Extracting Molecular Dynamics in Acetylene From Unzipped Dispersed Fluorescence Spectra

1 S.A.B. Solina, J.P. O'Brien, R.W. Field, W.F. Polik, Ber, Bunsen. Phys. Chem. accepted. 2. S.A.B. Solina, J.P. O'Brien, R.W. Field, W.F. Polik. Symposium on Molecular Spectroscopy, 1995 ""Unzipped Acetylene Dispersed Fluorescence: An unconventional form of Franck-Condon Analysis''Author Institution: Massachusetts Institute of Technology, Cambridge, MA 02139; Hope College, Holland, MI 49423Dispersed Fluorescence (DF) spectra of the $\tilde{A} \leftrightarrow \tilde{X}$ system of Acetlene have been recorded utilizing $\tilde{A}$ State intermediates that have 0, 2, and 3 quanta of excitation in the trans-bend Intramolecular Vibrational Redistribution is manifested in the spectra by the degree of fractionation of the zero-order state among the molecular eigenstates. Recently a method for unzipping complex fractionation patterns in the dispersed fluorescence spectra have been $employed^{12}$. Once the DF Spectrum is unzipped, two trends in IVR become apparent. For the particular chromostates selected in these experiments, IVR increases with increasing excitation in the trans-bend, $\nu_{4}$ and, surprisingly IVR decreases with increasing excitation in the CC stretch, $\nu_{2}$. Since the density of states increases with increasing $\nu_{2}$ and $\nu_{4}$ one would expect for statistical reasons to observe an increase in IVR with increasing $\nu_{4}$ consistent with the spectrum and increasing $\nu_{2}$. contrary to the spectrum. As opposed to statistical expectations, both IVR trends turn out to be local effects. The extent of IVR, for the initial states prepared in these expriments, is controlled by the strength of the off diagonal matrix elements, $H_{ij}$ and the zero-order energy differences. $\Delta E$, of the first few initial resonance steps, or tiers, and is independent of the number or density of states

Experimental and Computational Electrochemistry of Quinazolinespirohexadienone Molecular Switches – Differential Electrochromic vs Photochromic Behavior

Our undergraduate research group has long focused on the preparation and investigation of electron-deficient analogs of the perimidinespirohexadienone (PSHD) family of photochromic molecular switches for potential application as photochromic photooxidants for gating sensitivity to photoinduced charge transfer. We previously reported the photochemistry of two closely related and more reducible quinazolinespirohexadienones (QSHDs), wherein the naphthalene of the PSHD is replaced with a quinoline. In the present work, we report our investigation of the electrochemistry of these asymmetric QSHDs. In addition to the short wavelength and photochromic long-wavelength isomers, we have found that a second, distinct long-wavelength isomer is produced electrochemically. This different long-wavelength isomer arises from a difference in the regiochemistry of spirocyclic ring-opening. The structures of both long-wavelength isomers were ascertained by cyclic voltammetry and 1H NMR analyses, in concert with computational modeling. These results are compared to those for the symmetric parent PSHD, which due to symmetry possesses only a single possible regioisomer upon either electrochemical or photochemical ring-opening. Density functional theory calculations of bond lengths, bond orders, and molecular orbitals allow the rationalization of this differential photochromic vs electrochromic behavior of the QSHDs