Abstract
The ultrafast dynamics of unsubstituted spironaphthopyran (SNP) were investigated using femtosecond transient UV and visible absorption spectroscopy in three different solvents and by semi-classical nuclear dynamics simulations. The primary ring-opening of the pyran unit was found to occur in 300 fs yielding a non-planar intermediate in the first singlet excited state (S1). Subsequent planarisation and relaxation to the product ground state proceed through barrier crossing on the S1 potential energy surface (PES) and take place within 1.1 ps after excitation. Simulations show that more than 90% of the trajectories involving C-O bond elongation lead to the planar, open-ring product, while relaxation back to the S0 of the closed-ring form is accompanied by C-N elongation. All ensuing spectral dynamics are ascribed to vibrational relaxation and thermalisation of the product with a time constant of 13 ps. The latter shows dependency on characteristics of the solvent with solvent relaxation kinetics playing a role.
Highlights
Paper hybridisation of the spiro-carbon and conformational isomerisation
The modified-surface hopping dynamics are calculated using the adiabatic electronic energies and gradients calculated on the fly at time-dependent density functional theory (TDDFT) level of theory using the ab initio electronic structure package Gaussian 09.30 We treated the electronic structure of the SNP molecule with the use of density functional theory (DFT) for the ground electronic state in combination with
Combining semi-classical nuclear dynamics simulations and transient absorption spectroscopy in both the visible and the UV, the present study provides deeper insight into the mechanism and following product equilibration of the photochemical reaction of unsubstituted spiropyrans
Summary
Paper hybridisation of the spiro-carbon and conformational isomerisation. These structural changes yield a planar isomer called merocyanine (MC), in which the two moieties are linked by a methine bridge and become electronically coupled. The improved wave function overlap and delocalisation causes a reduction of the energy difference between the highest occupied and the lowest unoccupied molecular orbital giving rise to an intense absorption of the MC in the visible range. Eight different conformers are in principle possible due to an additional degree of freedom brought upon by the formation of the methine bridge. Which conformers are most stable depends on the respective spiropyran derivative[11] and the solvent environment.[12]
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