Structure-Dependent (Non)Linear Optical Excitons in Primary Cyclic Ammonium (C_nH_2n–1NH₂; n = 3–8)-Based Inorganic–Organic Hybrid Semiconductor Series

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This work reports the synthesis, structural, linear, and nonlinear optical investigations into low-dimensional naturally self-assembled inorganic–organic (IO) hybrid systems based on (C_nH_2n–1NH₃)⁺ (n = 3–8) cyclic moieties. The steric effects of cyclic ring sizes convert the IO hybrids from two-dimensional (2D) layered structures to 1D crystal packing. The crystal packing of this class of IO hybrid compounds of cyclic sizes from n = 3 to 6 shows a perfect 2D layered structural arrangement having a crystal structure (R–NH₃)₂PbI₄. On the contrary, n = 7 hybrid shows a 1D layered structural arrangement, but the adjacent chains are disconnected along the “c”-axis, resulting into (R–NH₃)₃PbI₅. Moreover, for n = 8 hybrids, the inorganic network structure is infinitely extended along the “a”-axis having (R–NH₃)PbI₃ 1D crystal structure. These structural changes may lead to defect states, which is verified by density functional theory (DFT) calculations. The linear and nonlinear optical probing of room-temperature optical excitons demonstrate the photoluminescence and absorption feature variation from 2D layered crystal packing (n = 3–6) to quasi-1D structures (n = 7, 8). A systematic correlation of one-photon (1PA)- and two-photon (2PA)-excited exciton photoluminescence (PL) features with a cyclic size is discussed and presented. While one-photon absorption-induced photoluminescence (1PA-PL) provides information about strong exciton emission from the top few perfectly aligned layers, two-photon excitation probes the deeper depths. This shows red-shifted PL (2PA-PL) from the structurally distorted crystal packing within the sample and traces of defect-induced emission. The DFT study shows that the I-vacancy defect creates the states at conduction band minimum (CBm), which leads to a sudden reduction in the band gap for n = 7 and 8. The systematic optical probing studies to determine the structural deviations in IO hybrid semiconductors will provide a new platform for advanced photonics and optoelectronic devices.