# Substitution Effects on the Optoelectronic Properties of Coumarin Derivatives
## 1 Introduction  
Coumarin is one of the most versatile and studied scaffolds. It exhibits interesting biological, pharmacological, biochemical, therapeutic, and photochemical properties with a wide range of applications [1, 2, 3, 4]. It is classified in the benzopyrone family, and formed by fusion of \(\alpha\)-pyrone ring with a benzene nucleus. Coumarin was first isolated in 1820 from tonka bean by [PERSON] and has since been reported to be present in seeds, roots, and leaves of many plant species, especially in green plants [5]. Coumarin molecule (2H-chromen-2-one) exerts diversified nature of interactions (hydrophobic, hydrogen bonds, electrostatics) with many key sites in organisms and therefore it is able to display wide-range of pharmacological activities [2, 6]. Furthermore, coumarin derivatives are synthesized for different application as dye-sensitized solar cells and dye lasers [7]. Moreover, a multifaceted utility of coumarin is possible thanks to flexible structural variations in the scaffold positions (3 and 4)by substitution on the aromatic ring [8]. Because of their synthetic versatility, phenylcoumarins (an extra phenyl ring is attached to any position of the coumarin scaffold) are considered as important molecular moiety with proved biological and pharmacological properties [9]. Notably, 3-phenylcoumarins (an extra phenyl ring attached to 3-position of the coumarin scaffold) has attracted more attention, since their molecular structure closely resembles combination of two pharmacophores (courmain-resveratrol). In these molecules, the 3,4-double bond of the coumarin nucleus fixes the _trans_ disposition of the \(t\)-resveratrol-type double bond. _Trans_-resveratrol displays pharmacological activities [10; 11] and protective effect against cardiovascular and neurodegenerative diseases [12; 13]. On the other hand, replacement of phenyl ring in position 3 with thiophene (a heterocyclic compound) resulted in a 3-heteroarylcumarin moiety [14], with promising applications in coumarin based dyes [15], florescence microscopy and nanoscopy [16]. A significant number of research studies have investigated the potential of 3-phenylcoumarins [17; 18; 19; 20] and 3-heteroarylcumarins [21; 22; 23] derivatives as inhibitors against many target enzymes that are associated to different biological diseases [18; 24]. In a recent study [13], authors investigated the inhibitory potential of twenty 3-phenylcoumarin and eight 3-heteroarylcumarin derivatives against xanthine oxidase enzyme, commonly associated to gout [25]. Computational methods can be employed to provide valuable molecular information for these coumarin derivatives. Therefore, the principal objective of this work was to investigate the structure, electronic and optical properties of these same derivatives (Figure 1), in order to establish the relationship between the chemical substitutions and computed molecular properties to aid in the design of materials with potential optoelectronic applications [26]. To do so, we employed density functional theory (DFT) [27], by far the most popular quantum mechanical method to calculate the structure and electronic properties of the molecules [28; 29; 30]. A detailed computational investigation was performed on the four coumarin-derivatives, which were selected on the basis of their enzyme inhibition capability [13]. Further, to provide a systematic investigation on the optical properties (absorption spectrum, electronic states transitions), time-dependent DFT (TD-DFT) [31] calculations were performed on the optimized ground state structures of the molecules in vacuum. Several computational DFT studies have investigated molecular properties of coumarin derivatives for applications such as optoelectronics [26; 32], solar cells [33; 34; 35; 36; 7], fluorescent dyes and lasers [7; 37]. In order to extract useful information about properties that determine the performance of molecules in solution, it is important to understand the role of solvents.