Conformational effects on resolved emission spectra of floppy aromatic molecules Sujit S. Panja Department of chemistry Indian Institute of Technology Kanpur India 60 th International Symposium on Molecular Spectroscopy Ohio State University

allylbenzene n-propylbenzene 2-phenylethanol 2-(p-fluorophenyl)ethanol  * electronic transitions are affected by ring-chain interaction in the conformers. We show that the Spectral features are sensitive to the nature and orientation of the side chain with respect to benzene ring.

Excitation spectra (37533 cm -1 ) (37633 cm -1 ) (37505 cm -1 ) (37074 cm -1 ) 2-(p-fluorophenyl)ethanol 2-phenylethanol allylbenzene n-propylbenzene

Panja and Chakraborty, J.Chem.Phys., 118, 6200 (2003) Dispersed fluorescence spectra of the two origin bands of allylbenzene Some modes (e.g., 6a) appears in different spectral positions. Some modes (e.g., 6b) appears in exactly at the same position but with different intensity- a conformer specific vibronic coupling effects for 6b mode.

Dispersed Fluorescence spectra of n-Propylbenzene Low frequency vibronic transitions appear only for gauche conformer. Positions of some of the bands are sensitive to the orientation of the chain and some are not. For few bands there are alteration in intensity. Panja and Chakraborty, J.Chem.Phys., 119, 9486 (2003)

b. ex = cm -1 (gauche) a. ex = cm -1 (trans) Alongwith the expected sequence origin band (622cm -1 ), other strong vibronic fundamentals also show up as sequence origin.

Other observation: Gauche (~65 MHz) and trans (~50 MHz) According to the Fermi Golden rule, the decay rate of an isolated molecule is governed by the following equation: k = (2π/Ћ) | V el | 2 F nuc Where, the nuclear part F nuc contains the dependence of the rate on the density of states and vibrational overlap integral. V el is the electronic matrix element and it depends on the electronic wave functions of the Initial and final states. From theoretical calculations (CIS/6-31G**) it is found that |μ trans | = 1.27x C m |μ gauche | = 1.42x C m V el is almost same for both conformers TM=

Dispersed Fluorescence spectrum of 2-phenylethanol Panja and Chakraborty, J.Phys.Chem.A, 107, (2003)

µ TM =∫Ψ * S1 |µ| Ψ S0 d τ Relative energies and TM alignments (in the parenthesis) of different conformers of 2-phenylethanol predicted by MP2/ G** level of calculations. 0.0 (4 degree) 6.46 (0 degree)

I (0.0) II(5.55) III(5.62)IV(6.12)V(8.92) 2-(p-fluorophenyl)ethanol

Dispersed Fluorescence spectrum of 2-(p-fluorophenyl)ethanol

Pump Sample He or Ar or N 2 gas UV light SHG Dye LaserNd:YAG Laser Delay Nozzle driver Spectrometer ICCD Detector Oscilloscope Boxcar averager Chart Recorder Computer O OO   Fluorescence Supersonic jet Experimental set up Photodiode

Summary:  The vibronic transitions are very much sensitive to the orientation of the substituent present in a molecule for different conformations or in other words, there is conformationally-induced change in electronic structure of the isolated molecules.  The ring-chain interactions produce changes in the electronic structure of the S 1 or S 0 state or both. And this interaction depend on the nature of the side chain.

Acknowledgment  Dr. Tapas Chakraborty Chayan K. Nandi, Montu K. Hazra, Partha Biswas, V. Ramanathan, Amit Samanta, Prasenjit Pandey, Biman Bandopadhay  Indian Institute of Technology Kanpur, INDIA  CSIR, INDIA  DST, INDIA