Accelerated and Efficient Photochemistry From Higher Excited Electronic States in Fulgide Molecules
The Journal Of Physical Chemistry, 2008, 112 (51), 13364-71 published on 02.12.2008
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 S1 (570 nm) and SN (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 S1 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 SN state shows an unexpected long lifetime of 0.5 ps. The photoreaction starting from the SN 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 S1 state is only populated with an efficiency of 38%. The observed behavior strongly differs from the expected picture with fast relaxation into the S1 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.