Computational Molecular Photophysics

National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, ViaArnesano 16,1-73100, Lecce, Italy eduardo.fabiano nano.cnr.it 

In this chapter we present an introductory overview of the basic theoretical concepts of computational molecular photoph rsics. First, the nature and properties of electronic excitations are considered, with special attention to transition moments and vibrational contributions. Then, the main photophysical processes involving the electronic excited states are examined, focusing in particular on nonradiative deactivation phenomena. Finally, we present a brief review of computational methods commonly applied for the description of molecular excitations. Special emphasis is given to the configuration-interaction (Cl) method and the time-dependent density functional theory (TD-DFT), discussing some technical details and outlining advantages and limitations. 

Recent years have witnessed a fast development of quantum-chemical methods for the calculation of molecular excited states. 

This makes now possible to perform computational studies of the excitations of realistic systems ranging from organic and organometallic dyes to biological systems. A detailed knowledge about the excited states of molecules is essential for the explanation and interpretation of molecular optical properties as well as for the description of photoinduced physical and chemical processes. 

Excitation spectra arise from transitions between different quantum states of the system, corresponding to different nondegenerate solutions of the Schrodinger equation. In quantum chemistry it is common practice to treat the solution of the Schrodinger equation within the Bom-Oppenheimer approximation [1] and separate the electronic and nuclear degrees of freedom. Consequently the excitation spectra are also separated into an electronic and a roto-vibrational spectrum. The former is studied mainly in optical (UV/vis) spectroscopy experiments and will constitute the main subject of this chapter the latter, which can be investigated by infrared, microwave or Raman spectroscopy measurements, provides fine-structure corrections to the electronic spectrum. 

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Molecular Photophysics Computing Absorption and Emission Spectra. 512... 

Sobolewski AL, Domcke W (2007) Computational studies of the photophysics of hydrogen-bonded molecular systems. J Phys Chem A 111 11725-11735... 

We have presented nonadiabatic ab initio molecular dynamics simulations of the photophysical properties of a variety of nucleobases and base pairs. In addition to the canonical tautomers a number of rare tautomers have been investigated. Moreover, effects of substitution and solvation have been studied in detail. The simulations of nonradiative decay in aqueous solution, in particular, demonstrate the strength of the na-AIMD technique employed here as it permits the treatment of solute and solvent on an equal footing. Condensed phase calculations can be directly compared with those in the gas phase because the same computational setup can be used. 

Photophysics depends critically on the availability of appropriate instrumentation and adequate computational protocols, as well as a ready supply of suitable molecular systems. As the systems become more complex it becomes necessary to design new instruments and to improve procedures for data analysis. A necessary part of this improvement concerns increasing the reliability and precision of existing facilities. 

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