Erich Tauer

ABSENCE OF DUAL FLUORESCENCE WITH 4-(DIMETHYLAMINO) PHENYLACETYLENE. A COMPARISON BETWEEN EXPERIMENTAL RESULTS AND THEORETICAL PREDICTIONS

Abstract From ab initio electronic structure calculations it has been predicted that 4-(dimethylamino)phenylacetylene (DACET) will undergo an exothermic intramolecular charge transfer (ICT) reaction with dual fluorescence even under isolated-molecule conditions. Photostationary and time-resolved fluorescence measurements reveal however that an ICT reaction does not occur with DACET under any condition of solvent polarity or temperature studied. DACET is similar to 4-(methyl)amino-benzonitrile, which shows only fluorescence from a locally excited state, but is clearly different from the dual fluorescent 4-(dimethyl)aminobenzonitrile (DMABN). The energy gap Δ E ( S 1 , S 2 ) of DACET is larger than that of DMABN, supporting the importance of this gap in these ICT reactions.

Tunnel effect on the kinetics of hydrogen shifts. The enol-ketone transformation of 2′-methylacetophenone

Abstract A large isotope effect on the enol—keto transition of 2′-methylacetophenone and a temperature-dependent activation energy for this process have been observed in flash experiments. A tunnel process and the existence of two metastable rotamers of the enol are proposed to describe the kinetcis of the reketonization process.

Internal conversion in 4-substituted 1-naphthylamines. Influence of the electron donor/acceptor substituent character

The thermally activated internal conversion (IC) taking place in 4-substituted 1-(dimethylamino)naphthalenes (14DMX) and 1-aminonaphthalenes (14ANX) with X=CN, Cl, H, CH3 and OCH3 was investigated in three solvents spanning the polarity scale, hexane, diethyl ether and acetonitrile. In both series 14DMX and 14ANX, the efficiency of the IC reaction decreases substantially when X changes from CN to OCH3, the order in which the electron donor character of the 4-substituent increases. Considerably larger IC reaction rate constants are obtained for the first group of compounds. This difference is connected with the ground state structure of the amino group, which is more strongly twisted for 14DMX (ca. 60°) than for 14ANX (ca. 20°), whereas both sets of 1-naphthylamines are planarised in the S1 excited state. The IC process slows down with increasing solvent polarity for each of the 14DMX and 14ANX molecules. The substituent X and the solvent polarity mainly affect the IC activation energy EIC. With 14DMX in hexane, EIC increases from 10 kJ mol−1 for X=CN to 34 kJ mol−1 for X=OCH3, whereas with, e.g., 14DMCL a solvent polarity dependent increase of EIC from 16 kJ mol−1 in hexane to 28 kJ mol−1 in acetonitrile is observed. The height of the barrier EIC is governed by the energy gap ΔE(S1,S2) between the two lowest excited singlet states. The influence of ΔE(S1,S2) on EIC is attributed to vibronic coupling caused by the proximity of the S1 and S2 states, which flattens the S1 potential energy surface and thereby lowers the IC barrier when ΔE(S1,S2) becomes smaller. It is assumed that the IC reaction of the 1-naphthylamines passes through a conical intersection, which exists as a consequence of the relative displacement of the S1 and S0 surfaces caused by the different amino twist angles in the two states.

On the viscosity effects in proton transfer systems. Structure and photophysics of 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethylindole.

Abstract Temperature effects on fluorescence quantum yields and lifetimes of 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethylindole (HBDMI) and 2-(2′-hydroxyphenyl)benzoxazole (HBO) in the series of hydrocarbons were studied. The results obtained allowed us to examine the role of the viscosity in the electronically excited proton transfer (PT) systems. The large-amplitude torsional motions operate in the initially excited and PT forms. The X-ray determination of the crystal structure revealed that HBDMI is distorted from planarity in the crystalline state. We propose that solvent frictional forces effectively hinder the formation of the coplanar structure required for the barrierless proton transfer in the excited state.

The utilization of tunnel effects for mechanistic studies of hydrogen transfer reactions

Abstract The non-oxidative photocyclization of an N-aryl enamine and the enolketo transition of several aromatic ketones were investigated flash photolytically. Large isotope effects and non-linear Arrhenius plots of the rate constants are interpreted as tunnel contributions to the hydrogen transfer reactions. The occurrence of proton tunnelling was used to distinguish between two possible routes of a sigmatropic hydrogen shift reaction and to investigate the mechanism of the enol re-ketonization reactions in detail. The existence of a pre-equilibrium between two enol rotamers is postulated and experimental evidence for its existence is presented.