Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide-Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods.

In order to obtain a long-lived charge separation (CS) state in compact electron donor-acceptor molecular systems, we prepared a series of naphthalenediimide (NDI)-phenothiazine (PTZ) triads, with phenylene as the linker between the donor and acceptor. Conformation restriction is imposed to control the mutual orientation of the NDI and PTZ units by attaching methyl groups on the phenylene linker to tune the electronic coupling between the donor and the acceptor. Moreover, the PTZ moiety was oxidized to sulfoxide to tune the ordering of the CS state and the 3LE state (LE: locally excited state). UV-vis absorption spectra indicate electronic coupling between NDI with the phenylene linker as well as the PTZ units, manifested by the appearance of a charge-transfer (CT) absorption band, whereas this coupling is devoid in the triads with conformation restriction imposed. Fluorescence is strongly quenched in the triads compared to the reference compound, indicating electron transfer upon photoexcitation. Femtosecond transient absorption spectra indicate that the CS takes 0.8 ps, and then the 3LE state is formed by charge recombination in 83 ps. Nanosecond transient absorption (ns-TA) spectra show that the 3NDI state was observed in nonpolar solvents such as cyclohexane (triplet state lifetime: 95.7 µs), whereas the CS state was observed in more polar solvents. The CS state lifetimes are up to 1.2 µs (in toluene). Time-resolved electron paramagnetic resonance spectra of the triads in toluene consist of two types of signals: CS states (narrower signals, ∼10 mT) and 3LE states (broader signals, ∼50 to 200 mT). In the spectra of the triads containing PTZ, the CS state signals dominate, whereas for the triads containing oxidized PTZ, the 3NDI signals (zero-field splitting D ≈ 2000 MHz) prevail, both observations being in agreement with the ns-TA spectral studies. The electron spin polarization phase pattern of the 3NDI states of the triads indicates that the intersystem crossing (ISC) mechanism is spin-orbit charge-transfer ISC. Considering the 3CS state as ion pairs, the electron-exchange energy (J) is determined to be -39 to -59 MHz, and the electron spin dipolar interaction is 83-92 MHz.

Near-Infrared BODIPY Photosensitizer for Modulating Mitochondrial Fusion Proteins and Inhibiting Choroidal Neovascularization.

Photosensitizers have emerged as cytotoxic reactive oxygen species (ROS) activators in photodynamic therapy (PDT), which induced cell apoptosis. As the major contributors to ROS and oxidative stress, mitochondria play an important role in cell apoptosis. Although there are many reports about near-infrared 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) as photosensitizers (PSs) for PDT, this kind of PS has rarely been used for treating mitochondrial function and choroidal neovascularization application at the same time. Herein, a novel near-infrared PS (BDP2) characterized by good water solubility, long wavelength excitation, and high ROS quantum yield has been made. Under near-infrared light irradiation, BDP2 would generate ROS with high yield, induce a mitochondrial morphology change, and trigger cell apoptosis by changing the fusion protein level. Deep investigation revealed that BDP2 can cause oxidative stress, break the balance between fusion and fission of mitochondrial dynamics protein through decreasing fusion protein MFN2 and OPA1 expression, and finally cause cell apoptosis. Due to these characteristics, the BDP2 PS was used to treat choroidal neovascularization in animal models and can inhibit neovascularization.

Photocatalytic Metal Hydride Hydrogen Atom Transfer Mediated Allene Functionalization by Cobalt and Titanium Dual Catalysis.

Catalytic metal hydride hydrogen atom transfer (MHAT) reactions have proven to be a powerful method for alkene functionalization. This work reports the discovery of Co-porphines as highly efficient MHAT catalysts with a loading of only 0.01 mol % for unprecedented chemoselective allene functionalization under photoirradiation. Moreover, the newly developed bimetallic strategy by the combination of photo Co-MHAT and Ti catalysis enabled the successful carbonyl allylation with a wide range of amino, oxy, thio, aryl, and alkyl-allenes providing expedient access to valuable ß-functionalized homoallylic alcohols in over 100 examples with exceptional regio- and diastereoselectivity. Mechanism studies and DFT calculations supported that selectively transferring hydrogen atoms from cobalt hydride to allenes and generating allyl radicals is the key step in the catalytic cycle.

Plenty of Room on the Top: Pathways and Spectroscopic Signatures of Singlet Fission from Upper Singlet States.

We investigate dynamic signatures of the singlet fission (SF) process triggered by the excitation of a molecular system to an upper singlet state SN (N > 1) and develop a computational methodology for the simulation of nonlinear spectroscopic signals revealing the SN → TT1 SF in real time. We demonstrate that SF can proceed directly from the upper state SN, bypassing the lowest excited state, S1. We determine the main SN → TT1 reaction pathways and show by computer simulation and spectroscopic measurements that the SN-initiated SF can be faster and more efficient than the traditionally studied S1 → TT1 SF. We claim that the SN → TT1 SF offers novel promising opportunities for engineering SF systems and enhancing SF yields.

Singlet Fission, Polaron Generation and Intersystem Crossing in Hexaphenyl Film.

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells.

Atomically Dispersed Indium-Copper Dual-Metal Active Sites Promoting C-C Coupling for CO2 Photoreduction to Ethanol.

Photoreduction of CO2 to C2+ solar fuel is a promising carbon-neutral technology for renewable energy. This strategy is challenged by its low productivity due to low efficiency in multielectron utilization and slow C-C coupling kinetics. This work reports a dual-metal photocatalyst consisting of atomically dispersed indium and copper anchored on polymeric carbon nitride (InCu/PCN), on which the photoreduction of CO2 delivered an excellent ethanol production rate of 28.5 µmol g-1 h-1 with a high selectivity of 92 %. Coupled experimental investigation and DFT calculations reveal the following mechanisms underpinning the high performance of this catalyst. Essentially, the In-Cu interaction enhances the charge separation by accelerating charge transfer from PCN to the metal sites. Indium also transfers electrons to neighboring copper via Cu-N-In bridges, increasing the electron density of copper active sites. Furthermore, In-Cu dual-metal sites promote the adsorption of *CO intermediates and lower the energy barrier of C-C coupling.

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads.

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τT =0.1 µs), these new adducts show a longer-lived triplet excited state (τT =9.5 µs). Based on the singlet oxygen quantum yield (ΦΔ =7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D1 ↔Q1 ) at ∼80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05∼0.10 eV).

Switching Pathways of Triplet State Formation by Twisted Intramolecular Charge Transfer.

With the aim of constructing efficient photoelectric organic materials, a pyrido[3,2-g]quinoline derivative named LA17b has been synthesized, and its photodynamic relaxation processes in solvents and films were studied by time-resolved fluorescence and femtosecond transient absorption techniques. The steady-state fluorescence spectra show pronounced red-shift with the increase of the solvent polarity as well as in binary solvent hexane/ethanol by increasing ethanol concentration. However, the strong red-shift does not lead to quenching of the fluorescence. This is explained in terms of a twisted intramolecular charge transfer (TICT) state. The TICT state of LA17b in ethanol is highly emissive with a long fluorescence lifetime: 1.1 ns. TICT state was shown to play an important role in enhancement of intersystem crossing rate. TD-DFT calculations confirm the pathways of relaxation of locally excited state via TICT and triplet states. In films, the photodynamic properties are similar to that of LA17b in hexane and the TICT state vanishes due to the rigid environment. The obtained optical properties of this molecule suggest that it can be a promising candidate for various optoelectronic applications.

Trap-free exciton dynamics in monolayer WS2via oleic acid passivation.

Two-dimensional transition metal dichalcogenides have attracted a great deal of attention in the past few decades owing to their attractive optoelectronic properties. However, their widespread utility in photonic devices and components is still limited owing to their weak photoluminescence. While various treating methods are in place to improve the photoluminescence yield, the impact of these treatments on the excited state (especially exciton) dynamics in these two-dimensional materials remains ill defined. In this work, exciton dynamics in pristine and oleic acid-treated monolayer WS2 were comprehensively studied through various ultrafast experimental techniques. We demonstrate that oleic acid effectively passivates the defect states in as-fabricated WS2, resulting in trap-free exciton dynamics and exciton annihilation rate reduction, which leads to stronger steady-state photoluminescence and longer photoluminescence lifetime. These results provide valuable information on the intrinsic exciton dynamics in monolayer WS2, which could also be applicable in other two-dimensional transition metal dichalcogenides and help improve optoelectronic device performance.

Toward efficient photochemistry from upper excited electronic states: Detection of long S2 lifetime of perylene.

A long 0.9 ps lifetime of the upper excited singlet state in perylene is resolved by femtosecond pump-probe measurements under ultraviolet (4.96 eV) excitation and further validated by theoretical simulations of transient absorption kinetics. This finding prompts exploration and development of novel perylene-based materials for upper excited state photochemistry applications.

Bodipy-Containing Porous Microcapsules for Flow Heterogeneous Photocatalysis.

Photocatalysis is a facile strategy for complex chemical transformations. Heterogeneous photocatalysis, especially in the flow system, has attracted much attention as it avoids the separation of catalysts. Herein, a kind of a Bodipy-containing porous microcapsule heterogeneous photocatalyst was rationally constructed with modulation on a multiscale. The diiodo-Bodipy with methacrylate (MA-2IBDP) was synthesized as a polymerizable photosensitizer. After immobilization in a polymer matrix, the intersystem crossing rate constant of MA-2IBDP increased to 2.7 × 1010 s-1 and its triplet excited-state lifetime prolonged to ∼1 ms. Porous structures in microcapsules were created to facilitate mass transfer. A flat plate flow reactor was constructed to fix the catalytic microcapsules and improve light utilization. With the combination of all the above benefits, the reaction rate constant (0.896 s-1) is 10 times faster than that of MA-2IBDP in a homogeneous system for juglone synthesis. The continuous production can last for 30 h without yield decrease. The photocatalyst can also be used in aza-Henry reaction, Alder-Ene reaction, and oxidation of thiols to disulfides with conversion rates above 95%. This study provides a means for the construction of heterogeneous catalysts and the flow reaction system.

Does Twisted π-Conjugation Framework Always Induce Efficient Intersystem Crossing? A Case Study with Benzo[b]- and [a]Phenanthrene-Fused BODIPY Derivatives and Identification of a Dark State.

The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[b]-fused BODIPY, BDP-B; and [a]phenanthrene-fused BODIPY, BDP-P), are studied with steady-state and time-resolved optical and electron paramagnetic resonance (TREPR) spectroscopic methods as well as with ADC(2) theoretical investigations. Interestingly, BDP-B has a planar π-conjugation framework, but it displays weaker UV-vis absorption (ε = 3.8 × 104 M-1 cm-1 at 569 nm) and fluorescence (ΦF < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τF = 0.2 ns), and a long-lived triplet state (τT = 132.3 µs). In comparison, the more twisted BDP-P shows stronger UV-vis absorption (ε = 9.8 × 104 M-1 cm-1 at 640 nm) and fluorescence (ΦF = 70%), longer singlet-excited-state lifetime (τF = 6.4 ns), and shorter triplet-state lifetime (τT = 18.9 µs). In contrast to helicenes (ΦT = ca. 90%), the ISC of BDP-P and BDP-B is nonefficient (ΦT < 23%). The electron spin selectivity of the ISC of the derivatives is different, manifested by the phase pattern of the TREPR spectra as AAEAEE and EEEAAA for BDP-B and BDP-P, respectively. The spatially confined T1 state wave function of the twisted molecule keeps the T1 state energy high (1.44-1.61 eV). A dark S1 state was identified for BDP-B. This work demonstrated that the twisted π-conjugated framework does not necessarily induce efficient ISC and we found a dark singlet state for BODIPY, which is rare.

Ultrafast spectroscopy reveals singlet fission, ionization and excimer formation in perylene film.

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications.

Spin-Orbit Charge-Transfer Intersystem Crossing in Anthracene-Perylenebisimide Compact Electron Donor-Acceptor Dyads and Triads and Photochemical Dianion Formation.

Perylenebisimide (PBI)-anthracene (AN) donor-acceptor dyads/triad were prepared to investigate spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Molecular conformation was controlled by connecting PBI units to the 2- or 9-position of the AN moiety. Steady-state, time-resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI-9-AN with orthogonal geometry shows weak ground-state coupling and efficient intersystem crossing (ISC, ΦΔ =86 %) as compared with PBI-2-AN (ΦΔ =57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long-lived PBI localized triplet state was observed (τT =139 µs). Time-resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI. Reversible and stepwise generation of near-IR-absorbing PBI radical anion (PBI-⋅ ) and dianion (PBI2- ) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near-IR absorption bands of PBI.- is unable to produce PBI2- .

Singlet fission from upper excited singlet states and polaron formation in rubrene film.

Femtosecond fluorescence up-conversion and transient absorption pump-probe setups are applied to study the relaxation dynamics of the lower and upper excited singlet electronic states in easy-to-make rubrene films. Upon 250 nm (4.96 eV) excitation, singlet fission was observed directly from S2 state bypassing S1 state within 30 fs i.e. breaking the classical Kasha rule. From the transient absorption measurements, polaron formation was also detected on the same time scale. Both singlet fission and polaron formation are accelerated from upper excited states compared with S1 state. Our work shows that rubrene films with low degree of crystallinity could display efficient singlet fission, notably in the case of excitation to upper lying electronic states. This can strongly expand the applications of rubrene in organic electronics. Moreover, our results will provide a new direction for synthesizing novel materials with optimized excited state properties for organic photovoltaic applications.

N

To study the effect of a stable radical on the photophysical properties of a phosphorescent Pt(II) coordination framework and the intramolecular magnetic interaction between radical ligands in the N^N Pt(II) bisacetylide complexes, we prepared a series of N^N Pt(II) bis(acetylide) complexes with oxoverdazyl radical acetylide ligands. The linker between the Pt(II) center and the spin carrier was systematically varied, to probe the effect on the sign and magnitude of the spin exchange interactions between the radical ligands and photophysical properties. The complexes were studied with steady-state and femtosecond/nanosecond transient absorption spectroscopy, continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) computations. The transient absorption spectral studies show that the doublet excited state of the radicals are short-lived (τD ≈ 2 ps) and nonfluorescent. Moreover, the intrinsic long-lived triplet excited state (τT = 1.2 µs) of the Pt(II) coordination center was efficiently quenched by the radical (τT = 6.9 ps for one representative radical Pt(II) complex). The intramolecular magnetic interaction between the radical ligands through the diamagnetic Pt(II) atom was studied with temperature-dependent EPR spectroscopy; antiferromagnetic exchange interaction (-J S1S2, J = -5.4 ± 0.1 cm-1) for the complex with the shortest radical-radical distance through bridge fragments was observed. DFT computations give similar results for the sign and magnitude of the J values. For complexes with larger inter-radical distance, however, very weak coupling between the radical ligands was observed (|J| < 0.7 cm-1). Our results are useful for the study of the effect of a radical on the photophysical properties of the phosphorescent transition-metal complexes.

Upper Excited State Photophysics of Malachite Green in Solution and Films.

Relaxation pathways of upper excited electronic states of malachite green (MG) in ethanol and in films are studied by steady-state and time-resolved spectroscopic techniques. In contrast to ethanol, where MG emits weak short-lived spectrally well separated S2 and S1 fluorescence with the lifetimes ∼0.3 and ∼0.9 ps, MG films show a much stronger broadband fluorescence within 430-700 nm, revealing multiexponential kinetics with the characteristic decay times τ1 ≈ 1 ps, τ2 ≈ 10 ps, τ3 ≈ 0.05-0.8 ns, and τ4 ≈ 2-3 ns. By the analysis of spectroscopic responses of MG in ethanol and in films as well as by theoretical modeling, we demonstrate that significant increase of fluorescence lifetimes and substantial enhancement of fluorescence intensity in MG films are stipulated by the decrease of efficiency of the S2 → S1 and S1 → S0 internal conversion, which in turn is caused by hindrance of rotation of MG's phenyl rings controlling the S2/S1 and S1/S0 conical intersections. These findings indicate that MG films may become promising non-Kasha materials (with reasonable S2 emission) with numerous photophysical and photochemical applications.

Direct Observation of Long-Lived Upper Excited Triplet States and Intersystem Crossing in Anthracene-Containing PtII Complexes.

Exceptionally long-lived T2 states (7 ns) were observed with the N^N PtII bisacetylide complex (Pt-1) and trans-bis(phosphine) PtII bisacetylide complexes (Pt-2, Pt-3) containing anthryl acetylide ligands. For Pt-1, fluorescence of the anthryl moiety (An) was quenched and phosphorescence was observed. Under 350 nm excitation, the upper long-lived triplet state T2 (3An) was populated via ultrafast intersystem crossing (ISC) of S1 (1An) → T2 (3An) (within 0.2 ps). Interestingly, Pt-3, after population of the S1 state, emits strong fluorescence (ΦF = 89%); the poor ISC is due to the high-lying T2 (3An, 3.36 eV) versus S1 (1An, 2.55 eV) state and the large energy gap between S1 (1An, 2.55 eV) and T1 (3An, 1.32 eV) states. The population of the upper excited state S2 (1LLCT, 3.49 eV) turns to an efficient S2 → T2 → T1, and ISC yield increases by 55% compared with S0 → S1 excitation. These results present new in-depth insights into fundamental photochemistry of upper excited states.

Singlet Fission from Upper Excited Electronic States of Cofacial Perylene Dimer.

Singlet fission directly from the upper excited vibrational and electronic states of cofacial perylene dimers, bypassing the relaxed state S1, was detected within 50 fs. This process competes well with vibrational cooling in S1 (4.7-7.0 ps) and S2 → S1 internal conversion (380 fs). The singlet fission has the energy threshold E = 3.06 eV. Other competitive relaxation processes are excimer and dimer cation formation on an ultrafast time scale. Excitation to higher energy levels (4.96 eV) leads to a higher efficiency of singlet fission.

Anthracene-Naphthalenediimide Compact Electron Donor/Acceptor Dyads: Electronic Coupling, Electron Transfer, and Intersystem Crossing.

We attached different electron donors of phenyl, anthryl, and alkylamino moieties, to electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads. The purpose is to study the effect of electron coupling (the magnitude is the matrix element, VDA) on the photophysical properties of UV-vis absorption, fluorescence emission, especially spin-orbit charge transfer intersystem crossing. We found that the magnitude of VDA depends on the electron donating strength of the aryl moieties ( VDA = 0.22-0.55 eV), as well as the molecular conformation, based on steady state and time-resolved transient absorption spectroscopies. We also found that electron coupling does not show the add-up (or synergetic) effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83 ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in toluene) produces triplet state with ΦISC = 19%. The triplet state lifetime is up to 22 µs. This result indicates that orthogonal geometry for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such as the energy gap between the CS state and triplet state also determine the ISC efficiency.