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University of Vienna |
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Institute for Theoretical
Chemistry
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Contents |
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Biphenyl
In conjugated oligo- and polymers consisting of aromatic rings connected by single bonds, typically the absorption spectrum is less structured than the luminescence spectrum (see Figur below). In order to understand this phenomenon we calculated the torsion potential curves for ground and excited states in the simplest case, biphenyl, which consists of two phenyl rings connected by one single bond (see Figure above).
Fluorescence(excitation at 5.32 eV)
and absorption spectra of biphenyl in cyclohexane at room temperature [1].
Potential Curves
Due to the D2 symmetry group of biphenyl, there exist four irreducible representations:
- A which is that of the groud state
- B1 which the dipole allowed first excitation is of
- B2 and B3 which results in almost forbidden transitions at lower energies
Potential curves for biphenyl as calculated by TD-DFT (B3-LYP/SVP).
Solid lines for ground state geometry, dashed lines for vibrationally relaxed excited states.Dotted lines represent bases on experimental data [2].
In what follows only the ground state (A symmetry), and the first excited 1B1 state are of interest.Note that the relaxed 1B1 state (dashed green line) has the lowest minimum. Consequently it is to assume as the fluorescent state, what fits well to the experimental data[2].
Torsional broadening
The thermal torsional motion creates a distribution of torsional angles (see below) which results in a corresponding distribution of transition energies.
Distribution of torsion angles in the ground (relaxed
excited) state with transitions (arrows) to the excited
(ground) state at
room temperature.
Since for the ground state distribution around the torsion angle 40° the gradient of the excited state 1B1 is much greater than around 0°, which corresponds to the minimum for the relaxed ecited state, the spectral broadening by torsional inhomgenety is much smaller in fluorescence than in absorption. Note that the ground state potential surface is almost flat compared to the excited state. Therefore in fluorescence spectral features as a (non-torsional) vibrational progression are clearer to see (see below).
Fluorescence
and absorption spectra of biphenyl (solid
lines) compared
with the respective torsionally broadened 11B1 transitions (dashed lines).
Notably, the calculated fluorescence peak fits to the highest vibrational band of the fluorescence spectrum. For the absorption spectrum the torsional broadening is of the same order of magnitude as the distance between the vibrational subbands and consequently the latter are not resolvable.
References
[1] H. Du, R. A. Fuh, J. Li, A. Corkan, J. S. Lindsey, Photochem. Photobiol. 68 (1998) 141-142
[2] H.-S. Im, E. R. Bernstein, J. Chem. Phys. 88 (1988) 7337-7347.