Hemant K. Sinha

Van der Waals complexes of xanthione and benzopyranthione with rare gases. S2S0 fluorescence excitation spectroscopy and microscopic solvation effects

The 1:1 and 2:1 van der Waals complexes of xanthione and benzopyranthione with He, Ne, Ar, Kr and Xe have been observed by S2S0 laser-induced fluorescence excitation spectroscopy in a supersonic jet. The adatom is located over the pyranthione ring in the 1:1 complex, and the 2:1 complex has a symmetrical sandwich structure. An analysis of the microscopic solvent shifts indicates that the thiones have substantially smaller electric dipole moments in the S2 state than in the ground state, in qualitative agreement with previous semi-empirical calculations. The dipole moment of xanthione S2 is estimated to be only 2.0 D, so that thione—adatom interactions are almost exclusively dispersive in the upper state.

Spectroscopy and photophysics of tropolone in condensed media

A systematic study of the spectroscopy and photophysics of tropolone in condensed media has been undertaken by measuring its UV–visible absorption, emission and emission–excitation spectra in a number of solvents of varying structure, at temperatures between 77 K and 295 K, and by measuring its quantum yields of emission, ϕem, and time-resolved emission decays as a function of temperature, T, and excitation wavelength, λex, in each solvent. In weakly interacting solvents, such as 3-methylpentane and perfluoro-2-n-butyltetrahydrofuran, the only species yielding significant fluorescence is the S1, 1(π,π*) state of the intramolecularly hydrogen-bonded neutral molecule. In acidic aqueous solution at pH <3, the emitting state is also 1(π,π*) S1, but the intramolecular hydrogen bond has been disrupted by the solvent. In basic aqueous solution at pH >8 the emitting species is the corresponding excited anion. In some solvents, emission from photochemical products prevents reliable measurements from being made at room temperature. Contrary to previous reports, no emission from solvent-stabilised 1(n,π*) excited states is observed. In glass-forming media the fluorescence quantum yields and the lifetimes of the emitting species increase in a sigmoidal fashion as the temperaure decreases, and are a factor of more than 100 greater at 77 K than at room temperature. The fluorescence quantum yields also decrease with increasing excitation energy in the UV, an effect which is most pronounced in more weakly interacting solvents at temperatures near room temperature. A mechanism is proposed. Both the spectra and the excited state temporal decays indicate that emission from short-lived, vibrationally unrelaxed S1 constitutes the majority of the fluorescence at room temperature, a small fraction of the total emission at 77 K, and an increasing fraction of the total emission with decreasing excitation wavelength. The variations of ϕem and lifetime with temperature are attributed to the effects of temperature-dependent solvent relaxation dynamics on the non-radiative decay, based on qualitative correlations between the observed parameters describing the effects in different media and known characteristics of the media, including their glass transition temperatures and polarity.

Ground and excited state dipole moments of pyranthione and xanthione by the electro-optic method

Abstract Electric field-induced changes in the absorption spectrum (electrochromism) have been used to obtain the ground (S 0 ) and second excited (S 2 ) state dipole moments of two sulphur-containing heteroaromatics, pyranthione and xanthione, which have C 2v symmetry. The magnitude of the change in the dipole moment (Δ μ = ¦ μ (S 2 ) − μ (S 0 )¦) was found to be nearly 2 D for both compounds, which is consistent with results reported previously based on microscopic solvent shifts. The transition dipole moment for the S 0 –S 2 transition is parallel to the direction of the ground state dipole moment ( μ g ), i.e. along the C 2 axis which contains the CS bond. For comparison the ground state dipole moments of both compounds have also been calculated using the AM1 method will full geometry optimization.

Laser-induced fluorescence excitation spectroscopy of jet-cooled tropolone-carbon monoxide van der Waals complexes

Abstract The 1:1 and 1:2 van der Waals complexes of tropolone (TRN) and tropolone-OD with CO [TRN · (CO n ( n = 1, 2)] have been synthesized in a supersonic expansion, and their S 1 −S 0 laser-induced fluorescence excitation spectra have been measured. Two distinct isomers of the 1:1 complex, TRN · CO(I) and TRN and CO(II), with their origins displaced 54.5 and 79.5 cm −1 respectively to the red of the origin of bare tropolone, have been identified. In TRN · CO(I), the CO is located over the seven-membered ring and is primarily dispersively bound. In TRN · CO(II), the CO is located in the plane of the ring and forms a hydrogen-bonded complex with the hydroxyl group of the chromophore. TRN · (CO) 2 has a spectral shift which is exactly twice that of the 1:1 TRN · CO(I) complex, indicating that the 1:2 complex has a symmetrical sandwich structure with one CO residing on each side of the chromophore. A large decrease in the proton tunnelling splitting, from 19.4 cm −1 in bare tropolone to 3.5 cm −1 in TRN · CO(I) is observed, indicating that CO strongly perturbs the reaction coordinate by significantly increasing the barrier height in the excited state. Strong coupling of the tunnelling mode to the in-plane CO torsional mode is suggested to be the likely source of this effect. No tunnelling doublets are observed in TRN · CO(II), suggesting that hydrogen bonding quenches excited state proton transfer. The results of parallel studies on tropolone-OD are completely consistent with this model. Empirical calculations of the geometries and binding energies of the complexes using Lennard-Jones 6–12 atom-atom pair potentials support the interpretations.

Proton tunneling in the tropolone-N2 van der Waals complex

Abstract Proton tunneling splitting has been measured for the S 0 → S 1 origin band of the tropolone: N 2 van der Waals complex by recording its laser-induced fluorescence excitation spectrum in a supersonic jet. A large decrease in the magnitude of the tunneling splitting, from 19.4 cm −1 in bare tropolone to 6.0 cm −1 in the 1:1 complex, has been observed, indicating that nitrogen perturbs the reaction coordinate by significantly increasing the barrier height in the excited state. Strong coupling of the tunneling mode to the in-plane N 2 torsional mode is the likely source of this effect.

S2S0 fluorescence excitation spectroscopy of van der Waals complexes of xanthione and benzopyranthione with N2 and CO

Abstract The S 2 S 0 fluorescence excitation spectra of xanthione (XT) and benzopyranthione (BPT) and their 1:1 and 1:2 complexes with N 2 and CO have been measured in a supersonic expansion. Remarkably long, harmonic progressions in van der Waals modes are observed and are assigned on the basis of the structural and dipole moment changes in the chromophores. The microscopic solvation shifts are rationalized in terms of dipolar and dispersive contributions to the binding energies.

Origins of the differences in solvation by alkanes and perfluoroalkanes: Evidence from the S2-S0 spectra of jet-cooled van der Waals complexes of xanthione and azulene

Abstract The S 2 -S 0 fluorescence excitation spectra ofxanthione (XT) and azulene (AZ) complexed with 1 or 2 molecules of the C 1 to C 10 n -alkanes and the C 1 to C 6 perfluoro- n -alkanes have been measured. The 1:1 complexes exhibit microscopic solvent shifts, Δ \ gn, which are larger for the alkanes than the corresponding perfluoroalkanes, despite the larger molecular polarizability of the latter. The values of Δ \ gn increase monotonically with carbon number of the adduct to C 10 in 1:1 n -alkane complexes with XT and to C 5 in 1:1 n -alkane complexes with AZ. However, Δ \ gn exhibits no further increase beyond C 2 in 1:1 perfluoro- n -alkane complexes with XT and beyond C 3 in 1:1 perfluoro- n -alkane complexes with AZ. The results are interpreted in terms of a model in which the n -alkanes stretch out along the long axis of the chromophore and ‘wet’ its surface whereas the perfluoro- n -alkanes with carbon numbers ⩾ 3 away from the surface of the chromophore and are ‘non-wetting’.