Accelerated and Efficient Photochemistry from Higher Excited Electronic States in Fulgide Molecules

Abstract

The photoinduced electrocyclic ring-opening of a fluorinated indolylfulgide is investigated by stationary and ultrafast spectroscopy in the UV/vis spectral range. Photoreactions, initiated by optical excitation into the S(1) (570 nm) and S(N) (340 nm) absorption band of the closed isomer, lead to considerable differences in reaction dynamics and quantum yields. Transient absorption studies point to different reaction pathways depending on the specific excitation wavelength: excitation into the S(1) state leads to the known reaction behavior with a picosecond decay to the ground state and a small quantum yield of 7% for the photoproduct. The S(N) state shows an unexpected long lifetime of 0.5 ps. The photoreaction starting from the S(N) state leads to a large extent directly to the product ground state and back to the educt ground state. This results in an increased reaction quantum yield of 28%. In contradiction to Kasha's rule, the S(1) state is only populated with an efficiency of 38%. The observed behavior strongly differs from the expected picture with fast relaxation into the S(1) state and a subsequent ring-opening reaction starting from the lowest excited electronic state. Quantum chemical calculations confirm and complement the experimental findings allowing a sound molecular interpretation to be obtained.

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

“…Although the relaxation rates show a dependence on the excess energy, the product formation yield is not influenced and yields similar amounts of the NCE photoproduct. Therefore, the excess energy does not lead to a different reaction outcome, as would for instance be possible by accessing higher-lying states from which the reaction proceeds differently 44,45 or by an altered branching due to wave packet splitting on the excited-state surface. 46,47 Having disclosed that the amount of product is independent of the excitation energy, we performed additional TA measurements of AA in the nonpolar solvent n-hexane and the polar aprotic solvent acetonitrile to understand the influence of the molecular environment.…”

On the excitation wavelength dependence and Arrhenius behavior of stilbene isomerization rates in solution

“…Thus at 300 K, the gas-phase rate is about 1 ns À1 whereas in hexane it is more than 10 times faster [7][8][9][10][11][12][13][14]. Various explanations of this observation have been proposed [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], the restricted IVR mechanism [11][12][13][14][25][26][27] being now widely accepted. It suggests that stilbene torsion about the C@C bond is weakly coupled to other vibrational modes, resulting for isolated molecules in a slow energy transfer, and accordingly slow isomerization rates.…”

Stability and reaction dynamics of trifluorinated indolylfulgides

“…Recently, reaction dynamics and efficiencies of the ring-opening and ring-closure reactions of a trifluorinated indolylfulgide (fulgide 1 in Scheme) were studied in several investigations and the following reaction model was proposed [5,14-17] (values for reaction times and quantum efficiencies were obtained for fulgide 1 dissolved in 1,4-dioxane at room temperature): Ring-closure reaction: The Z -form isomers exist in two pairs of enantiomers [2,18]. One pair can perform the ring-closure reaction and the other may not.…”