A nuclear chemical descriptor based on NMR databases for molecular information: characterization of π-conjugated oligomers

Context

Structure–property relationships of increasing systems, such as conjugated oligomers, atomic chains, and H-bonding networks, constitute a problem involving multiple variables in quantum chemical studies. Defining a single parameter that allows investigating different properties of these systems (or even those belonging to a specific chemical group) is not always a simple task. Thus, it is difficult to rationalize the evolution of molecular properties of increasing molecular systems, assigning relevant physicochemical information correlated to their size and structure. For conjugated oligomers, we have proposed a nuclear magnetic resonance (NMR)–based descriptor from the fluctuations of indirect spin–spin coupling constants (SSCCs) between two adjacent ¹³C nuclei (¹J_CC) in the π-conjugated system. This parameter is called J-coupling alternation (JCA) and systematically gathers information from structural and electronic properties, allowing a rational evaluation of the molecular properties of growing oligomers. We employ JCA to obtain correlations with vibrational, electronic, and optical properties of conjugated oligomers, belonging to the groups of polypyrrole (PPy), polyfuran (PFu), polythiophene (PTh), and polyselenophene (PSe). This analysis yields excellent linear structure–property relationships, confirming the ability of JCA to characterize realistically different families of π-conjugated oligomers and providing a reliable parameter to design efficient molecular materials.

Methods

All the oligomers, PPy(n), PFu(n), PTh(n), and PSe(n), with n = 2, 3, 5, 7, 9, and 11, respectively, were fully optimized, without symmetry constraints, using density–functional theory (DFT) at the PBE0/6–311 + G(d,p) level. Next, the vibrational properties were computed within harmonic approximation and NMR spin–spin coupling constants using the GIAO method. Finally, optical properties were obtained via TD-DFT with the CAM-B3LYP/6–311 + G(d,p) method.