To understand the carcinogenic effects upon absorption of UV radiation on DNA, it is very important to study the UV-induced photo-physics and photo-chemistry of DNA nucleic acid bases (NABs). NABs are the important constituents of life. The hydrogen bonding interaction between NABs such as adenine-thymine (A-T) and guanine-cytosine (G-C) in a Watson-Crick pair is the central of its stabilization along with the DNA double helix. People have extensively studied the photo-processes and the non-radiative deactivation mechanism of DNA bases experimentally as well as computationally. Over the last few decades, there has been significant effort towards expanding the genetic alphabet. The genetic information, or genetic code, is stored within the specific sequence of the DNA helix, which naturally contains only four NABs: adenine (A), thymine (T), cytosine (C), and guanine (G). When RNA is considered, uracil (U) replaces thymine as one of the bases, resulting in a five-base system across both DNA and RNA. Expanding the genetic alphabet beyond these natural bases could potentially increase the capacity for storing genetic information manyfold. This expansion has the potential to revolutionize various fields such as synthetic biology, biotechnology, and genetic engineering. Consequently, significant research has been directed toward understanding the different interactions that stabilize DNA bases and towards synthesizing new compounds that can be incorporated into DNA and remain stable in those environments. One of the major achievements in this field has been the development of unnatural base pairs (UBPs). These synthetic nucleotides are designed to form additional, non-natural base pairs within the DNA helix. Examples of successful UBPs include those developed by the Benner and Romesberg groups, which introduced new base pairs such as isoG-isoC and d5SICS-dNaM, respectively. The creation and integration of UBPs into DNA sequences present several challenges such as chemical stability, replication fidelity, compatibility with cellular machinery etc. Despite these challenges, expanding the genetic alphabet holds great promise. It could enable the creation of proteins with novel amino acids, leading to new functionalities and properties that are not possible with the 20 standard amino acids encoded by the natural genetic code. However, it has also been shown that significantly different chemical entities can be stabilized either by strong hydrogen bonds or by π-π and van der Waals interactions. For instance, some UBPs like xanthosine-5-(2,4-diaminopyrimidine) and isoguanine-thymine are stabilized through strong hydrogen bonding interactions. In contrast, other UBPs rely on different stabilizing forces. Pyrrole-2-carbaldehyde (Pa) and 7-(2-thienyl)imidazo [4,5-b]pyridine (Ds) form a pair of unnatural nucleic acid bases that are stabilized by van der Waals interactions, hydrophobic effects, and shape complementarity. The hydrophobic nature of the Ds-Pa pair helps them to make pair selectively with each other and reduces mispairing, such as forming Ds-Ds pairs. These properties make Ds-Pa a promising candidate for the expansion of the genetic alphabet. Their unique stabilizing interactions offer a different mechanism compared to the natural hydrogen-bonded base pairs, potentially increases the versatility and functionality of synthetic genetic systems. Two of the important aspects which need to be studied to understand the stability and instability of UNBs are their photo-activity and photochemical stability. Excited photoprocesses and non-radiavtive pathways present in NABs have been widely studied by many research groups. Not only NABs, UNBs have also been extensively studied experimentally as well as theoretically. The effect of solvation upon excitation of two such UNBs called d5SICS and dNaM have been studied by Pollum et al. A series of experimental work have been done to understand the photochemical properties of d5SICS, dNaM and dTPT3 and it has been found that triplet state is playing the crucial role for causing photodamage in UBP containing DNA. These experimental findings about d5SICS and dNaM have been theoretically supported by Bhattacharyya and Datta where they have used time-dependent density functional theory (TDDFT) and multi-state complete active space self-consistentfield followed by second-order perturabtive correction (SA-CASSCF/MS-CASPT2) level of theories. QM/MM studies on dTPT3 have been done by Cui et. al. where they have investigated two main excited state relaxation pathways that populate triplet state. They have also investigated the photophysical processes of unnatural base Z by using highly accurate multi-state complete active space second order perturbation (MS-CASPT2) theory. In our previous study, we studied about the excited state photoprocesses of UNB Pa.