Owing to their remarkable linear and nonlinear optical properties, conjugated polymers have attracted considerable attention as promising candidates for advanced photonic applications. However, achieving efficient multiphoton absorption through precise control of structural architecture remains a significant challenge. In this context, this work presents the synthesis and investigation of the photophysical properties, excited-state dynamics, and two- and three-photon absorption (2PA and 3PA) of a linear polyfluorene (PF) and a pyrene-centered branched polyfluorene (PFPy). The introduction of pyrene-centered branching significantly modulates the electronic structure in both solution and the solid state, leading to an average increase of ∼60% in the fluorescence quantum yield of PFPy relative to PF. A 4-fold lower nonradiative decay rate sustains this improvement in the branched system. Notably, PFPy exhibited 2PA and 3PA cross-sections of ∼4400 GM and ∼1.0 × 10–78 cm6·s2/photon2, respectively, about five- and two-folds higher than those of PF. Furthermore, normalizing the multiphoton absorption cross-sections as a function of the effective number of electrons highlights the intrinsic efficiency of the branched system. These findings indicate that the enhanced 2PA and 3PA responses of PFPy arise from the combined influence of the central pyrene chromophore and the multibranched architecture, with the 2PA analysis supporting a significant contribution from cooperative electronic coupling among the branches.
Abstract Selective electronic excitation of specific molecules or subpopulations within a mixture is challenging when their ultraviolet/visible (UV/VIS) spectra are strongly overlapping. Vibrationally Promoted Electronic Resonance (VIPER) excitation addresses this challenge by combining the selectivity of infrared (IR) excitation with UV/VIS excitation, which triggers the desired processes. The IR excitation modulates the UV/VIS spectrum of the selected subpopulation via vibronic couplings, shifting it into resonance with a subsequent off-resonant UV/VIS pump pulse. Here, we demonstrate that 2D vibrational electronic (2D-VE) spectroscopy is ideally suited to measure such IR-induced UV/VIS modulations. As a case study, we investigate the dynamic equilibrium of Schreiner’s thiourea catalyst binding a coumarin substrate. We find the substrate’s carbonyl (CO) stretching vibration to be vibronically coupled to the electronic transition of the coumarin and also to show distinct vibrational frequencies in the bound and free state, rendering it an excellent handle for future VIPER studies. Moreover, we show that direct vibronic coupling is not the sole pathway to subpopulation-selective excitation: vibrational energy transfer (VET) from the catalyst to the substrate through hydrogen bonds can likewise modulate the UV/VIS spectrum, providing an alternative mechanism for selective excitation. We further demonstrate that IR excitation of Fermi resonances generates additional UV/VIS modulations, which must be considered when planning VIPER-type experiments. Selective excitation of molecular subpopulations through vibronic couplings opens new possibilities to disentangle the kinetics of distinct species involved in photoreactions and advance our understanding of complex photocatalytic systems.
Abstract The development of efficient catalysts for environmental remediation and the conversion of chemical compounds into value-added products remains an important challenge in materials science. Herein, Cu-doped lithium niobate (Li1–xCuxNbO3; 0 ≤ x ≤ 0.025) was synthesized by a modified sol–gel method followed by heat treatment at 500 °C for 2 h. X-ray diffraction and Raman spectroscopy confirmed the predominant formation of trigonal LiNbO3 (R–3c), together with less than 5% monoclinic CuNb2O6 (P21/c) as a secondary phase in the doped samples. Cu incorporation enhanced visible-light absorption and decreased the optical bandgap from 3.95 eV for pure LiNbO3 to 2.96 eV for the x = 0.025 composition, likely owing to the introduction of intermediate electronic states. X-ray photoelectron spectroscopy confirmed the expected Li, Nb, O, and Cu surface chemical states. The materials were evaluated as piezocatalysts for the reduction of p-nitrophenol (PNP), mainly to p-aminophenol (PAP), which was supported by UV–vis and 1H NMR analyses. Cu-doped samples achieved PNP reduction above 93% within 5 min and retained this performance after four reuse cycles, whereas pure LiNbO3 exhibited conversion below 20%. Scavenger experiments indicated that electrons were the primary charge carriers involved in the reaction. These findings demonstrate that Cu-doped LiNbO3 is a promising reusable piezocatalyst for the rapid conversion of PNP under ambient conditions.
The protonated water trimer (H2O)3H+ and tetramer (H2O)4H+ are largely symmetric in the gas phase and strongly asymmetric when solvated in polar liquid solutions (as-(H2O)3H+ and as-(H2O)4H+). as-(H2O)3H+ features a shared-proton motif (a deformed Zundel arrangement) deformed by a strong interaction with a third water molecule. The fourth water molecule in both the star-like and linear (H2O)4H+ arrangements was found computationally to interact less strongly with the proton than the two water–water interactions within the as-(H2O)3H+ core-structure complex, thereby making the solution-phase structure of (H2O)4H+ highly asymmetric. We have tested these theoretical predictions by probing protonated water clusters (H2O)nH+ with predetermined sizes n = 2–6, in acetonitrile. We observe for as-(H2O)3H+ a previously unreported weak IR absorption band at 1820 cm–1, which also appears in the higher n = 4–6 complexes. The stability constants of protonated water clusters decrease as the cluster size increases from 2 to 3 and then to 4 water molecules. The weakest interaction in the 4-molecule water cluster resembles water–water interactions in bulk water predicted by theory. We argue, based on our new experimental evidence, that the as-(H2O)3H+ arrangement, which is a central part of the as-(H2O)4H+ arrangements, best represents the core structure of the hydrated proton in aqueous solutions with far-reaching consequences for all fields of chemistry.
Abstract The protonated water trimer (H2O)3H+ and tetramer (H2O)4H+ are largely symmetric in the gas phase and strongly asymmetric when solvated in polar liquid solutions (as-(H2O)3H+ and as-(H2O)4H+). as-(H2O)3H+ features a shared-proton motif (a deformed Zundel arrangement) deformed by a strong interaction with a third water molecule. The fourth water molecule in both the star-like and linear (H2O)4H+ arrangements was found computationally to interact less strongly with the proton than the two water–water interactions within the as-(H2O)3H+ core-structure complex, thereby making the solution-phase structure of (H2O)4H+ highly asymmetric. We have tested these theoretical predictions by probing protonated water clusters (H2O)nH+ with predetermined sizes n = 2–6, in acetonitrile. We observe for as-(H2O)3H+ a previously unreported weak IR absorption band at 1820 cm–1, which also appears in the higher n = 4–6 complexes. The stability constants of protonated water clusters decrease as the cluster size increases from 2 to 3 and then to 4 water molecules. The weakest interaction in the 4-molecule water cluster resembles water–water interactions in bulk water predicted by theory. We argue, based on our new experimental evidence, that the as-(H2O)3H+ arrangement, which is a central part of the as-(H2O)4H+ arrangements, best represents the core structure of the hydrated proton in aqueous solutions with far-reaching consequences for all fields of chemistry.
