Photophysical Insights Research Articles

Photophysical Insights into Nanomaterials-Based DNA-Mediated Superstructure Formation

Photophysical Insights into Nanomaterials-Based DNA-Mediated Superstructure Formation

Deciphering the Interplay of Structure and Charge Carrier Dynamics in Reduced-Dimensional Perovskites

Reduced-dimensional perovskites (RDPs) have advanced the development of perovskite solar cells (PSCs) due to their tunable energy landscape, structure, and orientation. Thus, we aim for an increase in the understanding of structure-photophysical properties of Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) RDPs with different dimensionalities. Our findings reveal that RP RDPs with lower dimensionality exhibits a dominant n=2 phase, preferential out-of-plane orientation, and longer charge carrier lifetime compared with DJ RDPs, as evidenced by X-ray scattering technique and transient absorption spectroscopy. In addition, we unveil the film growth of respective RDPs by in-situ X-ray scattering, showing the stoichiometry-determined phase growth. The formation of lower-n phases in RP RDPs with higher dimensionality is thermodynamically favored, while those phases are likely in the form of “intermediate phase”, which bridge the 3d-like and lower-n phases in DJ RDPs. DJ RDPs with higher dimensionality demonstrate comparable phase purity, giving rise to more sufficient energy transfer and longer charge carrier lifetime. As such, DJ-based PSCs (n=4) demonstrate better device stability under ISOS-I-L conditions compared to RP-based PSCs. Leveraging these structural and photophysical insights, our work paves the way for dictating the utilization of RDPs in 3D PSCs and the fabrication of quasi-2D solar cells.

Structural regulation and photophysical insights into zero-dimensional organic copper(I) halides with thiophene-substituted phosphonium cation

Here we report three zero-dimensional (0-D) copper cluster structures, (2-TTPS)[CuCl2] (1), (2-TTPS)2[Cu4Br6] (2), and (2-TTPS)4[Cu4I8] (3), formed by self-assembly of the 2-TTPSX ligand (2-TTPSX = triphenyl(thiophen-2-yl)phosphonium chloride/bromide/iodide) with neutral CuCl/CuBr/CuI, respectively. The CuX salts exhibit a propensity to form high nuclearity aggregates or clusters, leading to a specific structural richness. In 1, two [CuCl2]− units are staggered along the c-axis. In 2, the larger [Cu4Br6]2− inorganic unit is completely encapsulated by 2-TTPS+ cations, with each Cu+ located at the center of a [CuBr4]3− tetrahedral structure. The copper cluster in 3 is characterized by an unprecedented, centrosymmetric tetramer formation. The variation in copper cluster types is achieved through the control of a single crystal growth technique using different CuX salts for assembly. This work introduces the synthetic methods, crystallographic features, and optical properties of the three materials.

Synthesis and Multifaceted Exploration of 4-Phenylpiperidin-4-ol Substituted Pyrazole: Photophysical Insights with Biological Activity

Abstract In this study, we successfully synthesized a pyrazole derivative, specifically 4-phenylpiperidin-4-ol substituted pyrazole (CHP), through the reaction of Grignard reagents in combination with pyrazole. This newly synthesized molecule was subjected to a comprehensive evaluation for both its photophysical and biological applications. Notably, CHP exhibited promising invitro antifungal and antibacterial activities, primarily attributed to the presence of the 4-phenylpiperidin-4-ol moiety and resulting component contributed to an enhanced absorption rate of lipids, thereby improving the pharmacological activity of CHP. This correlation between structure and function was further supported by the outcomes of structure-activity relationship studies. Additionally, we conducted in silico studies to examine the molecular interactions of the synthesized molecule with key proteins, including DNA Gyrase, Lanosterol 14 α-demethylase, and KEAP1-NRF2. The results unveiled robust binding interactions at specific sites within these proteins, indicating potential therapeutic relevance. Furthermore, the photophysical properties of the synthesized compounds were thoroughly investigated using the ab-initio technique. This involved the determination of ground state optimization and HOMO-LUMO energy levels, all calculated with the DFT-B3LYP-6-31G(d) basis set. The assessment of the theoretically estimated HOMO-LUMO value provided insights into the global chemical reactivity descriptors, revealing that the synthesized molecule boasts a highly electronegative and electrophilic index. Taken together, our findings suggest that pyrazole derivatives with 4-phenylpiperidin-4-ol substitutions exhibit promising applications in both photophysical and biological contexts.

Enhancing c-Si Solar Cell Efficiency in the UV Region: Photophysical Insights into the Use of Eu3+ Complexes for Down-Shifting Layer Applications

[Eu(3DPIQC)3] (where DPIQC = 3-(diphenyl phosphoryl)-1-isoquinolinecarboxylate), a luminescent europium complex with antenna ligands, has been carefully embedded within a polyvinyl butyral (PVB) matrix and the resulting material was used to prepare films used as luminescent down-shifting layers (LDSLs) for crystalline Si-based solar cells. The films were characterized using photoluminescence spectroscopy, atomic force spectroscopy (AFM), UV-Vis spectroscopy, and fluorescence microscopy. The AFM analysis shows films with low surface roughness, while fluorescence microscopy revealed that the Eu complex embedded in PVB assumed a spheroidal configuration, a morphology especially beneficial for optical applications. The so-obtained LDSLs were utilized as energy converters in c-Si solar cells to enhance the utilization of high-energy photons, thereby improving their overall efficiency. The determination of photovoltaic parameters carried out before and after the deposition of the LDSLs on the c-Si cells confirms a positive effect on the efficiency of the cell. The Jsc increases from 121.6 mA/cm2 to 124.9 mA/cm2, and the open circuit voltage (Voc) is found to be unrelated to the complex concentration in the films. The fill factor (FF) remains constant with the Eu concentration. The EQE curves indicate an enhancement in the performance of the photovoltaic cells within the UV region of the spectrum for all coated devices. Electrochemical impedance spectroscopy (EIS) was also carried out in order to analyze the effect of the Eu complex in the charge transfer process of the devices.

Supramolecular interaction of PCBM with porphyrins in solution: Photophysical insights

This work reports the self-assembly between [6,6]-phenyl C71 butyric acid methyl ester (PCBM) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin (1)(and/2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin Zn(II) (2) in toluene. Ground state intermolecular interaction is evidenced from absorption spectrophotometric measurements. New absorption bands are observed in the visible region which may be identified due to charge transfer (CT). Several important physicochemical factors are enumerated for PCBM-1 and PCBM-2 systems. Fluorescence investigations elicit complex formation of PCBM with porphyrins (with both 1 and 2) and reveal considerable magnitude of binding constant (K) for PCBM-2 system, i.e., KPCBM-2 = 80,435 dm3⋅mol−1 compared to PCBM-1 system, i.e., KPCM-1 = 12,600 dm3·mol-1 as well as highly ratio of selectivity in binding (KPCBM-2/KPCBM-1 ∼ 6.4). Time resolved fluorescence experiments reveal that photoexcited decay from the excited singlet state of porphyrins (i.e., 1* and 2*) by PCBM is statically controlled compared to dynamic path. Magnitude of solvent reorganization energy indicates possibility of faster charge recombination in case of PCBM-2 system. Both 1H and 13C NMR measurements provide substantial support behind complexation of PCBM with porphyrins (both 1 and 2) in solution. Ab initio calculations in vacuo support the trend in K for PCBM-1 and PCBM-2 systems and establish the proper orientation of PCBM towards 1 (and/ 2) during complexation. Transient absorption measurements establish two different mode of energy transfer pathway from porphyrin to PCBM in toluene.

Band alignment driven efficient electron transfer and enhanced photoelectric property of gold nanoclusters sensitized graphene quantum dot composites

Gold nanoclusters (Au NCs) have received much attention due to their promising applications in many fields including optical imaging, catalysis, biomedical science and, more recently, energy conversion. Herein, we have constructed a model composite of Au NCs and graphene quantum dots (GQDs), and investigated the band alignment of their energetic levels, efficient electron transfer, and enhanced photoelectric properties of Au NCs/GQDs composites. The introduced Au NCs not only make up for the deficiency of light absorption of GQDs in the visible light region but importantly function as the electron donor to sensitize GQDs and improve the photoelectric properties of the composite. The results from steady-state and time-resolved measurements demonstrate a dramatically quenched fluorescence and a shortened lifetime of Au NCs in the composites, owing to the efficient electron transfer from Au NCs to GQDs. The electron transfer efficiency determined from the dynamical analysis is consistent with that of steady-state fluorescence quenching. The type-II band alignment of Au NCs and GQDs moieties effectively promotes the charge separation of excited Au NCs moieties and the electron transfer to GQDs. As a result, the photoelectric current of Au NCs/GQDs composite is enhanced by 2.69 and 4.80 times that of individual Au NCs and GQDs, respectively. These findings demonstrate a feasible strategy for constructing photoelectric composites with Au NCs as the sensitizer and carbon-based QDs as the electron acceptor and also provide photophysical insights into the structure-property relationship of Au NCs-based composites.

Shedding Novel Photophysical Insights Toward Discriminative Detection of Three Toxic Heavy Metal Ions and a hazard class 1 nitro-explosive By Using a Simple AIEE Active Luminogen.

In this work, we introduced a simple aggregation-induced emission enhancement (AIEE) sensor (PHCS) which can selectively detect and discriminate three environmentally and biologically imperative heavy metal ions (Cu2+, Co2+ and Hg2+) and a hazard class 1 categorized nitro-explosive picric acid (PA) in differential media. By virtue of its weak fluorescence attributes in pure organic medium owing to the synergistic operation of multiple photophysical quenching mechanisms, the molecular probe showcased highly selective 'TURN ON' fluorogenic response towards hazardous Hg2+ with a limit of detection (LOD) as low as 97nM. Comprehensive investigation of binding mechanism throws light on the cumulative effect of probe-metal complexation induced chelation enhanced fluorescence (CHEF) effect and subsequent AIEE activation within the formed probe-metal adducts. Noteworthily, the probe (PHCS) can be readily used in real water samples for the quantitative determination of Hg2+ in a wide concentration range. In addition, the probe displayed modest colorimetric recognition performances to selectively detect and discriminate two essential heavy metal ions (Cu2+ and Co2+) with a LOD of 96nM and 65nM for Cu2+ and Co2+ respectively, in semi-aqueous medium. Intriguingly, based on high photoluminescence efficiency, the AIEE active nano-aggregated PHCS displayed a remarkable propensity to be used as a selective and ultra-sensitive 'TURN-OFF' fluorogenic chemosensor towards PA with LOD of 34.4ppb in aqueous medium. Finally, we specifically shed light on the interaction of PHCS hydrosol towards PA using some unprecedented techniques, which helped uncover new photophysical insights of probe-explosive molecule interaction.

Organic Photovoltaic Stability: Understanding the Role of Engineering Exciton and Charge Carrier Dynamics from Recent Progress.

Benefiting from the synergistic development of material design, device engineering, and the mechanistic understanding of device physics, the certified power conversion efficiencies (PCEs) of single-junction non-fullerene organic solar cells (OSCs) have already reached a very high value of exceeding 19%. However, in addition to PCEs, the poor stability is now a challenging obstacle for commercial applications of organic photovoltaics (OPVs). Herein, recent progress made in exploring operational mechanisms, anomalous photoelectric behaviors, and improving long-term stability in non-fullerene OSCs are highlighted from a novel and previously largely undiscussed perspective of engineering exciton and charge carrier pathways. Considering the intrinsic connection among multiple temporal-scale photocarrier dynamics, multi-length scale morphologies, and photovoltaic performance in OPVs, this review delineates and establishes a comprehensive and in-depth property-function relationship for evaluating the actual device stability. Moreover, this review has also provided some valuable photophysical insights into employing the advanced characterization techniques such as transient absorption spectroscopy and time-resolved fluorescence imagings. Finally, some of the remaining major challenges related to this topic are proposed toward the further advances of enhancing long-term operational stability in non-fullerene OSCs.

Photophysical Insights of Halogenated Dipyrrolonaphthyridine-Diones as Potential Photodynamic Therapy Agents†.

We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X2 -DPNDs, X=Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spin-orbit coupling (SOC) dynamics in the investigated X2 -DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of 3 O2 to 1 O2 using X2 -DPND correlates with the rate of triplet population, kISC >1.6 × 108 s-1 for I2 -DPND vs kISC >2.9 × 109 s-1 for Cl2 -DPND and Br2 -DPND (where τISC =343 ± 3 ps for I2 -DPND and τISC =5-6 ns for Cl2 -DPND and Br2 -DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl2 -DPND and Br2 -DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl2 /Br2 -DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.

Charge generation in organic solar cells: Journey toward 20% power conversion efficiency

AbstractThe power conversion efficiencies of organic solar cells (OSCs) have routinely lagged far behind those of their inorganic counterparts. However, owing to the enormous contributions of many researchers, the power conversion efficiencies of OSCs have rapidly improved and now exceed 19%. The charge generation mechanisms in OSCs have been heavily debated during this period while acquiring valuable knowledge. This review highlights fundamental and cutting‐edge research that rationalizes why OSCs can generate photocurrent so efficiently. In particular, a photophysicist's views on exciton diffusion to donor:acceptor interfaces, charge transfer at the donor:acceptor interface, and long‐range spatial dissociation of charge transfer states are discussed. Although a general consensus in this area has not been reached yet, recent time‐resolved spectroscopic measurements provide important photophysical insights that can help achieving a better understanding of the charge generation mechanism in OSCs. Based on these observations, future research directions for realizing further improvements in OSC performance are discussed.

New Bis-Cyclometalated Iridium(III) Complexes with β-Substituted Porphyrin-Arylbipyridine as the Ancillary Ligand: Electrochemical and Photophysical Insights.

An efficient synthetic access to new cationic porphyrin-bipyridine iridium(III) bis-cyclometalated complexes was developed. These porphyrins bearing arylbipyridine moieties at β-pyrrolic positions coordinated with iridium(III), and the corresponding Zn(II) porphyrin complexes were spectroscopically, electrochemically, and electronically characterized. The features displayed by the new cyclometalated porphyrin-bipyridine iridium(III) complexes, namely photoinduced electron transfer process (PET), and a remarkable efficiency to generate 1O2, allowing us to envisage new challenges and opportunities for their applications in several fields, such as photo(catalysis) and photodynamic therapies.

(Invited) Cascaded Energy Landscape As a Key Driver for Slow Yet Efficient Charge Separation with Small Energy Offset in Organic Solar Cells

Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. I will talk about the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as an electron donor and acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ~0.1 eV. In this study, transient absorption spectroscopy was used to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. After hole transfer from Y6 to PM6, slow yet efficient spatial charge dissociation on a time scale of picoseconds was observed. Based on temperature dependence measurements, it is found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces.

Investigation of photophysical insights into the CsPbBr3-porphyrazine system in solution

In this work, a combination of steady-state and time-resolved fluorescence studies is used to examine molecular level excited-state interactions between CsPbBr3 QDs and porphyrazine in solution. Fluorescence quenching investigations show that the CsPbBr3 emission is quenched while the fluorescence of porphyrazine is enhanced. The association constant for the interaction between CsPbBr3 QDs and porphyrazine is determined to be 1.2 × 104 M−1. Time-resolved fluorescence quenching studies exhibit direct evidence of an energy transfer process. This research emphasizes the importance of optimizing both the donor and acceptor properties when designing energy-transfer light-harvesting assemblies.

Molecular physics of persistent room temperature phosphorescence and long-lived triplet excitons

Persistent room temperature phosphorescence (pRTP) is important to high-resolution imaging independent of autofluorescence and the scattering of excitation light for security and imaging applications. Although efficient and bright pRTP is crucial to imaging applications, photophysical processes from the triple states of heavy-atom-free chromophores have been explained by making many assumptions that are potentially based on incorrect photophysical explanations. This often confuses researchers in their efforts to control and enhance the pRTP characteristics. This paper introduces recent advances in our understanding of photophysical processes from the lowest triplet excited state of heavy-atom-free chromophores based on statistical evidence from experimental and theoretical viewpoints. After the introduction of two photophysical processes showing persistent RT emissions and the characteristics of the persistent emissions, physical parameters relating to pRTP and appropriate techniques for measuring the parameters are explained. For molecularly dispersed heavy-metal-free chromophores in a solid state, recent understandings of the physical parameters verified by correlations from optically estimated and theoretical viewpoints are summarized. Using the photophysical insights obtained for the dispersed chromophores, uncertainties regarding the photophysical processes of aggregated chromophores are discussed. After highlighting recently developed materials showing efficient pRTP, the potential advantages of pRTP over previous persistent emissions are discussed considering recent demonstrations of persistent emitters. This review quantitatively summarizes the relationship between the molecular backbone and physical parameters of pRTP characteristics and guides the reader in their efforts to appropriately design materials with efficient pRTP and control long-lived triplet excitons for promising applications.

Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

The photophysical insights of thermally activated delayed fluorescence emitters for OLEDs obtained by time-resolved EPR spectroscopy were summarized.

Photophysical insights on quantum dots-zinc porphyrazine system studied in solution

The present study reports spontaneous interaction of a quantum dots, namely, CdSxSe1-x/ZnS (QD) with zinc porphyrazine (1) in toluene. It is observed from steady state fluorescence measurements that photoluminescence of QD suffers quenching by 1. Time resolved fluorescence measurements reveal small change in the lifetime of QD (16.10 ns) following it interaction with 1 (15.77 ns). The magnitude of kq for QD-1 system, i.e., kq ​= ​5.25 ​× ​1012 ​L⋅mol−1⋅sec−1 (evaluated from the stern-volmer plot) establishes that photoexcited QD undergoes decay by 1 according to static quenching mechanism. The results emerging from above study confirm that QD-1 system may be judiciously applied as an energy storage material in near future.

Organic Spacers in 2D Perovskites: General Trends and Structure‐Property Relationships from Computational Studies

AbstractAddition of large organic molecules to halide perovskites has been shown to provoke dimensionality reduction and formation of two‐dimensional phases that demonstrate improved long‐term stabilities. Optoelectronic properties of the resulting 2D layered perovskites are strongly influenced by the chemical nature of the additive molecules, which opens immense possibilities for preparation of materials with tailored properties. However, given the huge chemical space of possible organic spacers, a systematic and exhaustive search for optimal compounds is impossible and general structure–property relationships that could guide a rational design are still largely absent. Here, we provide an overview of a series of recent computational studies from our group on different types of spacers. We first develop a simplified universal monovalent cation model to map out approximate structural stability maps as a function of the van der Waals radius and the magnitude of dispersion interactions to monitor the possible emergence of 2D phases. We further provide structural and photophysical insights from classical and first‐principles molecular dynamics simulations and density functional theory calculations on 2D hybrid perovskites based on a wide range of spacers with different chemical nature and varying conformational properties. Our computational predictions are validated through comparison with powder diffraction, conductivity and optical measurements. Such comparative study allows for providing some general structure–property correlations, which can serve as design guidelines in the search for optimal 2D and mixed 2D/3D perovskite photovoltaic materials.

Plasmon extinguishment by bandedge shift identified as a second-order spectroscopic differentiation

Abstract Optical excitation of metallic nanostructures induces strong intraband transitions, leaving transient depletion below the Fermi level, which allows transient interband transition to this depletion band. This is equivalent to the lowering of the threshold for interband transitions and pushes the plasmonic band to the red. As a result, localized surface plasmon resonance (LSPR) is “extinguished” or “quenched” around the bandedge, in contrast, the interband optical absorption becomes enhanced and redshifted. The corresponding transient absorption (TA) signals have equal lifetimes and opposite signs. Moreover, the TA spectrum is found to be a second-order differential of the steady-state optical extinction spectrum over the studied band. This is a commonly existing mechanism for metallic nanostructures and verified with gold in this work. Such a discovery is completely different from the optical-excitation-induced redshift of LSPR through enhanced electronic scattering and is important for understanding the ultrafast spectroscopic response of plasmonic nanostructures with clear photophysical insights, supplying solid basis for exploring optical logic device and optical data processing techniques.

Photophysical insights of highly transparent, flexible and re-emissive PVA @ WTR-CDs composite thin films: A next generation food packaging material for UV blocking applications

Herein, we have fabricated a highly transparent, flexible and re-emissive polyvinyl alcohol/waste tea residue carbon dots (PVA@WTR-CDs) composite thin films by simple solvent casting method. Incorporated WTR-CDs as a function of UV blocking (absorption of UV light and re-emission in the visible region) agent synthesized from waste tea residue powder as a sustainable material. The results revealed that, the WTR-CDs have the best compatibility with PVA. The incorporated fluorescent WTR-CDs impart fluorescence to the whole film and films are capable to not only block the UV region but also re-emit absorbed UV light into the visible region. The increased concentrations of WTR-CDs into PVA films enhances the UV blocking capability of the composite films. The composite, PVA@WTR-CDs-3 films were succeeded to block full (100%) of UV-C (230−280 nm) and UV-B (280−315 nm) region, while 20−60% of UV-A (315−400 nm). We achieved the maximum of UV blocking by PVA@WTR-CDs-5 composite films by controlling the transparency and thickness. By the successive addition of the UV blocking agent, there was no extreme changes observed in the intrinsic mechanical as well as tensile properties of PVA films. The thermal analysis study showed that, there was no adverse effect of WTR-CDs incorporation on the thermal stability and degradation profile of original PVA films. In detail, the possible mechanism of UV absorption and re-emission in the visible region by fabricated films is also discussed. Furthermore, practicability of the composite film was also investigated on a model (grape) fruit and thus can be used as a modern UV blocking material in packaging, pharmaceutical storage, wrapping, and coating etc.