Steady-state Techniques Research Articles

Practical Aspects in the Study of Biological Photosensitization Including Reaction Mechanisms and Product Analyses: A Do's and Don'ts Guide†.

The interaction of light with natural matter leads to a plethora of photosensitized reactions. These reactions cause the degradation of biomolecules, such as DNA, lipids, proteins, being therefore detrimental to the living organisms, or they can also be beneficial by allowing the treatment of several diseases by photomedicine. Based on the molecular mechanistic understanding of the photosensitization reactions, we propose to classify them in four processes: oxygen-dependent (type I and type II processes) and oxygen-independent [triplet-triplet energy transfer (TTET) and photoadduct formation]. In here, these processes are discussed by considering a wide variety of approaches including time-resolved and steady-state techniques, together with solvent, quencher, and scavenger effects. The main aim of this survey is to provide a description of general techniques and approaches that can be used to investigate photosensitization reactions of biomolecules together with basic recommendations on good practices. Illustration of the suitability of these approaches is provided by the measurement of key biomarkers of singlet oxygen and one-electron oxidation reactions in both isolated and cellular DNA. Our work is an educational review that is mostly addressed to students and beginners.

Indirubin: Nature finding efficient light-activated protective molecular mechanisms

Indirubin (INR), a red structural isomer of the blue indigo (IND) shows, for an organic dye, a high stability towards light. The nature of this photostability is unknown and is here proposed to be linked to efficient dark processes dominating the excited state deactivation. As with IND, an excited state proton transfer (ESPT) process, together with the possibility of rotation around the central carbon-carbon bond (with formation of a syn-rotamer) are, solvent dependent, active molecular mechanisms of the decay pathways in INR. To equate this behaviour, INR, together with N-methyl-(MINR) and N,N′-dimethyl-indirubin (DMINR) have been synthetized and fully investigated from a combination of steady-state, fast-transient absorption and fluorescence techniques together with TDDFT electronic calculations. To rationalize the behaviour of INR in water, the water-soluble sulfonated indirubin (5SINR) and the tri-sulfonated indigo (3SIND) were further investigated. For INR in viscous solvents (e.g. glycerol) ESPT is present. With the studied indirubin derivatives (exception made to DMINR), an additional pathway, to ESPT, exists involving rotation between the two indole-like moieties, with a syn-conformer, leading to a more efficient radiationless deactivation pathway, when compared to indigo. The mechanism of syn-conformer formation is found to involve a longer lifetime in the viscous solvent. This new deactivation process is not present in IND. ESPT, found to be enhanced in water via intermolecular water assisted proton transfer, with a kinetic isotopic effect (KIE = kH/kD), equal to 2 for 3SIND, is also found present for 5SINR with a KIE = 1.5. The common structural origin of the N–H⋯OC bonds in the DNA nucleotides and the indigoid dyes (INR and IND), introduces a new perspective for the study of the molecular protective mechanisms in these molecules.

Ultrafast Spectroscopies of Nitrophenols and Nitrophenolates in Solution: From Electronic Dynamics and Vibrational Structures to Photochemical and Environmental Implications.

Nitrophenols are a group of small organic molecules with significant environmental implications from the atmosphere to waterways. In this work, we investigate a series of nitrophenols and nitrophenolates, with the contrasting ortho-, meta-, and para-substituted nitro group to the phenolic hydroxy or phenolate oxygen site (2/3/4NP or NP-), implementing a suite of steady-state and time-resolved spectroscopic techniques that include UV/Visible spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy with probe-dependent and global analysis, and femtosecond stimulated Raman spectroscopy (FSRS), aided by quantum calculations. The excitation-dependent (400 and 267 nm) electronic dynamics in water and methanol, for six protonated or deprotonated nitrophenol molecules (three regioisomers in each set), enable a systematic investigation of the excited-state dynamics of these functional "nanomachines" that can undergo nitro-group twisting (as a rotor), excited-state intramolecular or intermolecular proton transfer (donor-acceptor, ESIPT, or ESPT), solvation, and cooling (chromophore) events on molecular timescales. In particular, the meta-substituted compound 3NP or 3NP- exhibits the strongest charge-transfer character with FSRS signatures (e.g., C-N peak frequency), and thus, does not favor nitroaromatic twist in the excited state, while the ortho-substituted compound 2NP can undergo ESIPT in water and likely generate nitrous acid (HONO) after 267 nm excitation. The delineated mechanistic insights into the nitro-substituent-location-, protonation-, solvent-, and excitation-wavelength-dependent effects on nitrophenols, in conjunction with the ultraviolet-light-induced degradation of 2NP in water, substantiates an appealing discovery loop to characterize and engineer functional molecules for environmental applications.

Thermophysical properties of 1D materials: transient characterization down to atomic level

The thermophysical properties of 1D micro/nanoscale materials could differ significantly from those of their bulk counterparts due to intensive energy carrier scattering by structures. This work provides an in-depth review of cutting-edge techniques employed for transient characterization of thermophysical properties at the micro/nanoscale scale. In terms of transient excitation, step Joule heating, step laser heating, pulsed laser heating, and frequency domain amplitude-modulated laser heating are covered. For thermal probing, electrical and Raman scattering-based physical principles are used. These techniques enable the measurement of thermal conductivity, thermal diffusivity, and specific heat from the sub-mm level down to the atomic level (single-atom thickness). This review emphasizes the advantages of these techniques over steady state techniques and their physics, challenges, and potential applications, highlighting their significance in unraveling the intricate thermal transport phenomena to the atomic level of 1D materials.

Thermally activated delayed fluorescence in a mechanically soft charge-transfer complex: role of the locally excited state.

Molecular design for thermally activated delayed fluorescence (TADF) necessitates precise molecular geometric requirements along with definite electronic states to ensure high intersystem crossing (ISC) rate and photoluminescence quantum yield (PLQY). Achieving all these requirements synchronously while maintaining ease of synthesis and scalability is still challenging. To circumvent this, our strategy of combining a crystal engineering approach with basic molecular quantum mechanical principles appears promising. A holistic, non-covalent approach for achieving efficient TADF in crystalline materials with distinct mechanical properties is highlighted here. Charge transfer (CT) co-crystals of two carbazole-derived donors (ETC and DTBC) with an acceptor (TFDCNB) molecule are elaborated as a proof-of-concept. Using temperature-dependent steady-state and time-resolved photoluminescence techniques, we prove the need for a donor-centric triplet state (3LE) to ensure efficient TADF. Such intermediate states guarantee a naturally forbidden, energetically uphill reverse intersystem crossing (RISC) process, which is paramount for effective TADF. A unique single-crystal packing feature with isolated D-A-D trimeric units ensured minimal non-radiative exciton loss, leading to a high PLQY and displaying interesting mechanical plastic bending behaviour. Thus, a comprehensive approach involving a non-covalent strategy to circumvent the conflicting requirements of a small effective singlet-triplet energy offset and a high oscillator strength for efficient TADF emitters is achieved here.

Synthesis, characterization, luminescence properties and deciphering the role of a terpyridyl-imidazole based ligand in the dissimilar luminescence sensitization of ternary lanthanide(III) tris-(β-diketonate) complexes.

We designed four ternary lanthanide tris-(β-diketonate) complexes of the form [Ln(tta)3(tpy-HImzphen)], where Ln = LaIII, EuIII, SmIII and TbIII; tta = (2-theonyltrifluoroacetonate) and tpy-HImzphen = 2-(4-[2,2':6',2'']terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole. All the complexes have been thoroughly characterized by standard analytical tools and spectroscopic techniques including single crystal X-ray diffraction. In situ generation of the complexes was also monitored via absorption and emission spectroscopy upon incremental addition of the respective lanthanide precursor {Ln(tta)3(H2O)2} to the dichloromethane solution of the terpyridyl-imidazole ligand. The photophysical behaviors of all the complexes were thoroughly investigated via absorption and both steady-state and time-resolved emission spectroscopic techniques. The emission spectral measurements were carried out at both room temperature and 77 K to understand the deactivation dynamics of the excited states and elucidate the distinctive luminescence responses from the four lanthanide metal ions owing to the introduction of the terpyridyl-based ancillary ligand.

Fluorescence of Bimolecular Guanine Quadruplexes: From Femtoseconds to Nanoseconds

The paper deals with the fluorescence of guanine quadruplexes (G4) formed by association of two DNA strands d(GGGGTTTTGGGG) in the presence of K+ cations, noted as OXY/K+ in reference to the protozoon Oxytricha nova, whose telomere contains TTTTGGGG repeats. They were studied by steady-state and time-resolved techniques, time-correlated single photon counting, and fluorescence upconversion. The maximum of the OXY/K+ fluorescence spectrum is located at 334 nm, and the quantum yield is 5.8 × 10-4. About 75% of the photons are emitted before 100 ps and stem from ππ* states, possibly with a small contribution of charge transfer. Time-resolved fluorescence anisotropy measurements indicate that ultrafast (<330 fs) excitation transfer, due to internal conversion among exciton states, is more efficient in OXY/K+ compared to previously studied G4 structures. This is attributed to the arrangement of the peripheral thymines in two diagonal loops with restricted mobility, facilitating the interaction among them and with guanines. Thymines should also be responsible for a weak intensity excimer/exciplex emission band, peaking at 445 nm. Finally, the longest living fluorescence component (∼2.1 ns) is observed at the blue side of the spectrum. So far, high-energy long-lived emitting states had been reported only for double-stranded structures but not for G4.

State and Prospects of Unbalanced, Compositionally Symmetric Flow Battery Cycling and Steady-State Amperometry Techniques for Electrolyte Stability Assessment: The Case of Methyl Viologen

An increasing number of redox-active substances with improved electrochemical stability is currently being reported in the literature for potential applications in redox flow batteries (RFBs). Consequently, the demand for methods capable of estimating the long-term stability of redox couples is increasing as well. It seems to be of particular importance to understand the limitations of existing stability assessment techniques. In this work, two recently established and highly promising stability assessment techniques are directly compared for the first time to measure the capacity fade rate of a N,N′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen or MV) molecule as a benchmark anolyte substance. These two methods are the unbalanced, compositionally symmetric flow cell cycling (symmetric cycling or UCSFCC) and the steady-state amperometric state of health (ASOH) measurement. The results of the symmetric cycling indicate a 3-fold higher stability for MV compared to a previous literature report as well as a high standard deviation for the measured capacity fade rate. More comprehensive investigations of the technique revealed that the influence of the MV purity, electrolyte leakage, and membrane cross-over could not cause the difference between the measured and earlier reported capacity fades and, thus, indicated insufficiency of the technique’s accuracy for the measurement of highly stable redox species. ASOH as the second technique exhibited similar capacity fade rates like the symmetric cycling as well as comparably high standard deviations. Furthermore, it is prone to temperature fluctuations and variations of the electrode radius. Thus, we demonstrate an approach for a temperature correction in this method, which enables significantly higher accuracy and reliability. Despite this, the variation of the electrode radius remains the main concern for this technique. Based on the obtained results for the MV anolyte, the state and prospects of both techniques for the stability assessment in RFB research are discussed in detail.

To twist or not to twist: From chromophore structure to dynamics inside engineered photoconvertible and photoswitchable fluorescent proteins.

Green-to-red photoconvertible fluorescent proteins (FPs) are vital biomimetic tools for powerful techniques such as super-resolution imaging. A unique Kaede-type FP named the least evolved ancestor (LEA) enables delineation of the evolutionary step to acquire photoconversion capability from the ancestral green fluorescent protein (GFP). A key residue, Ala69, was identified through several steady-state and time-resolved spectroscopic techniques that allows LEA to effectively photoswitch and enhance the green-to-red photoconversion. However, the inner workings of this functional protein have remained elusive due to practical challenges of capturing the photoexcited chromophore motions in real time. Here, we implemented femtosecond stimulated Raman spectroscopy and transient absorption on LEA-A69T, aided by relevant crystal structures and control FPs, revealing that Thr69 promotes a stronger π-π stacking interaction between the chromophore phenolate (P-)ring and His193 in FP mutants that cannot photoconvert or photoswitch. Characteristic time constants of ~60-67 ps are attributed to P-ring twist as the onset for photoswitching in LEA (major) and LEA-A69T (minor) with photoconversion capability, different from ~16/29 ps in correlation with the Gln62/His62 side-chain twist in ALL-GFP/ALL-Q62H, indicative of the light-induced conformational relaxation preferences in various local environments. A minor subpopulation of LEA-A69T capable of positive photoswitching was revealed by time-resolved electronic spectroscopies with targeted light irradiation wavelengths. The unveiled chromophore structure and dynamics inside engineered FPs in an aqueous buffer solution can be generalized to improve other green-to-red photoconvertible FPs from the bottom up for deeper biophysics with molecular biology insights and powerful bioimaging advances.

Unifying the Transformer Current in Multiple Phase Modulation Without Current Spike During Load Transients

To accelerate the dynamic response in a dual active bridge converter, feed-forward control can be applied in parallel to the conventional PI controller for closed-loop control. The transformer current thus changes significantly due to the phase shift change. A current spike can appear during load transients, particularly when using multiple phase shift modulation. Effort has been made in the previous literature to implement active compensation between two different steady-state operations to eliminate the transformer current spike; however, this results in a complicated control structure. This letter thus proposes a novel modulation method unifying the transformer current for dual phase shift and triple phase shift modulation to mitigate the transformer current spike when switching among various phase shift controls during load transients. By applying the proposed pulsewidth modulation strategy, the instantaneous value of the transformer current stays the same at the beginning of the switching period even with different steady-state modulation techniques. Also, full-operation-range zero-voltage switching can be realized for the primary side or the secondary side switches by combining with the proposed modulation strategy. An experimental prototype demonstration validates the proposed modulation strategy.

Evaluation of methods to accurately characterize the thermal conductivity of micro-and nanocellular polymers based on poly(methyl-methacrylate) (PMMA) produced at lab-scale

The characterization of the thermal conductivity of new and enhanced thermal insulators developed at lab-scale is a challenge. The small dimensions of the prototypes make impossible the use of the conventional techniques because steady-state methods require large samples. Furthermore, the accuracy of transient methods to measure the thermal conductivity is not clear. In this work, we compare four different approaches to measure the thermal conductivity of small prototypes of nanocellular poly(methyl-methacrylate) (PMMA). Both steady-state and transient techniques are used. Results show that the transient plane source method is not suitable for the characterization of these materials (the deviation from the steady-state methods is on average higher than 15%). In addition, two different approaches for measuring the thermal conductivity of small samples via a steady-state technique are proposed and validated.

Lipid composition dependent binding of apolipoprotein E signal peptide: Importance of membrane cholesterol in protein trafficking

Soluble secretory and membrane proteins contain a short stretch of signal peptide (SP) at their N-terminal end, which gets cleaved after reaching the destination organelle. However, the importance of SP in protein trafficking is not fully understood. The lipid compositions of cellular organelles are highly heterogeneous, and the preference of SP toward a particular lipid composition might play a key role in unidirectional trafficking of protein. In order to understand the preference of Apolipoprotein E (ApoE) toward endoplasmic reticulum (ER), we have studied the interaction of its SP with membranes of varying lipid compositions. The importance of cholesterol is paramount as subcellular organelles contain differential amount of cholesterol; endoplasmic reticulum (ER) contains the least amount of cholesterol. We have utilized batteries of steady-state and time-resolved fluorescence techniques to understand the affinity of ApoE signal peptide toward membranes of varying lipid compositions. We observed that the ApoE signal peptide binds tightly with membranes devoid of cholesterol, and binding affinity reduces with increasing concentration of membrane cholesterol. Our results clearly suggest the importance of membrane composition in the unidirectional movement of ApoE toward ER. This property of SP can further be utilized for the development of organelle specific cargo delivery.

Tracking the transient: Real-time exploration of electromaterials by dynamic single energy X-ray spectroelectrochemistry

Electrochemical characterization of an electromaterial normally combines steady-state and dynamic analysis techniques to identify and parameterize key charge–transfer processes along with coupled material transformations and chemical/catalytic reactions. On the other hand, highly informative complementary characterization methods, such as in situ X-ray spectroscopy, are most commonly applied in a potentiostatic mode, probing only quasi-stabilized states of the material. While being of immense importance to the field, such approaches provide very limited information on dynamic processes and their kinetics. This limitation can be resolved by combining the steady-state analysis with Dynamic Single Energy X-ray Spectroelectrochemistry – a technique of focus herein. The manuscript outlines the basic principles of this method, identifies the scope of its applications, and provides an overview of its progressively increasing use in in situ / operando research of electromaterials. • Dynamic spectroelectrochemistry is highly complementary with steady-state analysis. • Dynamic in situ XAS probes redox transformation kinetics in element-selective fashion. • Surface sensitivity and time resolution of the method can be increased in the future.

Capillary effects on curved solid–liquid interfaces: An overview

This overview begins with observations of capillary-mediated effects on crystal-melt interfaces in microgravity and attempts to interpret them using the LeChâtelier–Braun effect and Kelvin’s equation—both local equilibrium requirements applicable at curved interfaces. Numerical studies of interfacial kinetics followed, using Greens function distributions that simulated evolving sharp solid–liquid interfaces undergoing pattern formation. Those methods exposed a need for more incisive steady-state techniques. Grain boundary grooves were used as constrained, stationary, microstructures. Their steady-state profiles derive from variational calculus and were analyzed for their imputed Gibbs–Thomson thermopotentials for comparative thermodynamic studies. Variational profiles, however, have unrealistic zero-thickness transitions between phases, and thus lack fluxes of energy or solute that occur on real interfaces. The exact formulation of variational profiles, however, advantageously supports field-theoretic calculations of their first-order formation free energy, thermodynamic stability, capillary-mediated chemical potentials, interfacial gradients and scalar divergences. These linked fields all depend on an interface’s curvature distribution, i.e., its geometry, but others, for example tangential fluxes of energy and solute, also depend on interface thickness and structure, i.e., thermodynamics. We comparatively analyzed capillary-mediated fields up to 4th-order, including the surface Laplacian of a profile’s chemical potential. This Laplacian is proportional to scaled divergences of fluxes that appear on counterpart real or simulated microstructures with congruent shapes. Divergent energy fluxes manifest as cooling distributions, which cause depression of the thermochemical potential measured along diffuse-interfaces simulated with phase-field. Cooling distributions are visualized to explain qualitative and quantitative features of a microstructure’s steady-state thermal maps. Evidence is included of how thickness and shape of crystal-melt interfaces co-determine whether, and to what extent, interfacial transport occurs. Understanding the origin and actions of interfacial capillary fields might offer improved control of microstructure at mesoscopic levels, accessible with these deterministic fields through physical and chemical means.

Impact of the Anthryl Linking Mode on the Photophysics and Excited-State Dynamics of Re(I) Complexes [ReCl(CO)3(4'-An-terpy-κ2N)

Rhenium(I) complexes with 2,2′:6′,2″-terpyridines (terpy) substituted with 9-anthryl (1) and 2-anthryl (2) were synthesized, and the impact of the anthryl linking mode on the ground- and excited-state properties of resulting complexes [ReCl(CO)3(4′-An-terpy-κ2N)] (An—anthryl) was investigated using a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. Different attachment positions of anthracene modify the overlap between the molecular orbitals and thus the electronic coupling of the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores. Following the femtosecond transient absorption, the lowest triplet excited state of both complexes was found to be localized on the anthracene chromophore. The striking difference between 1 and 2 concerns the triplet-state formation dynamics. A more planar geometry of 2-anthryl-terpy (2), and thus better electronic communication between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores, facilitates the formation of the 3An triplet state. In steady-state photoluminescence spectra, the population ratio of 3MLCT and 3An was found to be dependent not only on the anthryl linking mode but also on solvent polarity and excitation wavelengths. In dimethyl sulfoxide (DMSO), compounds 1 and 2 excited with λexc > 410 nm show both 3MLCT and 3An emissions, which are rarely observed. Additionally, the abilities of the designed complexes for 1O2 generation and light emission under the external voltage were preliminary examined.

Sensing lysozyme fibrils by salicylaldimine substituted BODIPY dyes - A correlation with molecular structure

Quick and efficient detection of protein fibrils has enormous impact on the diagnosis and treatment of amyloid related neurological diseases. Among several methods, fluorescence based techniques have garnered most importance in the detection of amyloid fibrils due to its high sensitivity and extreme simplicity. Among other classes of molecular probes, BODIPY derivatives have been employed extensively for the detection of amyloid fibrils. However, there are very few studies on the relationship between the molecular structure of BODIPY dyes and their amyloid sensing activity. Here in a BODIPY based salicylaldimine Schiff base and its corresponding boron complex have been evaluated for their ability to sense amyloid fibrils from hen-egg white lysozyme using steady state and time-resolved spectroscopic techniques. Both dyes show fluorescence enhancement as well as increase in their excited state lifetime upon their binding with lysozyme fibrils. However, the BODIPY derivative which shows more emission enhancement in fibrillar solution has much lower affinity towards amyloid fibrils as compared to other derivative. This contrasting behaviour in the emission enhancement and binding affinity has been explained on the basis of differences in their photophysical properties in water and amyloid fibril originating from the difference in their molecular structure. Such correlation between the amyloid sensitivity and the molecular structure of the probe can open up a new strategy for designing new efficient amyloid probes.

Modulating Resonance Energy Transfer with Supramolecular Control in a Layered Hybrid Perovskite and Chromium Photosensitizer Assembly.

Recently, the low-dimensional organic-inorganic halide perovskites (OIHP) have been exploited heavily for their favorable exciton dynamics, broad-band emission, remarkable stability, and tunable band-edge excited-state energy compared to their 3D counterparts for potential optoelectronic applications. Low-dimensional perovskites are generally good candidates for utilization as room-temperature photoluminescence (PL) materials. Further, doping divalent transition metals like Mn2+ into OIHP is expected to introduce a 4T1-6A1-based low-energy luminescence emission around 600 nm; an optical property that is favorable for biomedical optoelectronics. Doping Mn2+ in the perovskite lattice is also expected to induce the generation of cytotoxic singlet oxygen species (1O2), a ROS that is being exploited for various therapeutic applications. To integrate these optical and therapeutic properties of a 2D (PEA)2PbBr4 (Pb PeV; PEA = phenylethylammonium cation) perovskite alloyed with Mn2+ ions (Mn:PbPeV) and the option for a photoinduced energy transfer process involving a Cr(III)-based 1O2 generating photosensitizer (CrPS), we designed a unique purpose-built nanoassembly (Mn:PbPeV@PCD) using the encapsulation properties of a water-soluble polymer derived from β-cyclodextrin (PCD). Here the PCD is observed to modulate the classical internal energy transfer of Pb2+ exciton to alloyed Mn2+ orange emission, resulting in the emergence of a new blue emission. The addition of CrPS into the Mn:PbPeV@PCD to generate the CrPS@Mn:PbPeV@PCD assembly results in restoring perovskite luminescence followed by the external energy transfer to CrPS. We have elucidated the mechanism of these cascade energy transfer processes between multiple components using steady-state and time-resolved luminescence techniques. Efficient ROS generation and its potential to induce an oxidation reaction of a biomolecule are realized using guanine as the target molecule. Further photoinduced cleavage studies with biomolecules confirmed the efficacy of the nanoassembly in inducing the cleavage of guanine-rich DNA. The study opens up a new direction in the field of perovskite for biomedical applications.

Effect of halide ions on the fluorescence properties of 3-aminoquinoline in aqueous medium.

Fluorescence (FL) quenching of 3-aminoquinoline (3AQ) by halide ions has been explored in an aqueous acidic medium using the steady-state and time-domain FL measurement techniques. The halide ions showed no significant change in the absorption spectra of 3AQ in an aqueous acidic medium. The FL intensity was strongly quenched by ions and the order of FL quenching by halide ions was . The decrease in FL lifetime along with the reduction in FL intensity of 3AQ suggested the dynamic nature of quenching. The obtained values were 328 for ions and 119 for ions and the values were ~ and , respectively. The observations suggested that the likely governing mechanism for FL quenching may be an electron transfer process and the involvement of the heavy atom effects.

Fundamental Charge Transfer Dynamics in 2D TMDCs for Use in Novel Heterostructures

Efficient transfer of charge carriers and/or excitons between small organic molecules and two-dimensional (2D) semiconductor interfaces is being explored for applications in photovoltaics and quantum information processing.Interfaces of small molecules and 2D semiconductors such as graphene, nanocrystals, quantum dots and transition metal dichalcogenides (TMDCs) are capable of charge, energy, and spin singlet transfer. The long excited state lifetimes of spin triplet excitons makes triplet exciton transfer across such interfaces advantageous for exciton harvesting and photon upconversion. However, to date, triplet energy transfer between physisorbed small molecules and monolayer TMDCs has only been reported in a single study. To our knowledge, the process where photoexcitation of a TMDC/organic interface yields triplet excitons in the TMDC layer has not been reported. In quantum dot (QD) systems, direct covalent attachment of organic molecules to QD surfaces has shown to facilitate triplet energy transfer across the QD/molecule interface, but this covalent approach has not been widely applied to TMDC/molecule interfaces.Here we present a systematic study of covalently functionalized TMDC/molecule interfaces employing synthetically tailored thiolated acenes. We create tunable amounts of sulfur vacancies, which allows for subsequent covalent acene functionalization. We study the impact of covalent bonding on the charge transfer (CT) dynamics of TMDC/organic interfaces with energy level offsets suitable for charge and energy transfer (ET) as well as triplet acceptance and sensitization. We characterize the interfaces with a variety of steady-state and time-resolved spectroscopic techniques and initial results show success in selective isolation of CT vs ET pathways. Figure 1

Ultrafast Charge-Transfer in Panchromatic, Push-Pull Porphyrin-Tetracyanobuta-1,3-Diene-Donor Conjugates

Light-induced electron transfer is a key step in solar energy conversion, and a process with myriad relevance in the fields of artificial photosynthesis, photocatalysis, photoconductivity, and molecular photovoltaics. In a donor-acceptor, D-A system, light absorption by a chromophore results in the formation of an excited state species, followed by a charge transfer between an electron donor (D) and an electron acceptor (A) leading to a charge-separated D .+ -A .- species.The optimization of these processes and comprehending their mutual interplay to ultimately achieve charge-separated species is of paramount importance for the development of efficient systems in solar light conversion schemes. As part of our continued effort in this field of research,1-3 using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H2P and ZnP) are decorated at their b-pyrrole positions with strong charge-transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-DONOR as a novel class of push-pull systems.4 The physicochemical properties of these compounds have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin p-system extended the absorption well into the near-infrared region. The occurrence of photoinduced charge transfer and separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. The role of central metal in the porphyrin cavity and the nature of the donor in governing these photo events will be discussed. Khan, Y. Jang, Y. Patil, R. Misra, and F. D’Souza, Photoinduced Charge Separation Prompted Intervalence Charge Transfer in a Bis(thienyl)diketopyrrolopyrrole Bridged Donor-TCBD Push-Pull System (hot article), Angew. Chem. Int. Ed. 2021, 60, 20518–20527.S. Yadav, A. Z. Alsaleh,R. Misra and F. D’Souza, ‘Excited State Charge Transfer Leading to Charge Stabilization via an Electron Exchange Mechanism in Symmetric, Central Triphenylamine Derived, Dimethylaminophenyl-Tetracyanobutadiene Donor-Acceptor Conjugates, Chem. Sci. 2021, 12, 1109-1120.Pinjari, A. Z. Alsaleh,Y. Patil, R. Misra and F. D’Souza, Interfacing High-Energy Charge Transfer States to a Near-IR Sensitizer for Efficient Electron Transfer upon Near-IR Irradiation’ Angew. Chem. Int. Ed. 2020, 59, 23697-23705.Sekaran, A. Dawson, Y. Jang, K. V. MohanSingh, R. Misra and F. D’Souza, Charge-Transfer in Panchromatic Porphyrin-Tetracyanobuta-1,3-diene-Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process, Chem. Eur. J. 2021, 27, 14335-14344 Figure 1