Asymmetric structure for the excited S₁ state of 2,2′‐bipyridine evidenced by picosecond time‐resolved resonance raman experiments and ab initio calculation

Abstract

ABSTRACT:The combination of ab initio calculation using the CIS method and experimental vibrational data from picosecond time-resolved Raman measurements leads to a confident picture of the structure of the lowest excited singlet state S 1 (n*) of 2,2Ј-bipyridine. The calculated structure is asymmetric with two dissimilar pyridyl moieties. It can be understood as resulting from a confinement of the excitation in one of the pyridyl rings, breaking the initial ground-state symmetry. The reliability of this structure is attested by good agreement between the derived theoretical vibrational data and the experimental Raman spectra.

The excited triplet (T1) state structure and vibrational properties of 2,2′-bipyridine

“…The optimized geometry is trans-planar, as predicted in previous semi-empirical [12] and ab initio [13] calculations, and characterized by an center of symmetry at the middle of the C 2 C 2 0 interring bond, the two pyridyl rings having identical geometries. There is thus no symmetry breaking effect by confinement of the excitation in one of the pyridyl rings, in contrast to what was observed for the S 1 state of 22BPY [23] and the T 1 state of 2,2 0 -biquinoline [21] and 2,2 0 -biquinoxaline [22]. The T 1 state geometrical parameters are given in Table 1 together with those calculated for the ground state and the variations between the two states.…”

“…Recently, a detailed analysis of TR 3 spectra recorded for the parent 22BPY molecule (h 8 ) and its 6,6 0 -dideuterio (d 2 ), 3,3 0 ,5,5 0 -quaterdeuterio (d 4 ), and perdeuterio (d 8 ) isotopomers (atom numbering shown in Scheme 1) has been proposed [27]. Qualitative assignments of the observed Raman peaks have been established, in the hypothesis of Franck-Condon resonance Raman mechanism, from the analysis of the frequency changes observed between the different isotopomers and by analogy with previous quantum calculations of the 22BPY ground state [28], excited S 1 state [23], and anion radical (R ÀÅ ) [29]. A structural interpretation of the changes in frequency on going from the ground state to the triplet state was proposed, suggesting for the T 1 species a trans-planar conformation with a structural distortion resembling that evidenced in the R ÀÅ ion, i.e., showing a semi-localized electronic configuration intermediate between a quinoidaltype form and a 3,5-dienic form.…”

“…In the structurally related 2,2 0 -biquinoline [21] and 2,2 0 -biquinoxaline [22] double molecules, low-temperature ODMR studies revealed the existence of two close-lying triplet states, one localized on one half of the molecule and the second one fully delocalized. Also, time-resolved resonance Raman spectroscopy (TR 3 ) and ab initio calculations demonstrated an asymmetric structure for the excited singlet state S 1 (np * ) of 22BPY resulting from the confinement of the excitation in one of the pyridyl rings only [23]. From ODMR data, the T 1 state structure of 22BPY in Shpolskii matrix at 1.8 K does not seem to show such symmetry breaking effect [19].…”

“…In fact, due to the low symmetry of the 22BPY molecules, the normal mode potential energy distributions (PEDs) are complex and may be subject to severe changes when modifying the electronic configuration of the molecule, which makes uncertain a correlation between the S 0 , T 1 , and R ÀÅ vibrational assignments in the absence of any normal mode calculation. In previous investigations of the 22BPY S 1 state [23] and R ÀÅ [29] Raman spectra, notable potential energy redistributions have been particularly evidenced for those vibrations that involve important contributions from the CH in plane bends, in the 1000-1600 cm À1 spectral domain.…”

Ligand influence on the electronic spectra of monocationic copper–bipyridine complexes

“…The photophysics of bipyridine has received much attention in the past 30 years. [3][4][5][6][7][8][9][10] In the present work, we exclusively use 2,2 0 -bipyridine (bipy), and we will restrict our discussion to this isomer. The lowest excited states of bipy are in the UV spectral region.…”