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One-state and two-state lasing is investigated experimentally and through numerical simulation as a function of temperature in microdisk lasers with Stranski–Krastanow InAs/InGaAs/GaAs quantum dots. Near room temperature, the temperature-induced increment of the ground-state threshold current density is relatively weak and can be described by a characteristic temperature of about 150 K. At elevated temperatures, a faster (super-exponential) increase in the threshold current density is observed. Meanwhile, the current density corresponding to the onset of two-state lasing was found to decrease with increasing temperature, so that the interval of current density of pure one-state lasing becomes narrower with the temperature increase. Above a certain critical temperature, ground-state lasing completely disappears. This critical temperature drops from 107 to 37 °C as the microdisk diameter decreases from 28 to 20 μm. In microdisks with a diameter of 9 μm, a temperature-induced jump in the lasing wavelength from the first excited-state to second excited-state optical transition is observed. A model describing the system of rate equations and free carrier absorption dependent on the reservoir population provides a satisfactory agreement with experimental results. The temperature and threshold current corresponding to the quenching of ground-state lasing can be well approximated by linear functions of saturated gain and output loss.

Vision is initiated by the rhodopsin family of light-sensitive G protein-coupled receptors (GPCRs). A photon is absorbed by the 11-cis retinal chromophore of rhodopsin, which isomerizes within 200 femtoseconds to the all-trans conformation, thereby initiating the cellular signal transduction processes that ultimately lead to vision. However, the intramolecular mechanism by which the photoactivated retinal induces the activation events inside rhodopsin remains experimentally unclear. Here we use ultrafast time-resolved crystallography at room temperature to determine how an isomerized twisted all-trans retinal stores the photon energy that is required to initiate the protein conformational changes associated with the formation of the G protein-binding signalling state. The distorted retinal at a 1-ps time delay after photoactivation has pulled away from half of its numerous interactions with its binding pocket, and the excess of the photon energy is released through an anisotropic protein breathing motion in the direction of the extracellular space. Notably, the very early structural motions in the protein side chains of rhodopsin appear in regions that are involved in later stages of the conserved class A GPCR activation mechanism. Our study sheds light on the earliest stages of vision in vertebrates and points to fundamental aspects of the molecular mechanisms of agonist-mediated GPCR activation., 視覚に関わるタンパク質の超高速分子動画 --薄暗いところで光を感じる仕組み--. 京都大学プレスリリース. 2023-03-23.

Recombination and regeneration dynamics in Fe-NHC-sensitized DSSCs revealed incomplete injection and the detrimental effect of photoinjected electron recapture by the I3-form of the redox electrolyte on performance. Importantly, the use of additives in the electrolyte allowed the best efficiency ever recorded for an iron-based DSSC to be reached.

Near-term quantum computers are expected to facilitate material and chemical research through accurate molecular simulations. Several developments have already shown that accurate ground-state energies for small molecules can be evaluated on present-day quantum devices. Although electronically excited states play a vital role in chemical processes and applications, the search for a reliable and practical approach for routine excited-state calculations on near-term quantum devices is ongoing. Inspired by excited-state methods developed for the unitary coupled-cluster theory in quantum chemistry, we present an equation-of-motion-based method to compute excitation energies following the variational quantum eigensolver algorithm for ground-state calculations on a quantum computer. We perform numerical simulations on H-2, H-4, H2O, and LiH molecules to test our quantum self-consistent equation-of-motion (q-sc-EOM) method and compare it to other current state-of-the-art methods. q-sc-EOM makes use of self-consistent operators to satisfy the vacuum annihilation condition, a critical property for accurate calculations. It provides real and size-intensive energy differences corresponding to vertical excitation energies, ionization potentials and electron affinities. We also find that q-sc-EOM is more suitable for implementation on NISQ devices as it is expected to be more resilient to noise compared with the currently available methods. Department of Energy (DOE) [DE-SC0019199]; NSF [1839136]; Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory (LANL) [20200056DR]; National Nuclear Security Administration of the U.S. Department of Energy [89233218CNA000001] Published version This research was supported by the Department of Energy (DOE). E. B. and N. J. M. acknowledge Award No. DE-SC0019199, and S. E. E. acknowledges support from the NSF, award number 1839136. A. A. would like to thank Dr Luke Bertels for helpful discussions. The authors thank the Advanced Research Computing (ARC) facility at Virginia Tech for the computational infrastructure. A. K., Y. Z., L. C., S. T., and P. A. D. are thankful for the support from the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory (LANL) under project number 20200056DR. LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (contract no. 89233218CNA000001).

The reactions of labile [Re(diimine)(CO)3(H2O)]+ precursors (diimine = 2,2′-bipyridine, bpy; 1,10-phenanthroline, phen) with dicyanoargentate anion produce the dirhenium cyanide-bridged compounds [{Re(diimine)(CO)3}2CN)]+ (1 and 2). Substitution of the axial carbonyl ligands in 2 for triphenylphosphine gives the derivative [{Re(phen)(CO)2(PPh3)}2CN]+ (3), while the employment of a neutral metalloligand [Au(PPh3)(CN)] affords heterobimetallic complex [{Re(phen)(CO)3}NCAu(PPh3)]+ (4). Furthermore, the utilization of [Au(CN)2]−, [Pt(CN)4]2–, and [Fe(CN)6]4–/3– cyanometallates leads to the higher nuclearity aggregates [{Re(diimine)(CO)3NC}xM]m+ (M = Au, x = 2, 5 and 6; Pt, x = 4, 7 and 8; Fe, x = 6, 9 and 10). All novel compounds were characterized crystallographically. Assemblies 1–8 are phosphorescent both in solution and in the solid state; according to the DFT analysis, the optical properties are mainly associated with charge transfer from Re tricarbonyl motif to the diimine fragment. The energy of this p...

A data survey on experimental fluorescence quantum yields of (multi)substituted dicyano-distyrylbenzenes in fluid solution evidences that non-radiative decay increases with the Franck-Condon energy (E-FC), being opposite to the conventional energy gap law. Quantum-chemistry indicates that this is controlled by access to the conical intersection (CI) following the Bell-Evans-Polanyi principle as a first-step approximation for this family of molecules; the variations in E-FC among the different compounds are found to be decisive, while those of E-CI are estimated to be weaker or even enhancing the effect. The current findings may have significant consequences for the design of molecules for organic solid state emitters.

In this article we revisit our recent picosecond and femtosecond X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) experiments, probing the ultrafast electronic and geometric evolution of photoexcited haem proteins, namely ferrous Nitrosyl Myoglobin (MbNO) and ferric Cytochrome c (Cyt c). We show through these two examples, combined with results from ultrafast optical spectroscopy, the universal behavior of the excited state dynamics of ferric and ferrous complexes. Regardless of the type of ligand, its dissociation or lack thereof, or the metal oxidation state, the photoexcited system relaxes through a cascade of excited spin states leading to formation of a high spin state, which in the case of the haem is a domed porphyrin.

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.

Different perylene-3,4:9,10-bis(dicarboximide) (abbreviated as perylenebisimides, or simply PBI) compounds (MT-PBI, DT-PBI, MP-PBI and DP-PBI) with thienyl and phenyl substituents at the bay position were prepared to study the effect of substitution on the photophysical properties, especially the intersystem crossing (ISC) efficacy. We found that the derivative with a monothienyl substituent (MT-PBI) shows efficient ISC (singlet oxygen quantum yield ΦΔ = 72%) and longer triplet state lifetime (τT = 64.4 μs) as compared to reference compounds (derivative with monophenyl substituent at bay position, MP-PBI, ΦΔ = 23% and τT = 48.5 μs). Theoretical computations reveal that the ISC efficiency is dependent on the magnitude of the spin orbit couplings (SOC) between the singlet and triplet excited states, and the SOC matrix element is up to 2 cm−1 for compound showing the highest ISC efficiency. Femtosecond transient absorption spectra show the ISC rate constant of 8 × 109 s−1 for MT-PBI, and no charge transfer exists. Photo-driven electron transfer was observed with the PBI derivatives in the presence of sacrificial electron donor triethanolamine (TEOA), and reversible formation of PBI radical anion (PBI−•) was observed, which shows absorption in the range of 600 nm–800 nm and 900 nm–1000 nm. Delayed fluorescence (P-type) in MT-PBI was also observed (τT = 31.0 μs). These finding are useful for the design of PBI-derived heavy atom-free compounds to be used as triplet photosensitizers.

To develop new and flexible CuI containing luminescent substances, we extend our previous investigations on two metal‐centered species to four metal‐centered complexes. These complexes could be a basis for designing new organic light‐emitting diode (OLED) relevant species. Both the synthesis and in‐depth spectroscopic analysis, combined with high‐level theoretical calculations are presented on a series of tetranuclear CuI complexes with a halide containing Cu4X4 core (X=iodide, bromide or chloride) and two 2‐(diphenylphosphino)pyridine bridging ligands with a methyl group in para (4‐Me) or ortho (6‐Me) position of the pyridine ring. The structure of the electronic ground state is characterized by X‐ray diffraction, NMR, and IR spectroscopy with the support of theoretical calculations. In contrast to the para system, the complexes with ortho‐substituted bridging ligands show a remarkable and reversible temperature‐dependent dual phosphorescence. Here, we combine for the first time the luminescence thermochromism with time‐resolved FTIR spectroscopy. Thus, we receive experimental data on the structures of the two triplet states involved in the luminescence thermochromism. The transient IR spectra of the underlying triplet metal/halide‐to‐ligand charge transfer (3 m/XLCT) and cluster‐centered (3CC) states were obtained and interpreted by comparison with calculated vibrational spectra. The systematic and significant dependence of the bridging halides was analyzed., Highly luminescent! The synthesis and an in‐depth photophysical characterization of highly luminescent tetranuclear CuI complexes are presented. For these promising candidates for organic light‐emitting diodes, a pronounced luminescence thermochromism with clearly separated emission bands is observed. The corresponding excited state structures are determined by applying temperature‐dependent time‐resolved step‐scan FTIR spectroscopy in combination with DFT calculations.

Chemistry - a European journal 27(38), 9905 - 9918 (2021). doi:10.1002/chem.202100766, A new base metal iron-cobalt dyad has been obtained by connection between a heteroleptic tetra-NHC iron(II) photosensitizer combining a 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine with 2,6-bis(3-methyl-imidazol-2-ylidene)-4,4���-bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)-cobalt(III) assembly has been extensively characterized by ground- and excited-state methods like X-ray crystallography, X-ray absorption spectroscopy, (spectro-)electrochemistry, and steady-state and time-resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited-state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal-to-ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the $^{3}$MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a $^{3}$MC state. Attachment of cobaloxime to the iron photosensitizer increases the $^{3}$MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two-component system, consisting of the iron photosensitizer and Co(dmgH)$_2$(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems., Published by Wiley-VCH, Weinheim

Excited states in the extremely neutron-deficient nucleus Pt-172 were populated via Ru-96(Kr-78, 2p) and Mo-92(Kr-83, 3n) reactions. The level scheme has been extended up to an excitation energy of approximate to 5MeV and tentative spin-parity assignments up to I-pi = 18(+). Linear polarization and angular distribution measurements were used to determine the electromagnetic E1 character of the dipole transitions connecting the positive-parity ground-state band with an excited side-band, firmly establishing it as a negativeparity band. The lowestmember of this negative-parity structure was firmly assigned spin-parity 3(-). In addition, we observed an E3 transition from this 3(-) state to the ground state, providing direct evidence for octupole collectivity in Pt-172. Large-scale shell model (LSSM) and total Routhian surface (TRS) calculations have been performed, supporting the interpretation of the 3(-) state as a collective octupole-vibrational state.

The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centered around 1160~\AA\/ in the blue wing of the Lyman-$\alpha$ line. It is extremely apparent in WD 1425+540 recently observed with HST COS. With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman-$\alpha$ seen in this and other white dwarf spectra. Our previous work predicting this absorption feature was limited by molecular potentials that were not adequate to follow the atomic interactions with spectroscopic precision to the asymptotic limit of large separation. A new set of potential energy curves and electronic dipole transition moments for the lowest electronic states of the H--He system were developed to account accurately for the behavior of the atomic interactions at all distances, from the chemical regime within 1~\AA\/ out to where the radiating H atoms are not significantly perturbed by their neighbors. We use a general unified theory of collision-broadened atomic spectral lines to describe a rigorous treatment of hydrogen Lyman-$\alpha$ with these potentials and present a new study of its broadening by radiative collisions of hydrogen and neutral helium. These results enable ab initio modeling of radiative transport in DBA white dwarf atmospheres., Comment: 9 pages, 7 figures

In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials.

Perylenebisimide-carbazole (PBI–Cz) dyads were prepared to study charge-recombination (CR) induced intersystem crossing (ISC) in electron donor/acceptor dyads. The distance and the mutual orientation of the perylenebisimide (PBI) and carbazole (Cz) moieties were varied to study their effect on photophysical properties. Steady state and time-resolved optical spectroscopies show that electronic coupling between the electron donor and acceptor is negligible at the ground state. The fluorescence of the PBI moiety is strongly quenched in the dyads, and a larger separation between the donor and the acceptor results in less fluorescence quenching. The Gibbs free energy changes of the electron transfer and the energy level of the charge transfer state were studied using the electrochemical and optical spectra data. The singlet oxygen quantum yields (ΦΔ) are up to 72% for dyads with the shortest separation between the donor and the acceptor. Nanosecond transient absorption spectra confirmed the formation of the PBI-localized long lived triplet state (the lifetime is up to 190 μs). Notably non-orthogonal dyads show efficient spin orbit charge transfer (SOCT-ISC), which is different from the previously proposed orthogonal molecular structure for SOCT-ISC. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that all three dyads give the same electron spin polarization of eae/aea, and thus the radical pair ISC (RP ISC) mechanism is excluded, which is different from the previously reported PBI-phenothiazine analogues. Efficient and reversible transformation of dyads to their radical anion was observed in the presence of sacrificial electron donor triethanolamine in an inert atmosphere with photo-irradiation.

Measuring viscosity and temperature on the microscale is a challening yet very important task, in materials sciences and in biology alike. In this perpsective we review and discuss fluorescent microviscosity sensors, termed ‘molecular rotors’, that offer a convenient way of measuring microscopic viscosity and sometimes may even be used to measure microscopic temperature in addition to viscosity. We discuss how temperature in combination with various solvent properties can affect microviscosity measurements and we review possible action mechanisms that make molecular rotors sensitive to multiple parameters of their environment. Overall, we reveal a complicated, yet exciting, behaviour of molecular rotors at different viscosity, temperature and solvent properties on the microscale and how this behaviour can be explained and exploited.

A detailed microscopic study of the local geometry was performed for the cation sites in fluoroelpasolite Cs2NaYF6 crystal doped with 3d ions (Cr3+ and Fe3+). Electronic structure of pure, Crdoped and Fe-doped Cs2NaYF6 has been studied by determining various properties: substitution energies of dopant ions, ionic Mulliken charges, atomic coordinates and unit cell parameters for the optimized structures. Also, the optimized local geometry of all three cation sites before and after the possible dopant ions substitutions has been deduced by DFT calculations. It is shown that the local symmetry of both Y3+ and Na+ sites maintains its cubic form after both Fe and Cr substitution; however, Fe ions lead quite larger distortion on ligand lengths at Y-site. The optimized structure has been used independently to calculate zero-field splitting parameters (ZFSPs) and crystal field parameters (CFPs) of both Cr3+ and Fe3+ centers. The calculated ZFSPs of Fe(3+ )ions indicate that the results predicted by SPM for both Y3+ site and Na+ site agree well with the previous experimental observations of ZFSPs. It can be inferred from the findings regarding both electronic structure and local optimized geometry that substitution of both Cr3+ and Fe3+ ions for Y3+ is more likely in C(s)2NaYF(6). (C) 2019 Elsevier Ltd. All rights reserved.

The first coordination sphere of Ir(III) 2,2′-bipyridine / 2-phenylpyridine complexes can be tuned to achieve either C– or N–chelation in ratios that range between 0/6 and 3/3. Of particular interest is the synthesis of Ir(III) complexes bearing a 2,2′-bipyridine ligand coordinated in a N,C3 pattern, leaving an exposed pyridine moiety, accessible for acid-base chemistry or coordination to a second transition metal center. The protonated forms of these “rolled-over” Ir(III) complexes were isolated in a straight-forward procedure using trifluoroacetic acid. The photophysical, photochemical and electrochemical properties of both the protonated and unprotonated Ir(III) complexes were investigated by steady-state and time-resolved spectroscopies, as well as by density functional theory calculations. The nature of the excited states was shown to depend on both the ligand coordination pattern and protonation state of the complex. In addition, the unprotonated and protonated analogues were efficiently quenched by hydroquinone and benzoquinone in acetonitrile with quenching rate constants close to the solvent diffusion limit. The results presented herein have direct implications for proton sensitive photoredox chemistry and the development of photo-acids and photo-bases.

[EN] Through-bond triplet exciplex formation in donor-acceptor systems linked through a rigid bile acid scaffold has been demonstrated on the basis of kinetic evidence upon population of the triplet acceptors (naphthalene, or biphenyl) by through-bond triplet-triplet energy transfer from benzophenone., Financial support from the Spanish Government (Grants SEV-2012-0267, CTQ2012-38754-C03-03, CTQ2013-47872-C2-1-P and JCI-2011-09926), EU (PCIG12GA-2012-334257), Generalitat Valenciana (Prometeo Program), and Technical University of Valencia (VLC/Campus, ASIC-UPV for computational facilities and Predoctoral FPI fellowship for P. Miro) is gratefully acknowledged.

A long-lived hot carrier population is critical in order to develop working hot carrier photovoltaic devices with efficiencies exceeding the Shockley–Queisser limit. Here, we report photoluminescence from hot-carriers with unexpectedly long lifetime (a few ns) in formamidinium tin triiodide. An unusual large blue shift of the time-integrated photoluminescence with increasing excitation power (150 meV at 24 K and 75 meV at 293 K) is displayed. On the basis of the analysis of energy-resolved and time-resolved photoluminescence, we posit that these phenomena are associated with slow hot carrier relaxation and state-filling of band edge states. These observations are both important for our understanding of lead-free hybrid perovskites and for an eventual future development of efficient lead-free perovskite photovoltaics., Hot carriers have excess energy that could be used to enhance solar cell efficiencies if their lifetimes are sufficiently long. Here, Fang et al. observe nanosecond hot carrier lifetime by photoluminescence in formamidinium tin triiodide perovskites which is 1000 times higher than lead based perovskites.

The challenges of simulating extreme ultraviolet (XUV)-induced dissociation dynamics of organic molecules on a multitude of coupled potential energy surfaces are discussed for the prototypical photoionization of benzene. The prospects of Koopmans' theorem-based electronic structure calculations in combination with classical trajectories and Tully's fewest switches surface hopping are explored. It is found that a Koopmans' theorem-based approach overestimates the CH dissociation barrier and thus underestimates the fragmentation yield. However, the nonadiabatic population dynamics are in good agreement with previous approaches, indicating that the Koopmans' theorem based potentials are well described around the Franck-Condon point. This is explicitly tested for the ground state potential of the benzene cation employing CASPT2 calculations, for which very good agreement is found. This work highlights the need for efficient electronic structure approaches that can treat medium-sized organic molecules with a multitude of coupled excited states and several dissociation channels.

Conjugation through space can give rise to aromaticity in the lowest excited triplet state, with impact for photochemistry., Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T1) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T1 homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical “lever” that is powered by relief of homoantiaromatic destabilization in the first singlet excited state.

International audience; The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump-probe X-ray techniques. In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(I) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [CuI2(NSSN)(2)](2+). In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump-probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS). Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range. DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, i.e. S-0, (MLCT)-M-1 and (MLCT)-M-3, respectively. Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states. In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and 4100 ps, which were assigned as internal conversion to the ground state (Sn -> S-0), intersystem crossing (MLCT)-M-1 -> (MLCT)-M-3, and subsequent relaxation of the triplet to the ground state, respectively. Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S-S bridges is cleaved, causing the movement of two halves of the molecule relative to each other. Extended X-ray absorption fine structure spectroscopy resolves changes in Cu-ligand bond lengths with precision on the order of 0.01 angstrom, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule. The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper-sulfur complexes and demonstrate the potential of combining the pump-probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes.

Through a combined quantum mechanics/molecular mechanics (QM/MM) approach, we evaluate the photodynamics of the mono- and dithiolated azobenzenes when they are chemisorbed on a gold surface. The analysis of steric effects suggests that the presence of the surface influences the photoisomerization process of the chemisorbed monothiolated azobenzene. In particular, the trans -> cis quantum yields decrease, and the n?* state lifetimes become longer. The approach to the twisted conical intersection needed for the isomerization is hindered when the molecule is attached to a substrate because of the van der Waals interactions with the surface. For the cis isomer, the cis -> trans photoisomerization quantum yield is almost unaffected, since this isomer is not flat, and thus the interaction with the surface is less remarkable. Dithiolated azobenzene can photoisomerize both trans -> cis and cis -> trans, also when doubly linked to the surface, preserving the two bonds with the gold atoms: the flexibility of the central azo-moiety enables the molecule to photoisomerize without any bond breaking. The quantum yields in this case are even higher than in the monothiolated case, probably because of the strained initial conformation, which must adapt to the available distances between the anchoring sites.

This study presents a computational investigation of the initial step of the dimethyldihydropyrene (DHP) to cyclophanediene (CPD) photoinduced ring-opening reaction using time-dependent density functional theory (TD-DFT). In particular, the photochemical path corresponding to the formation of the CPD precursor (CPD*) on the zwitterionic state is scrutinized. The TD-DFT approach was first validated on the parent compound against accurate ab initio calculations. It confirms that CPD* formation is efficiently quenched in this system by an easily accessible S2/S1 conical intersection located in the vicinity of the CPD* minimum and leading to a locally excited state minimum responsible for DHP luminescence. Increased ring-opening quantum yields were observed in benzo[e]-fused-DHP (DHP-1), isobutenyl-DHP (DHP-2), and naphthoyl-DHP (DHP-3). The calculations show that CPD* formation is much more favorable in these systems, either due to an inversion of electronic states in DHP-1, suppressing the formation of the locally excited state, or due to efficient stabilization of CPD* on the S1 potential energy surface in DHP-2 and DHP-3. Both effects can be combined in a rationally designed benzo[e]-fused-naphthoyl-DHP (DHP-4) for which we anticipate an unprecedented efficiency.

A series of 1,2,3-triazole linked donor–acceptor chromophores are prepared by Click Chemistry from ene–yne starting materials. The effects of three distinct chemical variations are investigated: enhancing the acceptor strength through oxidation of the sulphur atom, alteration of the double bond configuration, and variation of the triazole substitution pattern. A detailed photophysical characterization shows that these alterations have a negligible effect on the absorption while dramatically altering the emission wavelengths. In addition, strong solvatochromism is found leading to significant red shifts in the case of polar solvents. The experimental findings are rationalized and related to the electronic structure properties of the chromophores by time-dependent density functional theory as well as the ab initio algebraic diagrammatic construction method for the polarization propagator in connection with a new formalism allowing to model the influence of solvation onto long-lived excited states and their emission energies. These calculations highlight the varying degree of intramolecular charge transfer character present for the different molecules and show that the amount of charge transfer is strongly modulated by the conducted chemical modifications, by the solvation of the chromophores, and by the structural relaxation in the excited state. It is, furthermore, shown that enhanced charge separation, as induced by chemical modification or solvation, reduces the singlet–triplet gaps and that two of the investigated molecules possess sufficiently low gaps to be considered as candidates for thermally activated delayed fluorescence.

Conjugated porphyrin dimers have emerged as versatile viscosity‐sensitive fluorophores that are suitable for quantitative measurements of microscopic viscosity by ratiometric and fluorescence lifetime‐based methods, in a concentration‐independent manner. Here, we investigate the effect of extended conjugation in a porphyrin‐dimer structure on their ability to sense viscosity and temperature. We show that the sensitivity of the fluorescence lifetime to temperature is a unique property of only a few porphyrin dimers.

International audience; Singlet oxygen production upon photosensitization plays a critical role in drugs based on thionucleobases. While for immunosuppressants its yield must be near zero, for phototherapeutic drugs it should be near the unity. In this work, we apply high-level quantum chemical modelling to investigate the decay of the triplet state of thionucleobases, a main determinant of the singlet oxygen yield. Working on CASPT2 optimizations of two prototypical thiothymines (2-thiothymine and 6-aza-2-thiothymine), we showed that the T-1 state is characterized by two pi pi* minima and by the intersection of T-1 with the singlet ground state. On the basis of this topography, we propose a two-step mechanistic model, which, depending on the energetic balance between the two minima, may have as a determining step either a slow transition between minima or a faster intersystem crossing to S-0. Chemical kinetics modelling, as well as simulations of the transient absorption spectra, confirmed that the two-step model can explain the experimental results available for both molecules. Moreover, through additional investigations of 2-thiocytosine and 6-thioguanine, we show that such a T-1 topography is a common theme for nucleobases. We also discuss how the triplet-state topography may be used to control the singlet oxygen yield, aiming at different medical applications.

State-of-the-art femtosecond spectroscopies and quantum-chemical methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor unit, A) derivatives bearing dibutylamino groups as strong electron donors (D) and bithiophenes as highly effective π-rich spacers. The absorption spectra of C1 and C2 are broad and shifted to the red side of the visible spectral range. A significant negative solvatochromism was observed for the absorption bands of the investigated salts with increasing solvent polarity that was rationalized in terms of the change in electron density upon excitation. The absorption spectra of C2 are red-shifted with respect to those of C1, whereas the emission bands of the two compounds overlap, suggesting a localization of the excitation on just one branch of the quadrupolar compound, which becomes the fluorescent portion. This is in agreement with our quantum-mechanical calculations, which predict that the symmetry of C2 is broken in th...

[EN] Visible light photoredox catalysis is emerging as a versatile technique for a great variety of chemical transformations. Specifically,Ru(bpy)(3)(2+) has been widely used as a transition metal-based photocatalyst; however, little if any attention has been paid to the thermodynamic analysis of the photoredox processes that occur in the photocatalytic cycle of the studied reactions or, even more interestingly, to the examination of the kinetic feasibility of the involved processes. In addition, only a few studies on the progress of the reaction have been performed. Organic halides constitute a major concern for environmental remediation since they are reluctant towards aerobic oxidation. Therefore, p-halonitrobenzene (X-NB) derivatives have been selected in the present work as the model compounds to obtain a deeper understanding of their photocatalytic reduction using visible light and RuRu(bpy)(3)(2+). Thermodynamic estimations were made on the basis of the experimentally determined energy of the LUMO of RuRu(bpy)(3)(2+), which was determined to be 54.5 kcal mol(-1) from the cross-point of the normalized emission and excitation spectra, and redox potentials of X-NB and several sacrificial amines. As anticipated from chemical intuition, the feasibility of the global photoredox process increased upon going down in the group of halogens regardless of the participation of the oxidative or reductive quenching cycles. To unequivocally demonstrate the direct participation of the excited state of RuRu(bpy)(3)(2) in the photoreduction, steady-state and time-resolved experiments were carried out upon increasing X-NB or amine concentration; this allowed determining the quenching rate constants for the electron transfer processes, which were found to be in the range of 108 M-1 s(-1) for the X-NB and 106 M-1 s(-1) for the amines. Therefore, the main role of the oxidative quenching cycle has been demonstrated under the experimental conditions employed. A good correlation was found between the thermodynamic and kinetic parameters, in agreement with the expectations from Marcus theory. Upon optimization of the reaction conditions, reductive dehalogenation was found to occur leading to the parent nitrobenzene., Generous support from the Ministerio de Economia y Competitividad (Project CTQ2012-38754-C03-03 and SEV-2016-0683) and from the Generalitat Valenciana (Prometeo Program) is gratefully acknowledged.

We numerically investigate the nonlinear dynamic properties of an exclusive excited-state (ES) emission quantum dot (QD) laser under optical injection. The results show that, under suitable injection parameters, the ES-QD laser can exhibit rich nonlinear dynamical behaviors, such as injection locking (IL), period one (P1), period two (P2), multi-period (MP), and chaotic pulsation (CP). Through mapping these dynamic states in the parameter space of the frequency detuning and the injection coefficient, it can be found that the IL occupies a wide region and the dynamic evolution routes appear in multiple forms. Via permutation entropy (PE) calculation to quantify the complexity of the CP state, the parameter range for acquiring the chaos with high complexity can be determined. Moreover, the influence of the linewidth enhancement factor (LEF) on the dynamical state of the ES-QD laser is analyzed. With the increase of the LEF value, the chaotic area shrinks (expands) in the negative (positive) frequency detuning region, and the IL region gradually shifts towards the negative frequency detuning.

10.1039/c3dt52027g The spin crossover behavior of the two [Fe(mtz)6](2+) complexes occupying different lattice sites in Fe(mtz)6(BF4)2 is addressed by combining quantum chemical calculations with a careful analysis of the crystal structure. It is first established from the calculations that the energy difference between high spin and low spin states depends on the orientation of the tetrazole ligands; small rotation angles favor the low spin state, while for angles larger than ~20° the high spin state is more stable. The crystal structure shows that the two complexes have different average rotation angles of the ligands. It is larger for the site that remains HS down to low temperatures and smaller for the site that shows spin crossover to LS. The origin of the different rotation angles is found to be determined by a subtle interplay amongst steric repulsion between the ligands, HF interactions between the complex and the counterions, and intersite interactions involving NH contacts and p-p interactions between the N[double bond, length as m-dash]N double bonds of the tetrazole rings.

Several azine derivatives have been prepared and structurally characterized by spectroscopic and single-crystal X-ray diffraction analysis. Two of them, viz. naphthalene based (A10) and anthracene based (A11) show fluorescence enhancement in the presence of Ag+ in aqueous-methanol. Moreover, A11 efficiently tracks Ag+ in vitro endophytic bacteria infected rice root tissue. Experimental results have been substantiated by theoretical DFT calculations.

The new dinuclear (ZnDyIII)-Dy-II and trinuclear (ZnDyIII)-Dy-II-Zn-II complexes of formula [(LZnBrDy(ovan) (NO3)(H2O)](H2O)center dot 0.5(MeOH) (1) and [((LZnBr)-Zn-1)(2)Dy(MeOH)(2)](ClO4) (3) (L and L-1 are the dideprotonated forms of the N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato and 2-{(E)-[(3-{[(2E,3E)-3-(hydroxyimino)butan-2-ylidene ]amino}-2,2-dimethylpropyl)imino]methyl}-6-methoxyphenol Schiff base compartmental ligands, respectively) have been prepared and magnetostructurally characterized. The X-ray structure of 1 indicates that the Dy-III ion exhibits a DyO9 coordination sphere, which is made from four O atoms coming from the compartmental ligand (two methoxy terminal groups and two phenoxido bridging groups connecting Zn-II and Dy-III ions), other four atoms belonging to the chelating nitrato and ovanillin ligands, and the last one coming to the coordinated water molecule. The structure of 3 shows the central Dy-III ion surrounded by two L1Zn units, so that the Dy-III and Zn-II ions are linked by phenoxido/oximato bridging groups. The Dy ion is eight-coordinated by the six O atoms afforded by two L1 ligands and two O atoms coming from two methanol molecules. Alternating current (AC) dynamic magnetic measurements of 1, 3, and the previously reported dinuclear [LZnClDy(thd)(2)] (2) complex (where thd = 2,2,6,6-tetramethyl-3,5-heptanedionato ligand) indicate single molecule magnet (SMM) behavior for all these complexes with large thermal energy barriers for the reversal of the magnetization and butterfly-shaped hysteresis loops at 2 K. Ab initio calculations on 1-3 show a pure Ising ground state for all of them, which induces almost completely suppressed quantum tunnelling magnetization (QTM), and thermally assisted quantum tunnelling magnetization (TA-QTM) relaxations via the first excited Kramers doublet, leading to large energy barriers, thus supporting the observation of SMM behavior. The comparison between the experimental and theoretical magnetostructural data for 13 has allowed us to draw some conclusions about the influence of ligand substitution around the Dy-III on the SMM properties. Finally, these SMMs exhibit metal- and ligand-centered dual emissions in the visible region, and, therefore, they can be considered as magnetoluminescent bifunctional molecular materials.

International audience; We investigate the behaviour of a multimode two-color quantum dot laser subject to optical feedback. In particular, we focus on the effects of a variation of the external cavity length at the micrometer scale on the laser emission characteristics and especially on its optical spectrum. For each mode, we observe oscillations of the output power with different spectral amplitudes. No clustering or mode grouping effect is observed. Theoretically, we demonstrate a good agreement with a multimode two-color quantum dot laser model.

Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile-water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (-NMe2)-pull (-CN) substituents and those are closely overlapped for non-push-pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (-NMe2)-pull (-CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push-pull diphenylbutadiyne together cover the visible region (400-700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push-pull character.

The copper(I) complexes containing bisphosphines such as bis(2-diphenylphosphinoimidazol-1-yl) methane (1), bis(5-diphenylphosphinopyrazol-1-yl) methane (2) and bis(5-diphenylphosphino-1,2,4-triazol- 1-yl) methane (3) have been synthesized. The reaction of 1 with CuX (X = Cl, I) afforded homoleptic dinuclear complexes [Cu-2(mu-X)(2){CH2(1,3-C3H2N2PPh2)(2)}(2)-kappa P-2,P] (X = Cl, 4; I, 6). However, a mononuclear complex [CuBr(CH3CN){CH2(1,3-C3H2N2PPh2)(2)}-kappa P-2,P] (5) was isolated in the reaction of 1 with CuBr. The reaction of 2 with CuX (X = Cl, Br, I) also produced dinuclear complexes [Cu-2(mu-X)(2){CH2(1,2-C3H2N2PPh2)(2)}(2)-kappa P-2,P] (X = Cl, 7; Br, 8; I, 9). The reaction of 3 with CuCl yielded a novel 1D coordination polymer [Cu-2(mu-Cl)(2){CH2(1,2,4-C2HN3PPh2)(2)}-kappa P-2,N](n) (10), whereas homoleptic dinuclear complexes [Cu-2(mu-X)(2){CH2(1,2,4-C2HN3PPh2)(2)}(2)-kappa P-2,P] (X = Br, 11; I, 12) were obtained in the reactions of 3 with CuX (X = Br, I). Bisphosphines 1 and 2 showed bidentate (kappa P-2,P) chelating mode of coordination, whereas 3 has adopted bidentate (kappa P-2,N) chelating mode of coordination in 10. The crystal structures of 4-7 and 10-12 have been established by single crystal X-ray crystallography. (C) 2016 Elsevier B.V. All rights reserved.

International audience; The synthesis and new optical features of three indan-1,3-dione class structures is reported. The simple arylmethylene-1,3-indandione structure 2-(4-diethylaminobenzylidene)indan-1,3-dione was used to design two linked structures: 1,3-bis-{3-hydroxy-4-[4-diethylamino-1-(1,3-dioxoindan-2-ylmethylene) benzen-3-yloxy]-1-thiabuthyl}benzene and 4,4-bis-({3-hydroxy-4-[4-diethylamino-1-(1,3-dioxoindan-2-ylmethylene)benzen-3-yloxy]-1-thiabutyl}phenyl}sulfide. In contrast to the simple compound, which readily crystallized, the linked derivatives remained in an amorphous phase and are considered as molecular glasses with respective glass transition temperatures 88 and 100 degrees C. For non-linear optical investigations samples were prepared as a guest-host system in polycarbonate matrix (10%). The Maker-fringe technique was used to investigate the third harmonic generation at a wavelength of 355 nm (YAG laser). Third-order non-linear susceptibility chi((3)) values were extracted 5.75.10(-21) m(2) V-2, 8.60.10 (-21) m(2) V-2, 16.85.10(-21) m(2) V-2 for these respective indandiones while modeling the experimental results. To evaluate the susceptibility of the indan-1,3-dione derivatives third harmonic generation a comparative experiment for a reference azodye in polycarbonate was performed. The results show an important feature - higher molecular second order hyperpolarizability for the linked structures. NMR, MS, IR, UV-VIS, XRD and elemental analysis were used to structurally characterize the new compounds and ellipsometry was applied to interpret the non-linear optical results.

The behavior toward oxygen sensing of nano- composites made of the aluminum oxide-hydroxide nano- structured solid support (AP200/19) and neutral blue emitting cyclometalated iridium(III) complexes was studied. The results are compared with the same dyes immobilized in polystyrene films. Since the photoluminescence of the complexes is totally quenched for oxygen concentrations just over 10%, these systems using the blue region of the visible spectrum are promising for oxygen detection at low concentration. In particular, dyes supported into the AP200/19 provide the best sensitivity to oxygen concentration, with the possibility to detect oxygen below 1% O2 in gas (0.01 bar).

Light-driven molecular motors convert light into mechanical energy through excited-state reactions. Unidirectional rotary molecular motors based on chiral overcrowded alkenes operate through consecutive photochemical and thermal steps. The thermal (helix inverting) step has been optimized successfully through variations in molecular structure, but much less is known about the photochemical step, which provides power to the motor. Ultimately, controlling the efficiency of molecular motors requires a detailed picture of the molecular dynamics on the excited-state potential energy surface. Here, we characterize the primary events that follow photon absorption by a unidirectional molecular motor using ultrafast fluorescence up-conversion measurements with sub 50 fs time resolution. We observe an extraordinarily fast initial relaxation out of the Franck-Condon region that suggests a barrierless reaction coordinate. This fast molecular motion is shown to be accompanied by the excitation of coherent excited-state structural motion. The implications of these observations for manipulating motor efficiency are discussed.

Spectroscopic studies on jet cooled 3-methylisoquinoline using fluorescence excitation and dispersed fluorescence methods were carried out. The effect of methyl substitution on the excited state behavior of isoquinoline is focused. Methyl torsional behavior and the effect on torsional potential of methyl group due to excited states mixing are addressed. The possible explanations of the observed intensity pattern of the torsional transitions in the fluorescence excitation spectrum are also presented.

A detailed photophysical characterization of a couple of new perylene imide derivatives, a carboxylic trisimide PIx, and an asymmetrically substituted carboxylic bisimide PIa is presented. PIx and PIa have the lowest singlet excited state just below 2.6 eV. The dyes are remarkably fluorescent (phi(f) = 0.37 +/- 0.03 for PIa and phi(f) = 0.58 +/- 0.04 for PIx in toluene), but they also display an efficient intersystem crossing. This leads to typical excited triplet photophysics/photochemistry, with intense,triplet state absorption spectra and efficient singlet oxygen ((1)Delta(g)) photosensitization. (phi(Delta) = 0.68 +/- 0.02 for PIa and 0.44 +/- 0.02 for PIx in toluene). On the basis of the measured phi(Delta), a phi(isc) of 0.65 +/- 0.02 for PIa and 0.43 +/- 0.02 for PIx in toluene is derived. PIx reduces at 0.58, eV vs SCE, almost similarly to the,corresponding symmetrically substituted perylene bisimide PI0, but unlike the latter, it has the first oxidation potential above +1.9 V. PIa is more electron rich and displays a more difficult first reduction at -0.95 V with a more facile oxidation at +1.75 V, similar to that of the parent PI0. The absorption spectra of the excited singlet and triplet states and that of electrochemically generated monoanions are reported.

The synthesis and characterisation of two terpyridine based ruthenium/palladium heteronuclear compounds are presented. The photocatalytic behaviour of the Ru/Pd complex containing the linear 2,2':5',2 ''-terpyridine bridge (1a) and its analogue the non-linear 2,2':6',2 ''-terpyridine bridge (2a) are compared together with the respective mononuclear complexes 1 and 2. Irradiation of 1a with visible light (e.g., 470 nm) results in the photocatalytic generation of dihydrogen gas. Photocatalysis was not observed with complex 2a by contrast. A comparison with the photocatalytic behaviour of the precursors 1 and 2 indicates, that while for 1a the photocatalysis is an intramolecular process, for the mononuclear precursors it is intermolecular. The photophysical and electrochemical properties of the mono- and heterobinuclear compounds are compared. Raman spectroscopy and DFT calculations indicate that there are substantial differences in the nature of the lowest energy (MLCT)-M-3 states of 1a and 2a, from which the contrasting photocatalytic activities of the complexes can be understood.

Two new diimine-diselenolate PtII chromophores [Pt(bipy)(Me-dset)] (1) and [Pt(phen)(Me-dset)] (2) (Me-dset2- = N-methyl-2-thioxothiazoline-4,5-diselenolate) were synthesized and characterized. The effect of replacing sulfur with selenium was investigated by comparing the UV/Vis spectroscopic and electrochemical properties of 1 and 2 with those of the corresponding sulfur analogues, [Pt(bipy)(Me-dmet)] (3) and [Pt(phen)(Me-dmet)] (4), with a particular focus on their linear and nonlinear optical properties. Postprint Postprint

Sensitized triplet–triplet annihilation in multicomponent organic systems is already demonstrated to be suitable for obtaining effi cient up-conversion in solution with excitation power densities comparable to solar irradiance, but loses effi ciency in the solid state. Here, it is demonstrated that it is possible to reduce this limitation by incorporating a standard bicomponent system in polymer nanoparticles. The confi nement of all of the involved photophysical processes in a nanometer-scale volume makes each nanoparticle a single and isolated high-effi ciency up-converting unit. As a consequence, these dualdye-loaded nanoparticles can be used to produce drop-cast fi lms, as well as dopants for polymeric matrices, preserving the performances of the starting moieties in solution.

The syntheses and the electrochemical spectroscopic properties of a suite of asymmetrical bistridentate cyclometalated Ru(II) complexes bearing terminal triphenylamine (TPA) substituents are reported. These complexes, which contain structural design elements common to both inorganic and organic dyes that exhibit superior power conversion efficiencies in the dye-sensitized solar cell (DSSC), are broadly formulated as [Ru(II)(L-2,5'-thiophene-TPA-R(1))(L-R(2))](+) [L = tridentate chelating ligand (e.g., 2,2':6',2''-terpyridine (tpy); deprotonated forms of 1,3-di(pyridin-2-yl)benzene (Hdpb) or 6-phenyl-2,2'-bipyridine (Hpbpy)); R(1) = -H, -Me, -OMe; R(2) = -H, -CO(2)Me, -CO(2)H]. The following structural attributes were systematically modified for the series: (i) electron-donating character of the terminal substituents (e.g., R(1) = -H, -Me, -OMe) placed para to the amine of the "L-2,5'-thiophene-TPA-R(1)" ligand framework; (ii) electron-withdrawing character of the tridentate chelate distal to the TPA-substituted ligand (e.g., R(2) = -H, -CO(2)Me, -CO(2)H); and (iii) position of the organometallic bond about the Ru(II) center. UV-vis spectra reveal intense and broad absorption bands arising from a collection of metal-to-ligand charge-transfer (MLCT) and TPA-based intraligand charge-transfer (ILCT) transitions that, in certain cases, extend beyond 800 nm. Electrochemical data indicate that the oxidative behavior of the TPA and metal chelate units can be independently modulated except in cases where the anionic phenyl ring is in direct conjugation with the TPA unit. In most cases, the anionic character of the cyclometalating ligands renders a metal-based oxidation event prior to the oxidation of the TPA unit. This situation can, however, be reversed with an appropriately positioned Ru-C bond and electron-rich R(1) group. This finding is important in that this arrangement confines the highest occupied molecular orbital (HOMO) to the TPA unit rather than the metal, which is optimal for sensitizing TiO(2); indeed, a remarkably high power conversion efficiency (η) in the DSSC (i.e., 8.02%) is measured for the TPA-substituted pbpy(-) chelate where R(1) = -OMe. These results provide a comprehensive strategy for improving the performance of bistridentate Ru sensitizers devoid of NCS(-) groups for the DSSC.

A new family of highly soluble electrophosphorescent dopants based on a series of tris-cyclometalated iridium(III) complexes (1-4) of 2-(carbazol-3-yl)-4/5-R-pyridine ligands with varying molecular dipole strengths have been synthesized. Highly efficient, solution-processed, single-layer, electrophosphorescent diodes utilizing these complexes have been prepared and characterized. The high triplet energy poly(9-vinylcarbazole) PVK is used as a host polymer doped with 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) for electron transport. Devices with a current efficiency of 40 cd A(-1) corresponding to an EQE of 12% can thus be achieved. The effect of the type and position of the substituent (electron-withdrawing group (CF3) and electron-donating group (OMe)) on the molecular dipole moment of the complexes has been investigated. A correlation between the absorption strength of the singlet metal-to-ligand charge-transfer ((MLCT)-M-1) transition and the luminance spectral red shift as a function of solvent polarity is observed. The strength of the transition dipole moments for complexes 1-4 has also been obtained from TD-DFT computations, and is found to be consistent with the observed molecular dipole moments of these complexes. The relatively long lifetime of the excitons of the phosphorescence (microseconds) compared to the charge-carrier scattering time (less than nanoseconds), allows the transition dipole moment to be considered as a "quasi permanent dipole". Therefore, the carrier mobility is sufficiently affected by the long-lived transition dipole moments of the phosphorescent molecules, which are randomly oriented in the medium. The dopant dipoles cause positional and energetic disorder because of the locally modified polarization energy. Furthermore, the electron-withdrawing group CF3 induces strong carrier dispersion that enhances the electron mobility. Therefore, the strong transition dipole moment in complexes 3 and 4 perturbs both electron and hole mobilities, yielding a reduction in exciton formation and an increase in the device dark current, thereby decreasing the device efficiency. UK Research & Innovation (UKRI) Engineering & Physical Sciences Research Council (EPSRC) (EP/I013695/1)