Albin Kuriakose

Two new series of nonplanar π-extended difused porphyrins were synthesized; one bearing two antipodal phenylethynyl (PE) units, M(VCN)₂(PE)₂, (VCN = vinyl cyanide), and the other with one PE and one tetracyanobutadiene (TCBD) unit, MTCBD, where M = 2H, Zn(II), Cu(II), Ni(II), and Co(II). All compounds were fully characterized by various spectroscopic techniques and single-crystal XRD analysis. Ni(VCN)₂(PE)₂ and ZnTCBD adopted ruffled geometries with high twist angles of 28.46° and 20.25°, respectively. The synthesized porphyrins display a panchromatic absorption spectral feature with low-energy bands extending into the near-infrared (NIR) region, attributed to the extended π-conjugation. H₂(VCN)₂(PE)₂ and Zn(VCN)₂(PE)₂ exhibit pronounced emission red shifts compared to parent TPPs (tetraphenylporphyrins), whereas H₂TCBD and ZnTCBD are nonemissive, indicating strong fluorescence quenching and excited-state processes. Cyclic voltammetry reveals facile four-electron reductions and impressively narrowed HOMO-LUMO gaps compared to TPPs. Frontier molecular orbital analysis suggests a charge-transfer or charge-separated state in MTCBD, and all optimized structures display high ground state dipole moments (10.79-15.18 D). Third-order nonlinear optical (NLO) properties evaluated by Z-scan reveal exceptionally high two-photon absorption coefficients and cross sections, even at low laser power, outperforming recently reported macrocycles. These findings underscore their immense potential in optical limiters, advanced optoelectronic devices, and cutting-edge biomedical technologies.

The strong confinement of electromagnetic fields in photonic crystals influences light-matter interactions and is utilized for the generation, control, and manipulation of light at desired frequency bands. Here we demonstrate the unusual nonlinear optical responses of a:SiN_x-based spatially inhomogeneous quasi-periodic optical microcavity during intense femtosecond laser propagation. Aperiodic microcavities are realized using two quarter-wavelength nonstoichiometric Si- and N-rich a:SiN_x layers having contrasting refractive indices and bandgaps. The photonic miniband region of the aperiodic microcavity was probed with high-intensity femtosecond laser using wavelength-dependent (740 to 900 nm) and angle-resolved studies at the miniband edge (∼750 nm). The studies reveal unusual switching from one-photon-induced saturable absorption (SA) of defect-mediated energy levels to two-photon-induced reverse saturable absorption (RSA) around the photonic miniband edge. The strong overlap of the symmetric photonic confinement with the highly dense defect energies of fractal N-rich low refractive index a:SiN_x layers (3L) directly expedites such anomalous nonlinear optical switching. Extensive optical field simulations and comparative studies with its periodic microcavity counterpart strongly support the unusual nonlinearities. The fractal effect-induced resonant photonic modes in an aperiodic microcavity lead to unpredictable nonlinear optical responses, which establishes a novel and versatile route for photon manipulation and control. Thus, the present study provides deep insights into many different geometrical dominances in the evolution of photonic modes and consequent nonlinear responses, which further modulate photonic device capabilities.

A series of β -phenanthroline-appended porphyrins, MTPP(Phen) (M = 2H, Co, Ni, Cu, Zn), was synthesized for NLO studies. CoTPP(Phen) showed the highest TPA coefficient and cross-section due to its redox-active metal center.

To investigate the optoelectronic properties of extensively electron-deficient porphyrins, a series of mixed functionalized porphyrins with adjacent nitro (-NO₂) and cyano (-CN) functional groups at the β-pyrrolic positions were designed and synthesized via nucleophilic substitution reactions. The synthesized porphyrins were characterized by several spectroscopic techniques and confirmed by single-crystal X-ray diffraction analysis. The electron-deficient nature of porphyrins, as highlighted by their redox properties, makes the macrocycle a selective optical sensor for cyanide ions with a limit of detection of nearly 1 ppm. Furthermore, porphyrins were evaluated for their nonlinear optical responses. Femtosecond laser intensity-dependent third-order nonlinear optical studies were performed using a laser with the following specifications: 800 nm, 120 fs, and 84 MHz at an intensity of 3.0 × 10¹³ W/m². Among all analyzed porphyrins, free-base porphyrins exhibited higher nonlinear coefficients compared to metalated porphyrins. Moreover, highly substituted porphyrins showed high nonlinear coefficient values. 3-H₂ shows the highest (∼119 × 10^-11) two-photon absorption coefficient (β) and the highest negative nonlinear refractive index (-28.2 × 10^-17), making it suitable for optical limiting applications.

Meticulously engineered nonstoichiometric a:SiN_x-based photonic structures, such as optical microcavities, facilitate the emergence of pronounced photonic states that are predominantly influenced by defect and interface energies, resulting in diverse and complex electron-photon interactions. In this study, we present third-order nonlinear optical investigations on a:SiN_x-based optical microcavities employing femtosecond lasers across a broad wavelength range of 740-900 nm to examine the effects associated with photonic band edges, which are contingent upon defect and interface energies. The dispersion of nonlinear absorption (β) and the nonlinear refractive index (n₂) through wavelength- and angle-dependent studies strongly correlate with linear angular photonic mode dispersion. Intensity-dependent nonlinear assessments provide a variation of impact with the photonic band edge effects. The strong optical field confinements and layer-by-layer nonlinearities show substantial effects on the effective optical nonlinearities in the spectral domain. Such a significant impact of photonic modes and the marked dependence of the nonlinear optical properties on optical microcavity structures highlight promising avenues for designing various photonic interconnects suited for both linear and nonlinear optical applications.

A novel series of inorganic–organic (IO) hybrid semiconductors with varying crystal dimensions and structural reorganizations, arising from the unusual conformation of organic ammonium cation (R–NH3)+ within the MX6 inorganic octahedral networks, are investigated. Here, we report the systematic investigations of exciton behavior in a new IO hybrid series resulted out of phenyl group-based R = (C6H5–(CH2)n–NH3)+ (n = 1–4) organic moieties interactions with inorganic lead iodide (PbI2). The resultant IO hybrid semiconductors of n = 1 and n = 2 show an infinitely extended corner-sharing PbI6 octahedral 2D network, resembling natural multiple quantum wells, whereas the rest show staggered with edge sharing layered networks. Consequently, these hybrids demonstrate strong room-temperature Mott-type excitons having structural dependency. One-photon (1PA, ħω ≥ Eg) and two-photon (2PA, 2ħω ≥ Eg) absorption-induced photoluminescence (PL) studies highlight the presence of different types of excitons associated with variations in their bandgaps. The comprehensive analysis of transient absorption studies and PL spectral mapping reveal many important features of exciton behavior, specifically about their structural dependency, excited-state charge carrier dynamics, and relaxation pathways.

We designed and successfully synthesized highly substituted electron-deficient nonplanar Ni(II) porphyrins and their derivatives (1-7) in moderate to good yields. These derivatives were comprehensively characterized by various spectroscopic techniques and single-crystal X-ray diffraction (SCXRD) analysis. SCXRD analysis confirmed the structures of compounds 2, 4, and 7, adopting saddle-shape geometry. These nonplanar porphyrins demonstrated significant bathochromic shifts in their absorption spectra compared to parent NiTPP, attributed to the influence of bulky β-substituents and/or peripheral fusion. π-Extended porphyrins 6 and 7 displayed panchromatic absorption spectra extending into the NIR region. Porphyrins 6 and 7 demonstrated a profound anodic shift (∼400 mV) in their first reduction peak potentials compared to precursor NiTPP(NO₂)Br₆. The experimental absorption spectral pattern matches the simulated absorption spectra obtained from TD-DFT studies. The femtosecond laser intensity-dependent third-order nonlinear optical studies revealed that NiDFP(VCN)₂Br₆ (6) and NiDFP(VCN)₂(PE)₆ (7) displayed giant optical nonlinearities compared to the other porphyrins. Among all, NiDFP(VCN)₂Br₆ (6) possessed the highest two-photon absorption coefficient (β) and cross-section (σ_TPA) values in the range of 22-33 × 10^-10 m/W and 3.77-6.95 × 10⁶ GM, respectively. These findings suggest that the investigated nonplanar π-extended porphyrins are promising candidates for future optoelectronic applications.