Informing the design of amino-acid fingerprinting reagents using density functional theory

Improving the effectiveness of forensic fingerprinting reagents (FFRs) has been a challenge in the field of forensic chemistry for the previous four decades. Fluorescent methods offer significant improvement over their non-fluorescent counter parts and the focus of several previous studies have been to develop new fluorescent FFRs via a synthetic trial-and-error process. In this study, a computational Density Functional Theory (DFT) approach is used to work towards predicting the fluorescence activity of a system in an attempt to contribute to the development process. Aiming to inform the fluorescent FFR design process, this study asks: Is time-dependant DFT (TDDFT) able to replicate non-fluorescent behaviour of existing FFRs by identifying non-radiative features along their excited state potential energy surface (PES)? Reporting the validation of TDDFT as a predictive tool, this study presents the accurate calculation of optical properties belonging to products of the existing FFRs, 2,2-dihydroxyindane-1,3-dione (ninhydrin) and 1,8-diazafluoren-9-one (DFO), and has proposed chemical structures of the products of 1,2-indanedione and 2-hydroxy-1,4-napthoquinone (lawsone). Investigation of the PES representing a mechanism related to excited state intramolecular proton transfer (ESIPT) in the ninhydrin product, DYDA, and the DFO product revealed non-radiative characteristics in the former, indicating that an internal 10° dihedral rotation of DYDA leads to a non-radiative relaxation mechanism. The relevance of these findings with respect to the solid-state structures is also offered through analysis of the crystal packing in previously reported X-ray crystal structures. It is recommended that further investigation be carried out on a larger set of chromophoric systems to further validate the presented workflow, validation of which will offer an informative predictive tool to be utilised in a product-to-reactant approach to the design of new fluorescent FFRs.