Theoretical investigation of two model systems for molecular photoswitch functionality. I. 2-(4-nitropyrimidin-2-yl)ethenol

Abstract

The ground and five lowest-lying electronically excited singlet states of 2-(4-nitropyrimidin-2-yl)ethenol (NPE) have been studied theoretically using the complete active space self-consistent-field (CASSCF), Møller-Plesset second-order perturbation theory (MP2) and second-order multi-configurational perturbation theory (CASPT2) methods. The molecule can be regarded as being composed of a frame and a crane component and is characterised by the existence of two planar minima of similar energy on the ground-state potential energy (PE) surface. This work explores the possibility of an excited-state intramolecular hydrogen transfer (ESIHT) process in NPE. A hypothetical reaction coordinate has been constructed for NPE. State-averaged CASSCF and CASPT2 calculations of the six lowest-lying singlet states have been performed for the isomerisation. Adiabatic and simplified quasi-diabatic PE and transition dipole moment functions have been constructed. The computations indicate that there exist substantial barriers for the rotation process on the adiabatic PE surfaces of all the five excited states investigated. The six quasi-diabatic electronic states considered here decompose into two classes based on whether the electronic wave functions depend weakly or strongly on the nuclear displacement, subject to the effect of the torsion on relevant molecular orbitals.

Keywords:

• photoswitch
• photoisomerisation
• excited state dynamics
• vibronic coupling
• electronic structure calculations

Acknowledgements

Á. Vibók, G.J. Halász and A. Csehi would like to thank the Hungarian Research Council (NKTH grant number NO-1/2009) and the Centre for Theoretical and Computational Chemistry (CTCC) at the University of Tromsø for supporting several visits to Tromsø. A. Csehi is also grateful to the “National Excellence Program” (TÁMOP 4.2.4. A/2-11-1-2012-0001) supported by the European Union and the State of Hungary, co-financed by the European Social Fund.

This work has received continued support from the Mohn Foundation (C. W., grant number A32542), the Centre for Theoretical and Computational Chemistry (CTCC) at the University of Tromsø and the Research Council of Norway (grant number 177558/V30). Support of several visits at the University of Debrecen by the NKTH (grant Nr. NO-1/2009) is also acknowledged by C. Woywod.

We are further grateful to Norwegian High Performance Computing (NOTUR) and the John-von-Neumann Institute, Research Center Jülich (Project ID ehu01) for providing computational resources on the Titan Rock Cluster at the University of Oslo and on the Intel Xeon X5570 Cluster JUROPA, respectively.

This article is dedicated to Professor Rodney Bartlett.

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