Benzothiadiazole as a strategic central core or a better terminal acceptor for enhanced nonlinear optical and...

Benzothiadiazole derivatives (BTZ) have established themselves as benchmarks in nonlinear optical (NLO) research, owing to their exceptional charge-transfer capabilities and extensive electronic delocalization. With a unique blend of structural versatility and electronic tunability, these compounds hold immense potential for revolutionary modern optical applications. We developed a tailored set of BTZ derivatives, comprising four core-centered (C-1PhTDZ to C-2PhDTDZ) and four terminal-substituted (T-1PhTDZ to T-2PhDTDZ) compounds, by systematically modifying the acceptor unit size and placement to promote extended π-conjugation and enhance NLO responses. Overall, T-2PhDTDZ showed exceptional planarity (molecular planarity measurement; MPP 0.002 Å; span of deviation from plane, SDP 0.006 Å) due to enhanced intramolecular charge transfer (ICT) from terminal BTZ placement, boosting charge separation and performance. C-2PhDTDZ and T-2PhDTDZ exhibit the notable isotropic (α_iso) and anisotropic (α_aniso) polarizabilities, with values of 365.1 × 10^-24 esu, 93.8 × 10^-24 esu, 879.10 × 10^-24 esu, and 113 × 10^-24 esu, respectively. Within the terminal series, T-2PhDTDZ shows the highest total third-order NLO polarizability (<γ>) of 5128.8 × 10^-36 esu, which is approximately 1.2 to 6.4 times larger than other derivatives. While among core series, C-1PhDTDZ exhibits a remarkably enhanced NLO response, attaining the maximum γ value of 1906.0 × 10^-36 esu. Additionally, frequency-dependent γ was computed, with T-2PhDTDZ exhibiting the largest amplitudes for γ(-ω;ω,0,0) and γ(-2ω;ω,ω,0) at 1400 nm, with values of 247.90 × 10^-36 esu and 324.53 × 10^-36 esu, respectively. Moreover, the density of states (DOS) and natural bond orbital (NBO) analyses provide further insights into the electronic structure and charge transfer characteristics. We compared the V_oc values of our designed derivatives with those of PC₆₁BM. Notably, T-2PhDTDZ shows a pronounced offset value of 3.1 eV, which favors rapid electron injection and reduces energy loss. These findings suggest that our designed core-versus terminal-based BTZ derivatives possess significant advancements in NLO and photovoltaic technologies, paving the way for next-generation optoelectronics and photonic applications.