Resonance-Enhanced Two-Photon Absorption and Optical Power Limiting Properties of Three-Dimensional Perylene Bisimide Derivatives

Synergistic impact of organic and inorganic features in carborane hybrids for nonlinear optics and solvent-tunable properties

Carborane-based systems, renowned for their distinctive electronic structure, have recently attracted significant attention in the design of organic-inorganic hybrid materials. This study employs a quantum chemical approach to investigate the nonlinear optical (NLO) properties of novel V-shaped hybrid systems, especially derivatives of 1,2-diaryl-o-carborane as CB-1 to CB-6. Systematic donor group substitutions led to increased polarization and charge transfer. CB-6 displayed the highest isotropic and anisotropic polarizabilities (107.1 × 10-24 esu and 34.05 × 10-24 esu) and the highest second hyperpolarizability (334.7 × 10-36 esu), supported by TD-DFT simulations with the lowest transition energy of 3.928 eV. Analysis through unit sphere representation for CB-1 and CB-6 highlights the comparative dipole induced by replacing strong donor groups. Solvation studies for selected systems (CB-1, CB-4, and CB-6) reveal 〈γ〉 values ∼2-3 higher in implicit solvents compared to the gas phase, with explicit solvent results aligning closely. The results were more reliable in the explicit medium, where the 〈γ〉 for CB-6 was 374.7 × 10-36 esu, demonstrating consistency and correlation with the gas phase. To confirm its practical relevance, dynamic hyperpolarizability was evaluated over a broad frequency range (1947-510 nm) for CB-6. Furthermore, the charge transfer and reactivity of these systems were studied using frontier molecular orbitals (FMOs), and electrostatic potential maps were obtained, confirming efficient intramolecular charge transfer (ICT) from the donor groups toward the acceptor central borane cage. The excited-state charge transfer properties were studied based on hole-electron analysis, with higher reported values for the D-index ranging from 0.575 Å to 8.424 Å, indicating clear charge separation in these hybrid systems. All hybrids exhibited superior absorption compared to CB-1, with CB-6 peaking at 245 nm. Our findings pave the way for carborane-based hybrids to emerge as game-changers in the ever-evolving realm of laser technology.

Fluorescent molecular systems based on carborane-perylenediimide conjugates

This study presents the successful synthesis of two perylenediimide (PDI)-based ortho-carborane (o-carborane) derivatives, PDI-CB1 and PDI-CB2, through the insertion of decaborane into alkyne-terminated PDIs (PDI1 and PDI2). The introduction of o-carborane groups did not alter the optical properties of the PDI units in solution compared to their carborane-free counterparts, maintaining excellent fluorescence quantum yields of around 100% in various solvents. This was achieved by using a methylene linker to minimize electronic interaction between PDI and o-carborane, and by incorporating bulky o-carborane groups at imide- position to enhance solubility and prevent π-π stacking-induced aggregation. Aggregation studies demonstrated that PDI-CB1 and PDI-CB2 have greater solubility than PDI1 and PDI2 in both nonpolar and aqueous solvents. Despite the steric hindrance imparted by the o-carborane units, the solid state emission of PDI-CB1 and PDI-CB2 was affected by aggregation-caused fluorescence quenching. However, solid PDI-CB1 preserved bright red excimer-type emission, which persisted in water-dispersible nanoparticles, indicating potential for application as a theranostic agent combining fluorescence bioimaging with anticancer boron neutron capture therapy (BNCT) due to its high boron content.

Structure-activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells

An organelle-selective vision provides insights into the physiological response of plants and crops to environmental stresses in sustainable agriculture ecosystems. Biological applications often require two-photon excited fluorophores with low phototoxicity, high brightness, deep penetration, and tuneable cell entry. We obtained three aniline-based squaraines (SQs) tuned from hydrophobic to hydrophilic characteristics by modifying terminal pendant groups and substituents, and investigated their steady-state absorption and far-red-emitting fluorescence properties. The SQs exhibited two-photon absorption (2PA) ranging from 750 to 870 nm within the first biological spectral window; their structure-property relationships, corresponding to the 2PA cross sections (δ2PA), and structure differences were demonstrated. The maximum δ2PA value was ∼1220 GM at 800 nm for hydrophilic SQ3. Distinct biological staining efficiency and selective SQ bioimaging were evaluated utilizing the onion epidermal cell model. Contrary to the hydrophobic SQ1 results in the onion epidermal cell wall, amphiphilic SQ2 tagged the vacuole and nucleus and SQ3 tagged the vacuole. Distinguishable staining profiles in the roots and leaves were achieved. We believe that this study is the first to demonstrate distinct visualisation efficiency induced by the structure differences of two-photon excited SQs. Our results can help establish the versatile roles of novel near-infrared-emitting SQs in biological applications.

Comparative Observation of Distinct Dynamic Stokes Shifts in Diaryl BODIPY Triads with Broadband Two-Photon Absorption

Time-resolved evolution of excited states in the twist-conjugated chromophores is of great fundamental interest for photoluminescent applications. The four diaryl BODIPY triads modified with diverse end-cappers at 2,6-positions were investigated properly, and considerable two-photon absorption capabilities in the first biological spectral window were obtained. Fast relaxations from the initially twisted conformation to the planarized conformation in the excited state were resolved spectrally and kinetically, accompanied by the discernible phenomenon of the fluorescence dynamic Stokes shift (DSS). Along with increasing electron donating capabilities and solvent polarities, the characteristics of structural rearrangement and intramolecular charge transfer have been estimated by enhanced DSS behaviors. Especially, the blue-shifted DSS was rationalized as the sequence conversion between the planarized state and the twisted charge transfer state. A molecular-level picture for relaxation pathways in different polarities was depicted and supported by the theoretical simulations. Significant and fast structural motions in this work contribute to the excited-state dynamics and rational development of versatile BODIPY chromophores.

Interfacially confined preparation of fumaronitrile-based nanofilms exhibiting broadband saturable absorption properties

Interfacial nanofilms with nonlinear optical (NLO) properties were prepared via confined dynamic condensation of 4,4'-methylenedianiline (MDA) with the synthesized 2,3-bis(4-(bis(4-formylphenyl)amino)phenyl)fumaronitrile (BTFA). Investigated using the open-aperture Z-scan technique, BTFA showed reverse saturable absorption ascribed to the synergetic mechanisms of two-photon and excited-state absorption. In contrast, the as-prepared nanofilms demonstrated broadband saturable absorption within the spectral range of 720∼1700 nm. The characteristics of nonlinear absorption coefficient (β) decreased along with increasing the incident pulse intensity. Taking advantage of the flexibility and post-machinability properties, the folding layers of the nanofilms offered the feasibility to fine-tune the specific NLO responses. The optimal β value was found to be -10.1 cm/MW for eight-layer nanofilm as well as the normalized transmittance increased up to 35-fold at 800 nm. Utilized as a conceptual saturable absorber, the representative modulation depth and saturation intensity were observed to be around 2.4% and 7.37 GW/cm² at 800 nm, respectively, comparable to traditional two-dimensional (2D) materials. Aiming to clarify the possible underlying physical processes, a four-level model was employed to illustrate the fast relaxation of the excited states. Present work demonstrates that proper design of building blocks combined with interfacially confined dynamic condensation enables rational development of high-performance NLO materials.

Rigid Bay-Conjugated Perylene Bisimide Rotors: Solvent-Induced Excited-State Symmetry Breaking and Resonance-Enhanced Two-Photon Absorption

Intramolecular charge transfer and excited-state symmetry breaking have a significant effect on the nonlinear optical properties of multipolar chromophores. Rigid and nonplanar perylene bisimide derivatives (PBIs) functionalized at bay positions were comparatively and comprehensively investigated. In apolar solvents, two quadrupolar molecular rotors showed an obvious decrease of the A_0-0/A_0-1 ratios, suggesting strong exciton coupling with the adjacent PBI units initiated by the π-π stacking. The vanishment of the preferable dimer emission in polar solvents supported the plausible phenomena of excited-state symmetry breaking, thanks to the facile rotation around the rigid linkers. Comparative femtosecond transition absorption studies confirmed their notable differences in relaxation dynamics and the generation of radical anions (PBI^•-) and cations (PBI^•+). The maxima two-photon absorption (2PA) wavelengths obtained for the molecular rotors were slightly red-shifted to 670 nm with intrinsic resonance-enhanced characteristics, reflecting the synergistic effect of functional positions and molecular architectures. Meanwhile, the obvious increase of significant 2PA cross-section values in polar solvents illustrated the stabilization of the symmetry-broken dipolar states. Further femtosecond Z-scan also manifested the contribution of excited-state dynamics on the nonlinear optical properties of multipolar chromophores.