The presence of a high density of excited electronic states in the immediate vicinity of the optically bright state of a molecule paves the way for numerous photo-relaxation channels. In transition-metal complexes, the presence of heavy atoms results in a stronger spin–orbit coupling, which enables spin forbidden spin-crossover processes to compete with the spin-allowed internal conversion processes. However, no matter how effectively the states cross around the Franck–Condon region, the degree of vibronic coupling, of both relativistic and non-relativistic nature, drives the population distribution among these states. One such case is demonstrated in this work for the intermediate-spin Fe(III) trigonal-bipyramidal complex. A quantum dynamical investigation of the photo-deactivation mechanism in the Fe(III) system is presented using the multi-configurational time-dependent Hartree approach based on the vibronic Hamiltonian whose coupling terms are derived from the state-averaged complete active space self-consistent field/complete active space with second-order perturbation theory (CASPT2) calculations and spin–orbit coupling of the scalar-relativistic CASPT2 states. The results of this study show that the presence of a strong (non-relativistic) vibronic coupling between the optically bright intermediate-spin state and other low-lying states of the same spin-multiplicity overpowers the spin–orbit coupling between the intermediate-spin and high-spin states, thereby lowering the chances of spin-crossover while exhibiting ultrafast relaxation among the intermediate-spin states. In a special case, where the population transfer pathway via the non-relativistic vibronic coupling is blocked, the probability of the spin-crossover is found to increase. This suggests that a careful modification of the complex by incorporation of heavier atoms with stronger relativistic effects can enhance the spin-crossover potential of Fe(III) intermediate-spin complexes.
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7 April 2022
Research Article|
April 01 2022
Spin–vibronic coupling in the quantum dynamics of a Fe(III) trigonal-bipyramidal complex
Kishan Kumar Dakua
;
Kishan Kumar Dakua
a)
1
Department of Chemistry, Indian Institute of Technology Kharagpur
, Kharagpur, India
a)Author to whom correspondence should be addressed: mishra@chem.iitkgp.ac.in
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Karunamoy Rajak
;
Karunamoy Rajak
2
Centre for Theoretical Studies, Indian Institute of Technology Kharagpur
, Kharagpur, India
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Sabyashachi Mishra
Sabyashachi Mishra
a)
1
Department of Chemistry, Indian Institute of Technology Kharagpur
, Kharagpur, India
3
Centre for Computational and Data Sciences, Indian Institute of Technology Kharagpur
, Kharagpur, India
a)Author to whom correspondence should be addressed: mishra@chem.iitkgp.ac.in
Search for other works by this author on:
a)Author to whom correspondence should be addressed: mishra@chem.iitkgp.ac.in
J. Chem. Phys. 156, 134103 (2022)
Article history
Received:
December 02 2021
Accepted:
March 11 2022
Citation
Kishan Kumar Dakua, Karunamoy Rajak, Sabyashachi Mishra; Spin–vibronic coupling in the quantum dynamics of a Fe(III) trigonal-bipyramidal complex. J. Chem. Phys. 7 April 2022; 156 (13): 134103. https://doi.org/10.1063/5.0080611
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