n - π∗ Transition in Some Orthohalogenated Pyridines

Ultrafast Internal Conversion Dynamics of 2-Chloropyridine by Femtosecond Time-Resolved Photoelectron Imaging

“…These observations show that the lowest singlet state is the ππ* state, and the nπ* state shifts into the spectral region of the ππ* state in chloropyridine. 19,20 On the other hand, it was suggested by Sidman 32 that the nπ* shifts due to the electron donating resonance effect of the halogen substituent which increases the energy of the excited state relative to the energy of the ground state.…”

“…Hence, the main reason for the quantum yield decrease in the excited state is attributed to the relaxation from S1 to the ground state as the case of the S1 state in pyridine. 20,35,36 Therefore, we adopt the same order of levels as for 2-chloropyridine. It was concluded that the constancy of fluorescence emissions yield value of 2-fluropridine 34 can be related to a much smaller energy gap between the S1 and S2 states.…”

“…In polyatomic molecules excited to higher electronic states, internal conversion to vibronic levels of lower electronic states is one of the dominant mechanisms for deactivation. It has been established 19,20 that in pyridine and halogenated pyridines, there are at least one singlet-singlet (nπ*) electronic transition and one singlet-singlet (ππ*) electronic transition.…”