Photophysical Processes Research Articles

Pressure-controlled free exciton and self-trapped exciton emission in quasi-one-dimensional hybrid lead bromides

Hybrid metal halides represent a novel type of semiconductor light emitters with intriguing excitonic emission properties, including free exciton emission and self-trapped exciton emission. Achieving precise control over these two excitonic emissions in hybrid metal halides is highly desired yet remains challenging. Here, the complete transformation from intrinsically broadband self-trapped exciton emission to distinctively sharp free exciton emission in a quasi-one-dimensional hybrid metal halide (C2H10N2)8[Pb4Br18]·6Br with a ribbon width of n = 4, is successfully achieved based on high-pressure method. During compression, pressure-induced phonon hardening continuously reduces exciton–phonon coupling, therefore suppressing excitonic localization and quenching the original self-trapped exciton emission. Notably, further compression triggers excitonic delocalization to induce intense free exciton emission, accompanied with reduced carrier effective masses and improved charge distribution. Controlled high-pressure investigations indicate that the ribbon width of n > 2 is necessary to realize excitonic delocalization and generate free exciton emissions in similar quasi-one-dimensional hybrid metal halides. This work presents an important photophysical process of excitonic transitions from self-trapped exciton emission to free exciton emission in quasi-one-dimensional hybrid metal halides without chemical regulation, promoting the rational synthesis of hybrid metal halides with desired excitonic emissions.

Highly stoichiometry-deviating chalcopyrite quantum dots: synthesis and copper defects-correlated photophysical properties.

Copper indium selenide (CISe) is a prototype infrared semiconductor with low toxicity and unique optical characteristics. Its quantum dots (QDs) accommodate ample intrinsic point defects which may actively participate in their rather complex photophysical processes. We synthesize CISe QDs with similar sizes but with distinct highly stoichiometry-deviating atomic ratios. The synthesis condition employing Se-rich precursors yields the Cu-deficient CISe QDs with special photophysical properties. The photoluminescence exhibits monotonic red shift from 680 to 775 nm when the ratio of Cu's proportion to In's decreases. The luminescence is found to stem from the copper vacancy and antisite defects. The CISe QDs exhibit Raman activity at 5.6, 6.9, and 8.7 THz that is separately assigned to Cu-Se and In-Se optical phonon modes and surface mode.

Photo-induced dynamics with continuous and discrete quantum baths.

The ultrafast quantum dynamics of photophysical processes in complex molecules is an extremely challenging computational problem with a broad variety of fascinating applications in quantum chemistry and biology. Inspired by recent developments in open quantum systems, we introduce a pure-state unraveled hybrid-bath method that describes a continuous environment via a set of discrete, effective bosonic degrees of freedom using a Markovian embedding. Our method is capable of describing both, a continuous spectral density and sharp peaks embedded into it. Thereby, we overcome the limitations of previous methods, which either capture long-time memory effects using the unitary dynamics of a set of discrete vibrational modes or use memoryless Markovian environments employing a Lindblad or Redfield master equation. We benchmark our method against two paradigmatic problems from quantum chemistry and biology. We demonstrate that compared to unitary descriptions, a significantly smaller number of bosonic modes suffices to describe the excitonic dynamics accurately, yielding a computational speed-up of nearly an order of magnitude. Furthermore, we take into account explicitly the effect of a δ-peak in the spectral density of a light-harvesting complex, demonstrating the strong impact of the long-time memory of the environment on the dynamics.

Molecular Design Motifs of Non‐Fullerene Acceptors for Mitigating All Inherent Non‐Ideal Energy Losses in Organic Photovoltaics

AbstractNon‐fullerene acceptors (NFAs) have demonstrated a great potential for the transcendence of performance limits in organic photovoltaics. Despite the tremendous efforts, however, synthetic control of non‐ideal recombination losses on the photovoltage are still elusive and thus lag far behind the inorganic counterparts. Here, a rational design strategy of non‐fullerene acceptors is presented to overcome the limitations derived from the inherent excited state properties of organic molecules. Spectroscopic studies on a series of chemical analogs of ladder‐type fused acceptors reveal that the radiative efficiency of non‐fullerene acceptors is predominantly determined by the inherently strong electron‐vibration coupling of their bound excited states, rather than the redistribution of molecular orbitals itself. In this sense, regardless of intramolecular charge transfer characteristics, the incorporation of rigid, and planar moiety with wide π plane is found to promote the dominance of long‐lived and weakly‐bound excited states in the photophysical process, in which the mitigated non‐radiative decays of NFAs can be directly translated into that of charge‐transfer states. Furthermore, the incorporation of such chemical moiety can minimize the evolution of subgap broadening in the bulk heterojunction blends, lowering the contribution of non‐ideal radiative losses. As a result, the model systems demonstrate the non‐radiative loss down to 180–190 meV, without sacrificing photocurrent generations, reaching that of inorganic counterparts.

Binning method for artifact-free time-tag based correlation function calculations.

Correlation functions are nowadays routinely computed using time-tagged photon information instead of a hardware autocorrelator. The algorithm developed by Laurence et al. [Opt. Lett.31, 829 (2006)10.1364/OL.31.000829] is a powerful example. Despite its ease of implementation and fast computation process, it presents a prevalent noisy feature at the short time-lag range when computed on commonly used logarithmically spaced bins. We identified that arbitral logarithmic spacing produces the mismatch between the edges of generated bins and acquisition frequency, resulting in an aliasing artifact at the short time-lag range of the correlation function. We introduce a binning method that considers the acquisition frequency during the bin generation. It effectively eliminates the artifact and improves the accuracy of the autocorrelation. Applying the binning method herein can be particularly crucial when one extracts photophysical processes from fluorescence correlation spectroscopy or the diffusion coefficient of nanoparticles from dynamic light scattering at the time range below 10-5 s lag time.

Isomeric Effects of Au28(S-c-C6H11)20 Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State.

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au28i(CHT)20 and Au28ii(CHT)20. Based on their structures, analysis of 3O2 (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au28i(CHT)20, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au28ii(CHT)20 isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S1)-triplet (T1) gap in Au28ii than that in Au28i, and the higher T2 state plays a critical role in both isomers' photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.

Making Peace with Random Phases: Ab Initio Conical Intersection Quantum Dynamics in Random Gauges.

Ab initio modeling of conical intersection wave packet dynamics is crucial for various photochemical, photophysical, and biological processes. However, adiabatic electronic states obtained from electronic structure computations involve random phases, or more generally, random gauge fixings, which cannot be directly used in the modeling of nonadiabatic wave packet simulations. Here we develop a random-gauge local diabatic representation that allows an exact modeling of conical intersection dynamics directly using the adiabatic electronic states with phases randomly assigned during the electronic structure computations. Its utility is demonstrated by an exact ab initio modeling of the two-dimensional Shin-Metiu model with and without an external magnetic field. Our results provide a simple approach to integrating the electronic structure computations into nonadiabatic quantum dynamics, thus paving the way for ab initio modeling of conical intersection dynamics.

Capturing the Interplay Between TADF and RTP Through Mechanically Flexible Polymorphic Optical Waveguides

AbstractPolymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5′‐(4‐(diphenylamino)phenyl)‐[2,2′‐bithiophene]‐5‐carbaldehyde (TPA‐CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM‐tip‐based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal‐based photonic circuit technologies.

Capturing the Interplay Between TADF and RTP Through Mechanically Flexible Polymorphic Optical Waveguides.

Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TPA-CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM-tip-based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal-based photonic circuit technologies.

(Invited) Impact of Anchors on the Photophysical Processes in Water-Soluble Fullerene Conjugates

Fullerenes are excellent acceptors in both photoinduced electron- and energy-transfer reactions. Many studies on the photophysical processes of fullerenes have been reported in the last decades, especially aiming towards the development of electronic devices. However, only a few studies have been reported on biocompatible fullerene systems, mainly due to the extremely low solubility of C60 in both water and water-miscible solvents. In our previous studies, we reported the synthesis of fullerene-PEG conjugates and demonstrated their ability for photoinduced DNA damage and ROS generation upon visible light irradiation.2 In this work, we investigated the effect of the water-soluble anchors on the photophysical properties of fullerene molecules. The generation of ROSs in water-soluble fullerene-peptide conjugates were tested by electron paramagnetic resonance (EPR) methods in comparison to previously reported fullerene-PEG conjugates above. To better understand the mechanism in ROS generation, a comprehensive investigation was performed by transient absorption spectroscopic data, encompassing the dynamics of photon absorption, exciton formation, and charge transfer mechanisms. Additionally, insights derived from density functional theory (DFT) calculations and in vitro assessments of photocytotoxicity will be discussed in detail. Figure 1

(Digital Presentation) Cooperative Quantum Dynamics of Excitons in Colloidal Quantum Dots for Novel Optoelectronic Applications

Colloidal semiconductor quantum dots (QDs) are excellent materials for combining photonics and nanoscience. They have been extensively studied to understand the photophysics of quantum confined excitons and to develop efficient optoelectronic devices such as QD solar cells and light-emitting diodes. The photophysical processes of QD devices are governed by a large number of QDs constructing the devices. Therefore, cooperative quantum dynamics of QDs, i.e., synchronous responses of QD ensembles, have a great potential for boosting optoelectronic device performances.Before the use of cooperative quantum dynamics in QD devices, the understanding of quantum dynamics of excitons, i.e., coherent properties of multiexcitons, is required to draw out quantum cooperativity from QD ensembles. In order to elucidate coherent dynamics of multiexcitons during resonant laser-pulse excitation, we performed a quantum interference spectroscopy of QD responses. We found that multiexcitons generate the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called harmonic quantum coherence, and that the coherent responses govern the microscopic processes in the resonant multiphoton absorption [1-3]. Furthermore, it was demonstrated that the coherent responses were detectable as photocurrent signals in thin films of QD ensembles [4]. These developments build up an essential base of quantum cooperativities emerging in optoelectronics.Here, we report recent investigations of cooperative quantum dynamics in QDs. In optoelectronic devices, electronic couplings between QDs are expected to generate a new type of quantum cooperativities. To generate electronic coupling, we fabricated closely packed PbS QDs by using a ligand exchange method. We found that nonlinear photocurrent signals are strongly enhanced in the QD solids. The comparison analysis of different ligands clarified that the enhancement ratio increases monotonically with the decrease of the inter-QD distance [5]. This means that the signal enhancement originates from the electronic coupling and cooperative behavior of QDs. These findings show that the cooperative quantum dynamics provide a new route to advanced optoelectronic technology such as nonlinear amplifiers.Part of this work was supported by JSPS KAKENHI (JP19H05465, JP22H01990, and JP23K17877), JST PRESTO (JPMJPR23H3), and JST CREST (JPMJCR21B4).

Pressure-Induced Enhancement of Room-Temperature Phosphorescence in Heavy Halogen-Containing Imide and Polyimide.

To investigate the correlation between the aggregated state and photoluminescence (PL) mechanism of dual fluorescent (FL) and phosphorescent (PH) polyimides (PIs), the photophysical processes of FL-type BP-PI, PH-type DBrBP-PI, and their corresponding imide model compounds (BP-MC and DBrBP-MC) dispersed in poly(methyl methacrylate) (PMMA) films were analyzed at elevated pressures up to 8 GPa using a diamond anvil cell. Dibromo-substituted DBrBP-MC demonstrated a shorter wavelength absorption than BP-MC owing to the larger dihedral angle in the biphenyl moiety. Both MCs exhibited red-shifts in their absorption spectra with increasing pressure, indicating planarization occurred at the biphenyl moieties associated with the compression of the free volume in PMMA. The PL intensity of BP-MC increased with increasing pressure, while its quantum yield (ΦPL) decreased sharply due to the enhanced energy transfer via the Förster mechanism. In contrast, the PH quantum yield (ΦPH) of DBrBP-MC monotonically increased at lower pressures, while it showed excitation wavelength-dependent behaviors at higher pressures: ΦPH remained unchanged under excitation at 340 nm but gradually increased under excitation at 365 nm. This fact suggests that, at higher pressures, 365 nm excitation promoted intersystem crossing (ISC) from excited singlet states at higher energy levels. Using this phenomenon, a significant pressure-induced PH enhancement (PIPE) was observed for DBrBP-PI up to 0.9 GPa upon excitation at 365 nm, which is a rare phenomenon for organic polymers. This study indicates that even in colorless and optically transparent amorphous polymers, an enhancement of PH due to restricted molecular motion and intensified ISC outweighs the deactivation due to intermolecular energy transfer under certain pressures, leading to an increase in ΦPH.

Improving Photophysical Properties of Deazaflavin Derivatives by Acrylaldehyde Bridging: A Theoretical Investigation

AbstractThe electronic structure and photophysical properties of several acrylaldehyde‐bridged deazaflavin derivatives (cFLs) were investigated theoretically. The impact of acrylaldehyde bridging on photophysical properties of deazaflavin (cFL) is strongly site‐dependent. Specifically, the change of adiabatic energy of electronic transitions(ΔEad) and vibronic coupling promote fluorescent emission to be comparable to internal conversion of cFL and cFL4 (both C5−C6 and C9−N10 bridged, but C9−N10 bridged by propene), turning them eligible as fluorescent sensors. As El‐Sayed's rule is satisfied in cFL1(C5−C6 bridged), cFL2(C9−N10 bridged) and cFL3(both C5−C6 and C9−N10 bridged), intersystem crossing from first singlet excited state to triplet excited states (Tn) become dominant and the evolution of excited cFLs from T1 appears vital. The rate constants of photophysical processes indicate these cFLs are of dominantly high steady state T1 concentration and are potential triplet sensitizers. We expect the findings would pave the way for rational design of novel cFLs with extraordinary photophysical properties.

Effect of number of intramolecular double bonds on photophysical properties and ESIPT processes for Cha-NH2 and its derivatives: A theoretical study

In recent years, the research on excited-state intramolecular proton transfer (ESIPT) reactions has aroused increasingly attention theoretically and experimentally. For a deep exploration, we designed three derivatives of Cha-NH2 with different numbers of carbon–carbon double bonds: Cha-NH2-1, Cha-NH2-2, and Cha-NH2-3. Using density functional theory and time-dependent density functional theory methods, we systematically studied the photophysical properties and ESIPT processes of Cha-NH2 and its derivatives. Detailed analysis revealed that the number of carbon–carbon double bonds within the molecule has a significant impact on the ESIPT behavior and spectral properties of Cha-NH2 and its derivatives. Specifically, as the number of carbon–carbon double bonds increases, the strength of intramolecular hydrogen bond is gradually weakened, leading to a red shift in the spectrum and a increase in the forward energy barrier for ESIPT, which is unfavorable for the occurrence of the ESIPT reaction. Therefore, this finding opens up broad application prospects for the adjustment and optimization of future ESIPT process.

Magnetic fields reveal signatures of triplet-pair multi-exciton photoluminescence in singlet fission.

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.

Constructing Heavy-Atom-Free Photosensitizers for Hypoxic Tumor Phototherapy Based on Donor-Excited Photoinduced Electron-Transfer-Driven Type-I and Type-II Mechanisms.

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) photophysical process has shown great potential for constructing heavy-atom-free photosensitizers (PSs) for photodynamic therapy (PDT) of tumors. However, for almost all such PSs reported to date, the SOCT-ISC is driven by the acceptor-excited photoinduced electron transfer (a-PeT). In this work, for the first time the donor-excited photoinduced electron transfer (d-PeT)-driven SOCT-ISC mechanism is utilized to construct the heavy-atom-free PSs for PDT of tumors by directly installing the electron-deficient N-alkylquinolinium unit (as an electron acceptor) into the meso-position of the near-infrared (NIR) distyryl Bodipy chromophore (as an electron donor). In the less polar environment, the PSs exist as the monomer and promote the production of singlet oxygen (1O2) (Type-II) relying on the d-PeT-driven population of the triplet excited state via SOCT-ISC, whereas in the aqueous environment, they exist as nanoaggregates and induce the generation of superoxides (O2-•) and hydroxyl radicals (HO•) (Type-I) via the d-PeT-driven formation of the delocalized charge-separated state. The PSs could rapidly be internalized into cancer cells and induce the simultaneous production of intracellular 1O2, O2-•, and HO• upon NIR light irradiation, endowing the PSs with superb photocytotoxicity with IC50 values up to submicromolar levels whether under normoxia or under hypoxia. Based on the PSs platform, a tumor-targetable PS is developed, and its abilities in killing cancer cells and in ablating tumors without damage to normal cells/tissues under NIR light irradiation are verified in vitro and in vivo. The study expands the design scope of PSs by introducing the d-PeT conception, thus being highly valuable for achieving novel PSs in the realm of tumor PDT.

Spin-related excited-state phenomena in photochemistry.

The spin of electrons plays a vital role in chemical reactions and processes, and the excited state generated by the absorption of photons shows abundant spin-related phenomena. However, the importance of electron spin in photochemistry studies has been rarely mentioned or summarized. In this review, we briefly introduce the concept of spin photochemistry based on the spin multiplicity of the excited state, which leads to the observation of various spin-related photophysical properties and photochemical reactivities. Then, we focus on the recent advances in terms of light-induced magnetic properties, excited-state magneto-optical effects and spin-dependent photochemical reactions. The review aims to provide a comprehensive overview to utilize the spin multiplicity of the excited state in manipulating the above photophysical and photochemical processes. Finally, we discuss the existing challenges in the emerging field of spin photochemistry and future opportunities such as smart magnetic materials, optical information technology and spin-enhanced photocatalysis.

A Fluorescent Conjugated Polar Polymer for Probing Charge Injection in Multilayer Organic Light-Emitting Transistors.

Ambipolar organic light-emitting transistors (OLETs) are extremely appealing devices for applications from sensing to communication and display realization due to their inherent capability of coupling switching and light-emitting features. However, their limited external quantum efficiency (EQE) and brightness under ambipolar bias conditions hamper the progress of OLET technology. In this context, it was recently demonstrated in multi-stacked devices that the engineering of the interface between the topmost electron-transporting organic semiconductor (e-OS) and the emission layer (EML) is crucial in optimizing the recombination of the minority charges (i.e., electrons) and to enhance EQE and brightness. Here, we introduce a new light-emitting conjugated polar polymer (CPP) in a multi-stacked OLET to improve the electron injection from e-OS to EML and to study, simultaneously, electroluminescence-related processes such as exciton formation and quenching processes. Interestingly, we observed that the highly polar groups present in the conjugate polymer induced polarization-related relevant charge-trapping phenomena with consequent modulation of the entire electrostatic field distribution and unexpected optoelectronic features. In view of the extensive use of CPPs in OLETs, the use of multifunctional CPPs for probing photophysical processes at the functional interfaces in stacked devices may speed up the improvement of the light-emission properties in OLETs.

Spectral properties and photophysical processes of triphenylamine-porphyrin-pyrimidine triads

Porphyrins have attracted considerable attention as an important family of photosensitizers used for dye-sensitized solar cells (DSSCs). In this work, a novel triphenylamine-porphyrin-pyrimidine triad with a triple bond linkage was synthesized and characterized. The sensitizer presents broad absorption in solution and on TiO2 films across the UV-Vis region benefiting from the extended conjugation. The acetenyl [Formula: see text]-conjugated bridge stimulates the photoinduced electron transfer (PET) process and brings a charge-separated state lifetime of around 4 μs. DSSC devices based on the sensitizer achieve a power conversion efficiency (PCE) of 3.77% thanks to the prolonged charge-separated state lifetime and suppressed charge recombination. The understanding of the structural, optical, and electrochemical properties of triphenylamine-porphyrin-pyrimidine triads provides new insights into the rational design of porphyrin sensitizers for various applications.

Uncovering the sensing mechanism of a zinc ion sensor: Fluorescence enhancement induced by the elimination of the TICT state

Precise detection of zinc ion is of fundamental importance in the fields of environment protection and food safety. A comprehensive understanding of the sensing mechanism will help to the design of such sensors. The detailed photophysical process of a zinc ion sensor as well as the sensing mechanism are uncovered with the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT). Both the ground state and first excited state potential energy surfaces (PES) of the sensor are carefully explored to reveal the photo-physical process of the sensor. Excited state intramolecular proton transfer (ESIPT) is observed on the S1 state PES. Then, the twist motion of C=N double bond is triggered after the ESIPT process, which leads to a twisted intramolecular charge transfer (TICT) state. This TICT state is found to make the sensor non-emissive. With the addition of Zn2+, the TICT state is eliminated which greatly enhances the fluorescence of the sensor and achieves zinc ion detection. The interaction of the sensor with Cd2+ and Hg2+ are also explored, which well explains the good selectivity of the sensor.