Time-dependent quantum wave-packet description of the¹πσ* photochemistry of pyrrole

Abstract

The photoinduced hydrogen elimination reaction in pyrrole via the conical intersection of the 1B1 (1pi sigma*) excited state with the electronic ground state has been investigated by time-dependent quantum wave-packet dynamics. A two-dimensional model potential-energy surface has been constructed as a function of the NH stretching and the hydrogen out-of-plane bending mode, employing multi-reference ab initio electronic-structure methods. The branching ratio of the reactive flux at the conical intersection has been investigated in dependence on the initial vibrational state of the molecule. The results suggest that laser control of the photodissociation of pyrrole via mode-specific vibrational excitation should be possible.

Role of vibrational energy relaxation in the photoinduced nonadiabatic dynamics of pyrrole at the conical intersection

“…For pyrrole, ab initio calculations have confirmed that the four lowest excited states ½ 1 A 2 ðpr à Þ, 1 B 1 ðpr à Þ, 1 B 2 ðpp à Þ, 1 A 1 ðpp Ã Þ play a central role in the photoinduced dynamics [2][3][4][5][6]11,14]. The two 1 pp à states are responsible for the broad and intense absorption band near 6 eV.…”

“…Upon stretching of the NH bond, the 1 pr à states are essentially repulsive. They intersect with the electronic ground state, resulting in the 1 A 2 ðpr à Þ-S 0 and 1 B 1 ðpr à Þ-S 0 conical intersections [11,14]. These two conical intersections provide the mechanism for ultrafast internal-conversion (that is, radiationless decay to the electronic ground state), as well as hydrogen detachment [11,14].…”

“…Heteroaromatic systems, such as pyrrole and its derivatives [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], are often considered as simplified models for the investigation of the photophysical and photochemical properties of important biomolecules, such as DNA bases or aromatic amino acids. Previous experimental observations and theoretical calculations have revealed that the near-UV photolysis of these heteroaromatic molecules essentially involves ultrafast radiationless decay processes governed by nonadiabatic transitions at conical intersections.…”

Cluster‐Controlled Photofragmentation: The Case of the Xe–Pyrrole Cluster

“…Concerning the excitation at long wavelengths, some have suggested that this channel results from the dissociation on the S 0 (X 1 A 1 ) ground state following internal conversion from the initially excited 1 ps* state [15] or from adiabatic dissociation on the 1 ps* state. [12] Others have proposed dissociation on the 1 ps* state following relaxation within this excited state. [14] For excitation at short wavelengths, dissociation on the S 0 (X 1 A 1 ) ground state after internal conversion from the initially excited S 2 ( 1 pp*, 1 B 2 ) state has been documented.…”

Theory and computations of two-photon absorbing photochromic chromophores

“…In this approach adiabatic surfaces are treated classically, while transitions between them estimated using quantum mechanics. Another approximation involves reducing dimensionality of the system to two degrees of freedom and treating the resulting two-dimensional problem using wave packets [14]. Further simplification is called Pathway Approach [15].…”