Symmetry Breaking and Charge Delocalization as Key Drivers Toward Tuning Electronic, Static and Frequency-Dependent NLO Properties in Alkali Metal-Substituted [Mn; M =Li, Na, K; n =1–4] Tetracyclopentatetraphenylene Derivatives: A DFT Investigation for Electro-Optic Applications

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Nonlinear optical (NLO) materials are critical in advancing photonic technologies and enabling electro-optic applications. To meet the growing demand for highly efficient NLO materials, this study investigates the electronic, static and frequency-dependent NLO properties of alkali metals-substituted tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP) derivatives ([M[Formula: see text]; M [Formula: see text] Li, Na, K; [Formula: see text] 1–4]), focusing on symmetry breaking and charge delocalization. The dynamic polarizability [Formula: see text],[Formula: see text], electro-optical Pockels effect (EOPE) [Formula: see text],[Formula: see text],0) and second-harmonic generation (SHG) [Formula: see text]2[Formula: see text],[Formula: see text],[Formula: see text], frequency-dependent investigations were carried out at 532 nm and 1064 nm laser wavelengths. Potassium-doped derivatives, particularly TCPTP-K2, exhibited exceptional EOPE [Formula: see text],[Formula: see text],0) (9.08 × 105 a.u. at 532 nm and 8.50 × 104 a.u. at 1064 nm), whereas its SHG values were recorded at [Formula: see text]2[Formula: see text],[Formula: see text],[Formula: see text] 2.91 × 105 a.u. and 1.97 × 105 a.u. for the corresponding wavelengths. DFT and TDDFT-based simulations analysis, including DOS, MEP, FMO, TDM and NBO, were employed to comprehend and confirm the key properties such as polarizability [Formula: see text],[Formula: see text], EOPE [Formula: see text],[Formula: see text],0) and SHG [Formula: see text]2[Formula: see text],[Formula: see text],[Formula: see text] at 532 nm and 1064 nm laser wavelengths. The NBO analysis revealed a charge transfer spectrum spanning from 0.74956 a.u. to 3.19624 a.u. for alkali metals, while the TCPTP core exhibited a range of −0.74956 a.u. to −3.19624 a.u., underscoring a notable increase in electron density and polarization. A significant reduction in the energy gap (from 1.54 eV to 0.69 eV) and red-shifted UV–Vis absorption ([Formula: see text] up to 1312 nm) confirmed enhanced charge delocalization. These improved results can be attributed to a significant redistribution of charges, effective donor–acceptor interactions and the transition to a noncentrosymmetric structure brought about by potassium doping. These results suggest that alkali metal-substituted TCPTP derivatives may compete with future photonic technologies by optimizing NLO properties through strategic molecular engineering. This study advocates the potential of TCPTP derivatives for modern electro-optic applications.