Short Excited-state Lifetime Research Articles

Luminescent rare earth complexes of Ce3+, Eu2+and Yb2+with spiro-bis(pyrazolyl)borate ligands containing C–H···M interactions

Luminescent rare earth complexes based on interconfigurational 5d-4f transition have characteristics of adjustable emission spectra and short excited state lifetimes, showing potential applications in display, lighting and other fields. In this work, nine Ce3+, Eu2+ and Yb2+ complexes with spiro-bis(pyrazolyl)borate ligands, where pyrazolyl stands for pyrazolyl, 3-methylpyrazolyl, or 3,5-dimethylpyrazolyl, were designed and synthesized. Ce3+ complexes show emission colors of blue, with photoluminescence quantum yields (PLQYs) of 94%–100%; Eu2+ complexes show emission colors of yellow-green, with PLQYs of 60%–78%; Yb2+ complexes show emission colors of orange or red, with PLQYs of 3%–4%. In addition, the crystal structures and theoretical calculation results show C–H···M interactions between bridge-head H atoms of the spirane and central metal ions in these complexes. It is found that the introduction of spirane with large steric hindrance and the existence of C–H···M interactions can improve the stability of the complexes in air atmosphere.

Concentration-Dependent Nonradiative Decay Mechanisms in Fluorescence Self-Quenching: Insights from Azulene and 1,3-Dichloroazulene

A series of steady-state and time-resolved spectroscopies were performed on azulene (Az) and its 1,3-dihalogenated analogue, 1,3-dichloroazulene (DCAz). Understanding the intra/inter-molecular interactions in these types of systems is the key to unlocking the potential of these molecules and enabling their application in a range of future devices. Therefore, the concentration dependence of their self-quenched fluorescence intensities and of the reduced lifetimes of their second-excited singlet states were examined. A Stern-Volmer analysis was implemented based on these experiments, which resulted in DCAz exhibiting a higher bimolecular quenching constant, compared to Az, due to its shorter excited-state lifetime. The highly efficient self-quenching at room temperature was a result of a short-range energy transfer mechanism.

Photocatalytic H2O2 production over boron-doped g-C3N4 containing coordinatively unsaturated FeOOH sites and CoOx clusters

Graphitic carbon nitride (g-C3N4) has gained increasing attention in artificial photosynthesis of H2O2, yet its performance is hindered by sluggish oxygen reduction reaction (ORR) kinetics and short excited-state electron lifetimes. Here we show a B-doped g-C3N4 (BCN) tailored with coordinatively unsaturated FeOOH and CoOx clusters for H2O2 photosynthesis from water and oxygen without sacrificial agents. The optimal material delivers a 30-fold activity enhancement compared with g-C3N4 under visible light, with a solar-to-chemical conversion efficiency of 0.75%, ranking among the forefront of reported g-C3N4-based photocatalysts. Additionally, an electron transfer efficiency reaches 34.1% for the oxygen reduction reaction as revealed by in situ microsecond transient absorption spectroscopy. Experimental and theoretical results reveal that CoOx initiates hole–water oxidation and prolongs the electron lifetime, whereas FeOOH accepts electrons and promotes oxygen activation. Intriguingly, the key to the direct one-step two-electron reaction pathway for H2O2 production lies in coordinatively unsaturated FeOOH to adjust the Pauling-type adsorption configuration of O2 to stabilize peroxide species and restrain the formation of superoxide radicals.

Fast singlet excited-state deactivation pathway of flavin with a trimethoxyphenyl derivative

Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet 1(π,π*) excited state deactivation pathway through nonadiabatic transition to the 1(n,π*) excited- state, and subsequent deactivation to the ground electronic state (S0), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited–state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T1 state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C(2)-O distance elongates by 0.16 Å in the 1(n,π*) state, with respect to the ground state. Intermediate–state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques.

The Molecular Design and Electroluminescent Performance of Near-Infrared (NIR) Iridium(III) Complexes Bearing Isoquinoline-, Phthalazine- and Phenazine-Based Ligands.

Near-infrared (NIR) light has characteristics of invisibility to human eyes, less background interference, low light scattering, and strong cell penetration. Therefore, NIR luminescent materials have significant applications in imaging, sensing, energy, information storage and display. The development of NIR luminescent materials thus has emerged as a highly dynamic area of research in the realm of contemporary materials. To date, NIR luminescent materials are roughly divided into inorganic materials and organic materials. Compared with inorganic materials, organic NIR luminescent materials have become a hot research topic in recent years due to their rich sources, easy control of structure, simple preparation process, low cost, and good film-forming properties. Among them, iridium(III) [Ir(III)] complexes exhibit excellent properties such as thermal stability, simple synthesis, easy color modulation, short excited state lifetimes, and high brightness, thus becoming one of the ideal luminescent material systems for preparing high-quality organic light-emitting diodes. Therefore, how to obtain Ir(III) complexes with NIR emission and high efficiency through molecular design is a necessary and promising research topic. This work reviews the research progress of representative NIR Ir(III) complexes bearing isoquinoline-, phenazine-, and phthalazine-based ligands reported in recent years and introduces the design strategies and electroluminescent performances of NIR Ir(III) complexes.

Gold Coordination-Accelerated Multi-Resonance TADF Emission for Efficient Solution-Processible Ultrapure Deep-Blue OLEDs.

Multi-resonance (MR) type emitters have emerged as highly promising candidates for high-resolution organic light-emitting diodes (OLEDs). However, thermally activated delayed fluorescence (TADF) emissions with simultaneous short excited state lifetimes and ultrapure blue color (a CIEy close to 0.046 and an emission peak > 440 nm) have rarely been obtained for MR emitters. Herein, we report a design of dual gold-coordinated MR molecules to achieve efficient and short-lived ultrapure blue TADF emission. The dinuclear Au(I) complex, namely iPrAuBN, shows a narrowband deep-blue emission with a peak maximum of 448 nm and a full width at half maximum (FWHM) of 29 nm in doped film. The coordination with two Au atoms significantly shortens the delayed fluorescence lifetime to 7.8 μs in comparing with 60.6 μs for the organic analogue. Solution-processed OLED doped with iPrAuBN demonstrates an ultrapure blue electroluminescence with a peak maximum of 442 nm, a FWHM of 19 nm, Commission International de l'Éclairage (CIE) coordinates of (0.154, 0.036), and a maximum external quantum efficiency of 14.8%.

Excited-State Intramolecular Proton Transfer toward Conical Intersections in Indigo, Epindolidione, and Indirubin.

Indigo exhibits a high degree of photostability, experimentally supported by observations such as quenching of fluorescence and an exceptionally short excited-state lifetime. Epindolidione, a structural isomer of indigo, is highly fluorescent in contrast to indigo, while indirubin, another structural isomer, exhibits weak fluorescence similar to that of indigo. To elucidate the origin of the difference in photophysical and photochemical behavior, potential energy profiles of the excited-state intramolecular proton transfer in indigo, epindolidione, and indirubin are computationally studied by quantum chemical calculations using the TDDFT and extended MS-CASPT2 (XMS-CASPT2) methods. As a result, it is found that indigo and indirubin exhibit little energy barrier for the single proton transfer (SPT) in the S1(ππ*) state from the diketo to keto-enol form and low energy of the S1/S0 conical intersection (CI) in the latter form with a planar molecular structure. Epindolidione, on the other hand, exhibits much higher barriers for SPT and access to CI. These results suggest that the excited-state SPT and subsequent nonradiative deactivation via CI are more likely to occur in indigo and indirubin than in epindolidione, which is consistent with the experimental observations described above. For indigo and epindolidione, the deactivation channels via the second SPT from the keto-enol to dienol form are also compared.

Manganese Complexes with Consecutive Mn(IV) → Mn(III) Excitation for Versatile Photoredox Catalysis.

Manganese complexes stand out as promising candidates for photocatalyst design, attributed to their eco- and biocompatibility, versatile valence states, and capability for facilitating multiple electronic excitations. However, several intrinsic constraints, such as inadequate visible light response and short excited-state lifetimes, hinder effective photoinduced electron transfer and impede photoredox activation of substrates. To overcome this obstacle, we have developed a class of manganese complexes featuring boron-incorporated N-heterocyclic carbene ligands. These complexes enable prolonged excited-state durations encapsulating both Mn(IV) and Mn(III) oxidation stages, with lifetimes reaching microseconds for Mn(IV) and nanoseconds for Mn(III), concurrently exhibiting robust redox capabilities. They efficiently catalyze direct, site-selective cross-couplings between diverse arenes and aryl bromides, at a low catalyst loading of 0.5 mol %. Their proficiency spans an extensive array of substrates including both highly electron-rich and electron-deficient molecules, which underscore the superior performance of these manganese complexes in tackling intricate transformations. Furthermore, the versatility of these complexes is further highlighted by their successful applications in various photochemical transformations, encompassing reductive cross-couplings for the formation of C-P, C-B, C-S and C-Se bonds, alongside oxidative couplings for creating C-N bonds. This study sheds light on the distinctive photoredox properties and the remarkable catalytic flexibility of manganese complexes, highlighting their immense potential to drive progress in photochemical synthesis and green chemistry applications.

Lead-free Ce-doped perovskite scintillators with high figure of merit

Lead halide perovskite scintillators have recently received extensive research attention owing to their short fluorescence lifetimes, low detection limits, and ease of fabrication compared to traditional scintillators. The nontoxic cerium-doped lead-free perovskites with intrinsically efficient and short lifetime d–f transitions are a prospective replacement for the toxic Pb2+. Here, we demonstrated Ce-doped cesium lanthanide chloride perovskites (Cs3LnCl6, Ln = Gd, Y, Lu) synthesized through a facile solution method for the first time. These perovskites exhibit blue-violet emission, which arises from Ce 5d → 4f transitions. Among three types of Cs3LnCl6 perovskites, Ce:Cs3LuCl6 exhibited high photoluminescence quantum yield (PLQY) of 82% and a short excited-state lifetime of approximately 34 ns. When utilized as X-ray scintillators, Ce:Cs3LuCl6 crystals display a high light yield of 8120 photons per MeV and a low detection limit of 36.8 nGy air s−1. Importantly, the figure of merit (FoM), representing the ratio of light yield to decay time, reaches 239, which is the highest reported value for lead-free perovskite scintillators up to now. Additionally, the fabrication of perovskite/PMMA films was undertaken for practical demonstrations in X-ray imaging, resulting in the attainment of a resolution of up to 8.38 lp/mm. We anticipate that this work will inspire the utilization of Ce-doped Cs3LnCl6 perovskites in ultrafast scintillation applications such as high-energy physics, nuclear reaction monitoring, and dynamic X-ray imaging.

Cobalt-Mediated Photochemical C-H Arylation of Pyrroles.

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth-abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short-lived excited states, which has led researchers to prioritize the pursuit of longer excited-state lifetimes through careful molecular design. However, we report herein that the C-H arylation of pyrroles and related substrates (which are benchmark reactions for assessing the efficacy of photoredox catalysts) can be achieved using a simple and readily accessible octahedral bis(diiminopyridine) cobalt complex, [1-Co](PF6)2. Notably, [1-Co]2+ efficiently functionalizes both chloro- and bromoarene substrates despite the short excited-state lifetime of the key photoexcited intermediate *[1-Co]2+ (8 ps). We present herein the scope of this C-H arylation protocol and provide mechanistic insights derived from detailed spectroscopic and computational studies. These indicate that, despite its transient existence, reduction of *[1-Co]2+ is facilitated via pre-assembly with the NEt3 reductant, highlighting an alternative strategy for the future development of 3d metal-catalyzed PRC.

Ferromagnetically Coupled Chromium(III) Dimer Shows Luminescence and Sensitizes Photon Upconversion.

There has been much progress on mononuclear chromium(III) complexes featuring luminescence and photoredox activity, but dinuclear chromium(III) complexes have remained underexplored in these contexts until now. We identified a tridentate chelate ligand able to accommodate both meridional and facial coordination of chromium(III), to either access a mono- or a dinuclear chromium(III) complex depending on reaction conditions. This chelate ligand causes tetragonally distorted primary coordination spheres around chromium(III) in both complexes, entailing comparatively short excited-state lifetimes in the range of 400 to 800 ns in solution at room temperature and making photoluminescence essentially oxygen insensitive. The two chromium(III) ions in the dimer experience ferromagnetic exchange interactions that result in a high spin (S=3) ground state with a coupling constant of +9.3 cm-1. Photoinduced energy transfer from the luminescent ferromagnetically coupled dimer to an anthracene derivative results in sensitized triplet-triplet annihilation upconversion. Based on these proof-of-principle studies, dinuclear chromium(III) complexes seem attractive for the development of fundamentally new types of photophysics and photochemistry enabled by magnetic exchange interactions.

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short.

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

Optimizing photovoltaic performance of squaraine derivative dyes: a DFT study on different anchoring groups.

In this study, we designed squaraine-based dyes with a 2-amino pyrrole donor unit and acene groups like anthracene and pentacene. These dyes incorporate three different electron-withdrawing groups - cyanoacrylate (A1), phosphonate (A2) and boronic acid (A3) - as linkers to the TiO2 semiconductor. The spectroscopic, electronic and photochemical properties of these compounds were investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) simulations. Compared to the squarylium dye, SQD, the UV-vis data indicate excellent absorption especially for pentacene-based dyes, which extended beyond 920 nm, enhancing the panchromatic effect. The calculated excited-state lifetimes of these dyes were notably longer than SQD, particularly for those containing pentacene and either A1 or A2 withdrawing groups, with lifetimes approximately four times longer. In contrast, boronic acid derivatives had shorter excited-state lifetimes, hindering charge transfer. Simulations suggest all sensitizers can inject electrons into TiO2 and be efficiently regenerated by electron transfer from the electrolyte. The best results were achieved with pentacene and A1 or A2 as linkers, notably A1 dyes achieve superior short circuit photocurrent, J sc, and power conversion efficiency, PCE, with over 50% improvement compared to SQD. Phosphonate derivatives exhibited the highest energy adsorption on TiO2 while still achieving significant open-circuit voltage, V oc, J sc, and PCE values. After surface adsorption, all dyes displayed efficient electron recovery, with HOMO levels significantly dropping below -4.8 eV. Our study demonstrates that computational design can significantly enhance experimental work, offering valuable insights to improve dye design and boost the performance of dye-sensitized solar cells.

Sterically Encumbered Heteroleptic Copper(I) β-Diketiminate Complexes with Extended Excited-State Lifetimes.

One of the main challenges in developing effective copper(I) photosensitizers is their short excited-state lifetimes, usually attributed to structural distortion upon light excitation. We have previously introduced copper(I) charge-transfer chromophores of the general formula Cu(N^N)(ArNacNac), where N^N is a conjugated diimine ligand and ArNacNac is a substituted β-diketiminate ligand. These chromophores were promising regarding their tunable redox potentials and intense visible absorption but were ineffective as photosensitizers, presumably due to short excited-state lifetimes. Here, we introduce sterically crowded analogues of these heteroleptic chromophores with bulky alkyl substituents on the N^N and/or ArNacNac ligand. Structural analysis was combined with electrochemical and photophysical characterization, including ultrafast transient absorption (UFTA) spectroscopy to investigate the effects of the alkyl groups on the excited-state lifetimes of the complexes. The molecular structures determined by single-crystal X-ray diffraction display more distortion in the ground state as alkyl substituents are introduced into the phenanthroline or the NacNac ligand, showing smaller τ4 values due to the steric hindrance. UFTA measurements were carried out to determine the excited-state dynamics. Sterically encumbered Cu5 and Cu6 display excited-state lifetimes 15-20 times longer than unsubstituted complex Cu1, likely indicating that the incorporation of bulky alkyl substituents inhibits the pseudo-Jahn-Teller (PJT) flattening distortion in the excited state. This work suggests that the steric properties of these heteroleptic copper(I) charge-transfer chromophores can be readily modified and that the excited-state dynamics are strongly responsive to these modifications.

A-Band Absorption Spectrum of the ClSO Radical: Electronic Structure of the Sulfinyl Group.

Sulfur oxide species (RSOx) play a critical role in many fields, ranging from biology to atmospheric chemistry. Chlorine-containing sulfur oxides may play a key role in sulfate aerosol formation in Venus' cloud layer by catalyzing the oxidation of SO to SO2 via sulfinyl radicals (RSO). We present results from the gas-phase UV-vis transient absorption spectroscopy study of the simplest sulfinyl radical, ClSO, generated from the pulsed-laser photolysis of thionyl chloride at 248 nm (at 40 Torr of N2 and 292 K). A weak absorption spectrum from 350 to 480 nm with a peak at 385 nm was observed, with partially resolved vibronic bands (spacing = 226 cm-1), and a peak cross section σ(385 nm) = (7.6 ± 1.9) × 10-20 cm2. From ab initio calculations at the EOMEE-CCSD/ano-pVQZ level, we assigned this band to 12A' ← X2A″ and 22A' ← X2A″ transitions. The spectrum was modeled as a sum of a bound-to-free transition to the 12A' state and a bound-to-bound transition to the 22A' state with similar oscillator strengths; the prediction agreed well with the observed spectrum. We attributed the vibronic structure to a progression in the bending vibration of the 22A' state. Further calculations at the XDW-CASPT2 level predicted a conical intersection between the excited 12A' and 22A' potential energy surfaces near the Franck-Condon region. The geometry of the minimum-energy conical intersection was similar to that of the ground-state geometry. The lack of structure at shorter wavelengths could be evidence of a short excited-state lifetime arising from strong vibronic coupling. From simplified molecular orbital analysis, we attributed the ClSO spectrum to transitions involving the out-of-plane π/π* orbitals along the S-O bond and the in-plane orbital possessing a σ/σ* character along the S-Cl bond. We hypothesize that these orbitals are common to other sulfinyl radicals, RSO, which would share a combination of a strong and a weak transition in the UV (near 300 nm) and visible (400-600 nm) regions.

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby.

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby mer-[Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine), which shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only 10 luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer-MoX3(ddpd) (1, X = Cl; 2, X = Br) and cisfac-[Mo(ddpd)2]3+ (cisfac-[3]3+), an isomeric heavy homologue of the prototypical molecular ruby. For cisfac-[3]3+, we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer-1, mer-2, and cisfac-[3]3+ are much lower in energy than those in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in NIR-II, the emission of cisfac-[3]3+ peaking at 1550 nm is particularly low in energy. Femtosecond transient absorption spectroscopy reveals a short excited-state lifetime of 1.4 ns, 6 orders of magnitude shorter than that of mer-[Cr(ddpd)2]3+. Using density functional theory and ab initio multireference calculations, we break down the reasons for this disparity and derive principles for the design of future stable photoactive molybdenum(III) complexes.

Photocoupled Electroreduction of CO2 over Photosensitizer-Decorated Covalent Organic Frameworks.

Introducing an external visible-light field would be a promising strategy to improve the activity of the electrocatalytic CO2 reduction reaction (CO2RR), but it still remains a challenge due to the short excited-state lifetime of active sites. Herein, Ru(bpy)3Cl2 struts as powerful photosensitive donors were immobilized into the backbones of Co-porphyrin-based covalent organic frameworks (named Co-Bpy-COF-Rux, x is the molar ratio of Ru and Co species, x = 1/2 and 2/3) via coordination bonds, for the photo-coupled CO2RR to produce CO. The optimal Co-Bpy-COF-Ru1/2 displays a high CO Faradaic efficiency of 96.7% at -0.7 V vs reversible hydrogen electrode (RHE) and a CO partial current density of 16.27 mA cm-2 at -1.1 V vs RHE under the assistance of light, both of which were far surpassing the values observed in the dark. The significantly enhanced activity is mainly attributed to the incorporation of a Ru(bpy)3Cl2 donor with long excited-state lifetime and concomitantly giant built-in electric field in Co-Bpy-COF-Ru1/2, which efficiently accelerate the photo-induced electron transfer from Ru(bpy)3Cl2 to the cobalt-porphyrin under the external light. Thus, the cobalt-porphyrin active sites have a longer excited-state lifetime to lower the rate-determining steps' energy occurring during the actual photo-coupled electrocatalytic CO2RR process. This is the first work of porphyrin-based COFs for photo-coupled CO2RR, opening a new frontier for the construction of efficient photo-coupled electrocatalysts.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes.

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Nanosecond Intra-Ionic Chloride Photo-Oxidation.

Transition-metal photocatalysts capable of oxidizing chloride are rare yet serve as an attractive means to controllably generate chlorine atoms, which have continued to garner the interest of researchers for notable applications in photoredox catalysis and solar energy storage. Herein, a new series of four Ir-photocatalysts with different dicationic chloride-sequestering ligands were synthesized and characterized to probe the relationship between chloride binding affinities, ion pair solution structures, and rate constants for chloride photo-oxidation in acetonitrile at room temperature. The substituents on the quaternary amines of dicationic bipyridine ligands had negligible effects on the photocatalyst excited-state reduction potential, yet dramatically influenced the affinity for chloride binding, indicating that synthetic design can be utilized to independently tune these important properties. An inverse correlation was observed between the equilibrium constant for chloride ion pairing and the rate constant for intra-ionic chloride oxidation. Exceptions to this trend suggest structural differences in the ion-paired solution structures, which were probed by 1H NMR binding experiments. This study provides new insights into light-induced oxidation of ion-paired substrates, a burgeoning approach that offers to circumvent diffusional constraints of photocatalysts with short excited-state lifetimes. Ground-state association of chloride with these photocatalysts enables intra-ionic chloride oxidation on a rapid nanosecond timescale.

Fluorescent Properties of Cyanine Dyes As a Matter of the Environment.

In non-viscous aqueous solutions, the cyanine fluorescent dyes Cy3 and Cy5 have rather low fluorescence efficiency (the fluorescence quantum yields of Cy3 and Cy5 are 0.04 and 0.3, respectively [1, 2]) and short excited state lifetimes due to their structural features. In this work, we investigated the effect of solubility and rotational degrees of freedom on the fluorescence efficiency of Cy3 and Cy5 in several ways. We compared the fluorescence efficiencies of two cyanine dyes sCy3 and sCy5 with the introduction of a sulfonyl substituent in the aromatic ring as well as covalently bound to T10 oligonucleotides. The results show that because of the different lengths of the polymethine chains between the aromatic rings of the dyes, cis-trans-isomerization has a much greater effect on the Cy3 molecule than on the Cy5 molecule, while the effect of aggregation is also significant.