Frans W. Langkilde

The effect of solvent environment on molecular electronic transition moment directions: Symmetry lowering in pyrene

Measurements of polarized fluorescence and phosphorescence of pyrene, 2‐fluoropyrene, and 2‐methylpyrene in 3‐methylpentane glass at 77 K reveal significant differences among the three molecules. These are interpreted in terms of solvent‐induced mixing of the Lb and La states. It is proposed that such symmetry‐lowering effects of the solvent environment may well be important contributors to the often observed discrepancies between the theoretical limiting values of polarization ratios and the values actually observed.

The lowest triplet state of 1,3,5-hexatrienes : Quantum chemical force field calculations and experimental resonance Raman spectra

Theoretical and Raman spectroscopic studies are presented of E and Z‐1,3,5‐hexatriene and their 3,4‐ and 2,5‐dideuteriated analogs in ground and excited triplet states. The T1 potential energy surface is calculated from extended SCF‐LCAO‐MO‐CI theory. Energy minima and equilibrium geometries are determined in T1 . Frequencies and normal modes of vibration are calculated for the minima of the T1 and S0 states. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and the Tn level corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra of the T1 state under the assumption of a predominant Franck–Condon scattering mechanism. The results indicate that the planar E and Z forms of hexatriene and its analogs are the only ones contributing substantially to the T1→Tn absorption and the T1 resonance Raman spectr...

Resonance Raman spectra and quantum chemical vibrational analysis of the C7H7⋅ and C7D7⋅ benzyl radicals

Time‐resolved resonance Raman (RR) spectra of the benzyl radical and its perdeutero isotopomer are presented. The radicals are created by laser flash photolysis (λ=266 nm) of benzylchloride in solution. The spectra are excited at a wavelength of 315 nm, in resonance with the intense D3(2 2A2)←D0(1 2B2) transition. Twenty Raman bands, both polarized and depolarized, are observed. The RR spectra are analyzed through quantum chemical force field for π electrons (QCFF/PI) and ab initio calculations of equilibrium geometries, vibrational frequencies, Franck–Condon factors, and vibronic interactions. Polarized intense bands are assigned to totally symmetric a1 modes, and a number of depolarized bands to fundamentals of b1 modes. The observed activity of b1 modes suggests vibronic coupling, which is confirmed theoretically by calculations of vibronic interactions between the D3(2 2A2) and D5(4 2B2) states. The results from semiempirical and ab initio calculations are compared with experiment. The contributions o...

Time-resolved resonance Raman spectra of trans- and cis-stilbene in the lowest excited triplet state

Abstract Time-resolved resonance Raman spectra are obtained of the lowest excited triplet state T 1 produced from each of the trans and cis isomers of stilbene in methanol at room temperature, and from trans-stilbene in glycerol at 203 K. The room-temperature spectra are found to be identical for the two isomers. The spectra are compared with preliminary results of a quantum chemical force field calculation of frequencies of a g modes a planar T 1 trans-stilbene. The T 1 spectra are tentatively ascribed to a planar or near-planar trans geometry. Implications for the stilbene T 1 potential energy surface are discussed.

Time-resolved resonance raman spectrum of all-trans-diphenylbutadiene in the lowest excited singlet state

Abstract The resonance Raman spectrwn of all-trans-diphenylbutadiene in its lowest excited S 1 state excited in resonance with the S 1 → S n absorption band at 650 nm in non-polar solvents is reported. Three vibrational bands at 1572, 1481 and 1165 cm −1 are observed. A possible assignment of the the 1481 cm −1 band to the CC double-bond stretching mode may support the lowest S 1 state being of 1 A g *− symmetry.

A quantum chemical anaylsis of the time‐resolved resonance Raman spectrum of 2,5‐dimethyl–2,4‐hexadiene in the T1 state

Theoretical calculations are presented on 1,3‐butadiene and 2,5‐dimethyl–2,4‐hexadiene (tetramethyl–butadiene or TMB) in ground and excited triplet states. The T1 potential energy surfaces are calculated from extended self‐consistent‐field–linear combination of atomic orbits–molecular orbital–configuration interaction (SCF‐LCAO‐MO‐CI) theory. Energy minima and equilibrium geometries are determined in T1. Frequencies and normal modes of vibration are calculated for planar geometries in S0 and T1 and for a geometry in T1 twisted 90° around one of the formal C■C double bonds. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and Tn levels corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra in T1 under the assumption of a predominant Franck–Condon scattering mechanism. For TMB, the calculated reson...

Time-resolved absorption and resonance raman spectra of the lowest excited triplet state of all-trans-1,3,5-heptatriene

Abstract The lowest excited triplet state of all-trans-1,3,5-heptatriene has been studied by time-resolved absorption and resonance Raman spectroscopy. The difference absorption spectrum of the triplet state has a maximum around 315 nm, and the triplet state decays by first-order kinetics with k = (3.4 ± 0.3) × 10 6 s −1 . Time-resolved resonance Raman spectra of the heptatriene triplet excited at 317.5 nm showed bands at 1574, 1298, 1275, 1252, 1209, and 1132 cm −1 .

Resonance Raman study of the benzyl radical

Abstract Time-resolved resonance Raman spectra are obtained of benzyl radicals created by laser flash photolysis of benzylchloride and diphenylacetone in solution. The spectra are obtained in resonance with the intense 2 2 A 2 —1 2 B 2 transition of benzyl. The strong Raman bands are assigned to totally symmetric a 1 modes. The remaining observed bands are tentatively assigned to fundamental modes of b 1 , a 2 , and b 2 symmetry, and to overtones and combinations. The resonance Raman spectra are found to be quite different from previous fluorescence spectra of benzyl, and the origins of these differences are discussed.

Through-bond interaction in the radical cation of N,N-dimethylpiperazine. Resonance Raman spectroscopy and quantum chemical calculations

Abstract The radical cation of N,N-dimethylpiperazine is investigated by resonance Raman spectroscopy and ab initio calculations. The calculations strongly support the assignment of the vibrational spectrum to a chair conformation. It is shown that a dramatic geometry relaxation following ionization allows a much more efficient interaction between the nitrogen ‘lone pairs’ than in the neutral ground state.

Time-resolved resonance Raman study of proton transferring systems in the excited triplet state: 2,2′-bipyridine and 2,2′-bipyridine-3,3′-diol

Abstract Time-resolved resonance Raman (RR) spectra of the excited triplet state T 1 of 2,2′-bipyridine (BP), 2,2′-bipyridine-3,3′-diol BP(OH) 2 , and 5,5′-dimethyl-2,2′-bipyridine-3,3′-diol Me 2 BP(OH) 2 are obtained, and interpreted by comparison with their ground-state Raman spectra and the T 1 spectrum of biphenyl. The BP T 1 RR spectrum is assigned assuming C 2h molecular symmetry. The T 1 RR spectra of BP(OH) 2 and Me 2 BP(OH) 2 are ascribed to diketo tautomers that are products of double proton transfer in the S 1 state.