Structure-Dependent (Non)Linear Optical Excitons in Primary Cyclic Ammonium (CnH2n–1NH2;n= 3–8)-Based Inorganic–Organic Hybrid Semiconductor Series

This work reports the synthesis, structural, linear, and nonlinear optical investigations into low-dimensional naturally self-assembled inorganic–organic (IO) hybrid systems based on (CnH2n–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.