### What Is Photochemistry?

Photochemistry is a branch of chemistry that studies the permanent chemical effects caused by the interaction of light with matter. Photochemical processes are ubiquitous, such as photosynthesis of plants, photodenaturation of coatings and polymer materials, photocatalysis of organic chemical reactions, etc.

Different from general thermochemical reactions, photochemical reaction processes are often more complex, involving the competition of multiple processes, such as non-radiative transitions and radiative transitions, spin-allowed and spin-forbidden, adiabatic and diathermic, etc. Understanding photochemistry inevitably involves the study of electronically excited states, which remains a daunting task for experimentalists. Fortunately, with advances in computer technology and computational strategies, scientists can assist our understanding of photochemical processes through computer simulations. For example, in the field of photocatalysis, a great deal of research has achieved the understanding of the reaction mechanism and catalyst discovery of organic photocatalysts through computational chemistry.

EA/IP* and IP/EA* values for a number of polymers were predicted using density functional theory (DFT). [1]

### What We Do in Photochemistry

Alfa Chemistry's computational chemists use their extensive computational and modeling experience to provide you with the most efficient and cost-effective services using advanced computational chemistry methods. Our computational photochemistry approach includes:

**Time-Dependent Density Functional Theory (DT-DFT) Method**

Thanks to its simple use, high calculation efficiency, and the ability to calculate larger systems, TD-DFT is currently the most popular method for calculating excited states. For example, the emission wavelength of the organic light-emitting diode material Alq3 can be calculated using the functional B3LPY, which agrees well with experimental observations. In addition, combining long-range correction functionals such as CAM-B3LYP can effectively improve the insufficiency of DT-DFT method in calculating long-range charge transfer excited states.

**Single Reference State****Ab Initio Algorithm**

A variety of theories and methods can be used to study photochemical reactions, such as Moller-Plesset perturbation theory based on Hartree-Fock (HF) wave function, configuration interaction method (CI) for single and double excitation, coupled electron pair approximation (CEPA) and coupled cluster (CC) method. Among the single reference state methods, CIS (CI-Singles) method is the most widely used one. Furthermore, the coupled cluster method (EOM-CCSD) is the most accurate way to calculate the energy and transitions of an excited state if it can be well described by a single reference state wave function.

**Multiple Reference State Ab Initio Algorithm**

Complete activation space self-consistent field (CASSCF) and multi-reference state second-order perturbation theory (CASPT2) methods are widely used in theories of photodissociation, photoisomerization, proton (electron) transfer, and photostability (light resistance), etc. calculate.

**Other Calculation Methods**

Other combination and improvement methods include ONIOM method and QM/MM method, DFT/MRCI method, MRCC (multireference coupled cluster) method and RASPT2 (restricted active space multiconfigurational second-order perturbation) method, etc.

### Choose Alfa Chemistry

We know that the photochemical process is a complex process, which means that the study of photochemistry often requires a lot of time and effort. That's why Alfa Chemistry is committed to developing our computational chemistry solutions to serve your photochemical research projects. Our mission is to bring new possibilities to your research on photochemical processes by combining powerful computing resources with advanced theories and algorithms.

References

- Andrew W. Prentice,
*et al. Advanced Energy Materials*, 2021, 11, 9, 2100709. - Halls M D,
*et al. Chem. Mater.*, 2001, 13, 2632-2640.