity of the calculations to different DFT functionals (B3LYP, LC-BLYP and CAM-B3LYP) and basis sets (aug-cc-pVDZ and aug-cc-pVTZ) was explored and compared to accurate experimental relative intensities of the vibrational bands up to energies of 2000 cm-1 above the band origin obtained by recording the S1(A 1B2) and #61663; S0(X 1A1) electronic transition using resonance-enhanced multiphoton ionization spectroscopy (1+1 REMPI) in a molecular beam and cavity-ringdown (CRD) absorption spectroscopy in a slit jet-cooled sample. The long-range corrected functional CAM-B3LYP resulted in a 5 times bigger equilibrium transition moment than B3LYP, a result more closely matching the observations. The LC-BLYP result is yet 36% higher than CAM-B3LYP. It was observed that CAM-B3LYP produced a good match to most of the experimental spectrum. However, a superior match was observed when the CAM-B3LYP equilibrium transition moment and the moment derivatives from the B3LYP calculation were combined in the simulation. Results from the CRD experiment showed errors in relative intensities derived from the REMPI spectra, showing that REMPI spectroscopy is a less than accurate technique in this regard. Further pump-probe photoionization experiments not only confirmed Hofstein et al.'s observations but showed that some of strong assigned vibrational modes (ΝΒcc and Ν15) enhanced the formation of the long-lived species compared to others. The motion of the atoms in ΝΒcc and Ν15, along with the computational analysis of the energies of electronic states of different PA isomers with respect to the S1 state of PA., suggest isomer formation as a possible explanation for the observed long-lived species. The triplet ground state of Cycloocta-1,3,5-trien-7-yne (COTY), for example, appears to be a good candidate However, the evidence is still far from conclusive and additional experimental work is required to investigate the intriguing complexity in the photophysics of PA.