Femtosecond Dynamics of Pyridine in the Condensed Phase:  Valence Isomerization by Conical Intersections

Abstract

In this work, we combined femtosecond transient absorption (population and anisotropy) spectroscopy with ab initio electronic structure methods to study excited-state deactivation pathways of the pyridine molecule in liquid solutions. Studies of the effects of excitation energy, deuteration and substitution, solvent polarity and viscosity, and protonation of pyridine were performed. The experiments reveal the dynamics of S1(nπ*) and S2(ππ*) excited states of pyridine. The photoexcitation of the S2(ππ*) state leads to formation of the prefulvenic form of pyridine, a valence isomer, in ∼2.2 ps, while nonradiative deactivation of the S1(nπ*) state occurs in 9−23 ps and is to a large extent due to intersystem crossing. Using ab initio methods at the CASSCF and time-dependent DFT levels, we calculated the potential energy surfaces of the ground and S2(ππ*) states. A conical intersection was found responsible for the ultrafast deactivation of the pyridine molecule.

Nonradiative decay of the lowest excited singlet state of 2-aminopyridine is considerably faster than the radiative decay

“…Experimental and theoretical studies for the 1 ‫ء‬ state deactivation of the pyridine molecule have led to the conclusion that along the prefulvenic pathway, there was an ultrashort internal conversion to the ground state. 20 In this work, a similar reaction pathway, named as ring deformation reaction, is found for the 1 ‫ء‬ state of 2-AP molecule. A transition state with a barrier height of 0.13 eV is identified by frequency analysis, which has a prefulvenelike structure.…”

“…Such a rate corresponds to a very short lifetime of 1.1 ps, which is much lower than the fluorescence lifetime of 19.7 ns. One thing seems to be clear that photochemical property of 2-AP molecule is similar to that of pyridine and DNA bases, 20,22 for which the nonradiative process ͑internal conversion via the S 1 / S 0 conical intersection͒ also dominates the whole decay processes. The combination of both radiative ad nonradiative lifetimes leads to a value of 3.3 ps, which is much smaller than the experimental value of 1.5 ns.…”

“…9,10,[16][17][18][19][20][21] The surface intersection between the S 1 and S 0 states was found to play a crucial role for radiationless deactivation process. An extremely facile internal conversion has been identified for all nucleobases because of their genuine molecular property.…”

“…Femtosecond transient absorption spectroscopy and ab initio calculations for excited-state deactivation of pyridine molecule led to the conclusion that along a "prefulvenic" pathway, there was an ultrashort internal conversion between the S 1 state and the ground state. 20 Recently, an out-of-plane ring deformation was confirmed to be a deactivation pathway for the 6-AP molecule in the S 1 state. 21 To resolve the uncertainty related to the lifetime of S 1 state of 2-AP molecule, we have carried out ab initio calculations to identify the contributions from possible radiative and nonradiative channels.…”

Dynamics of excited tetrakis(dimethylamino)ethylene solvated by argon atoms

“…It mostly concerns gas phase, femtosecond pump-probe investigations in organic molecules documenting ultrafast energy relaxation processes [15][16][17][18][19][20][21], isomerisation reactions [22][23][24][25] and ring opening reactions [26][27][28][29][30][31]. Ethylene [18,32], substituted ethylene compounds [18][19][20][21] and polyenes [22] received a particular attention.…”