Structural Analysis and Excited-State Dynamics of Highly Polar Aromatic Species Multiply and Selectively Labeled by Stable Isotopes.

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Vibrational Analysis of p -N,N- dimethylaminobenzonitrile (DMABN) has been performed for a series of isotope-substituted DMABNs in the ground saate. Infrared and Raman spectra of DMABNs have been recorded and analyzed on the basis of normal coordinate calculation on an empirical force field and of the Hartree-Fock and the density functional calculations. It has been found that a strong interaction between the dimethylamino group and the aromatic ring is directly reflected in the force field.Picosecond transient infrared spectra have been recorded for the excited singlet state of a series of isotope-substituted DMABNs in the fingerprint region. The band at 1276cm^<-1> for normal species shifts to low frequencies on ^<15>N, ^<13>C, and ^2H substitution of the dimethylamino group, and therefore assigned to the ring C-N(CH_8)_2 single-bond stretching. Other bands observed are attributable to vibrational modes of the benzonitrile moiety. The observed results suggest that the electronic structure of the charge-transfer excited singlet state is basically of the benzenoid nature with a significant contribution from the quinoid structure.