Charge Transfer Character Research Articles

Pyrimidine-based thermally activated delayed fluorescent materials with unique asymmetry for highly-efficient organic light-emitting diodes

We have designed and synthesized three thermally activated delayed fluorescent (TADF) emitters as 2-phenypyrimidine (PPM)-10H-spiro[acridan-9,9′-fluorene] (SPAc) hybrids, collectively named X-SPAc-PPM, featured an asymmetrical D-A-D molecular architecture, through the para-/ortho-, para-/meta-, and para-/ortho- positions of the phenyl spacer in SPAc with PPM unit. The effect of the hybrid connection at ortho-, meta-, or para-positions on the different properties of pyrimidine derivatives is detailed discussed. The investigation suggests that the localization of the HOMO has the preferential choice tendency in asymmetric X-SPAc-PPM, and following the order of para-position arms, ortho-position arms, which adjust effectively the intramolecular charge transfer (ICT) character between SPAc and PPM unit. As a result, three isomers have the smaller ΔEST of 0.05–0.16 eV, and exhibit obvious TADF character. In addition, p,m-SPAc-PPM endowed its OLED device with impressive performance, including the maximum external quantum efficiency around 25.1%, and reduced efficiency roll-offs. The smaller ΔEST, over 25% EQE, and lower efficiency roll-offs of three isomeric X-SPAc-PPM emitters further prove the great potential of asymmetrical design strategy with the same donor for highly efficient TADF devices.

Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective.

In this work, we present a computational protocol that is able to predict the experimental absorption and emission spectral shapes of Eu2+-doped phosphors. The protocol is based on time-dependent density functional theory and operates in conjunction with an excited-state dynamics approach. It is demonstrated that across the study set consisting of representative examples of nitride, oxo-nitride, and oxide Eu2+-doped phosphors, the energy distribution and the band shape of the emission spectrum are related to the nature of the 4f–5d transitions that are probed in the absorption process. Since the 4f orbitals are very nearly nonbonding, the decisive quantity is the covalency of the 5d acceptor orbitals that become populated in the electronically excited state that leads to emission. The stronger the (anti) bonding interaction between the lanthanide and the ligands is in the excited state, the larger will be the excited state distortion. Consequently, the corresponding emission will get broader due to the vibronic progression that is induced by the structural distortion. In addition, the energy separation of the absorption bands that are dominated by states with valence 4f–5d and a metal to ligand charge transfer character defines a measure for the thermal quenching of the studied Eu2+-doped phosphors. Based on this analysis, simple descriptors are identified that show a strong correlation with the energy position and bandwidth of the experimental emission bands without the need for elaborate calculations. Overall, we believe that this study serves as an important reference for designing new Eu2+-doped phosphors with desired photoluminescence properties.

Accelerating symmetry-breaking charge separation in a perylenediimide trimer through a vibronically coherent dimer intermediate

Understanding the photophysics and photochemistry of molecular π-stacked chromophores is important for utilizing them as functional photonic materials. However, these investigations have been mostly limited to covalent molecular dimers, which can only approximate the electronic and vibronic interactions present in the higher oligomers typical of functional organic materials. Here we show that a comparison of the excited-state dynamics of a covalent slip-stacked perylenediimide dimer (2) and trimer (3) provides fundamental insights into electronic state mixing and symmetry-breaking charge separation (SB-CS) beyond the dimer limit. We find that coherent vibronic coupling to high-frequency modes facilitates ultrafast state mixing between the Frenkel exciton (FE) and charge-transfer (CT) states. Subsequently, solvent fluctuations and interchromophore low-frequency vibrations promote CT character in the coherent FE/CT mixed state. The coherent FE/CT mixed state persists in 2, but, in 3, low-frequency vibronic coupling collapses the coherence, resulting in ultrafast SB-CS between the distal perylenediimide units.

Synthesis of novel luminescent cyclometallated platinum (IV) complexes with a quinoline Schiff bases ligand and their photophysical properties

The synthesis, characterization, electro and photophysical properties of eight cyclometallated platinum (IV) complexes 6a–6d and 7a–7d derived from quinoline Schiff base ligands are reported. Spectral studies revealed that nonparticipation of nitrogen atom and participation of carbon atom of the quinoline ring in the bond formation through [NN′C] mode of coordination for complex 6, whereas [N S O] mode of coordination with octahedral geometry for complex 7. The absorption and emission spectra studied in acetonitrile medium at room temperature showed that the ligand charge transitions within the quinoline Schiff base ligands with Metal Ligand Charge Transfer (MLCT) character. The broad band appeared in the range of 460–502 nm is attributed to the higher emission intensities of cyclometallated platinum (IV) compounds. The HOMO, LUMO and energy gap of the synthesized complexes was found to be − 2.60 eV, −4.75 eV and 2.15 eV through Density Functional Theory (DFT).

Interface engineering of ZnO/In2O3 Z-scheme heterojunction with yolk-shell structure for efficient photocatalytic hydrogen evolution

Photocatalytic activities based on charge transfer and separation at interfaces can be effectively promoted by matching chemical Z-scheme heterojunctions and electronic structures. However, the fabrication of morphology-controlled heterojunctions has been a challenge to many researchers. Herein, hollow spheres ZnO/In2O3 was constructed via hydrothermal strategy by fine-tuning the concentration of metal nitrate precursors, and special yolk-shell morphology was generated by the symmetrical Ostwald ripening process. The fabricated Z-Scheme heterojunction exhibited characteristic multi-channel charge transfer properties, which favors the spatial separation of carriers. The ZnO/In2O3 hollow spheres increased the utilization of light source, and the yolk-shell structure increased the interface area and the active sites. Benefiting from the synergistic effect, the ZnO/In2O3 exhibited excellent photocatalytic activities for H2 production, which was comparatively 26.7 times that obtained using ZnO nanoparticles. From calculations based on DFT, the Z-scheme photogenerated charge transfer mechanism was proposed. The proposed mechanism was verified by analyzing the chemical properties (surface) and the •O2− and •OH free radical concentrations before and after the photoreaction. The mechanism presented the ZnO/In2O3 with strong capabilities for H2 production and elucidated the improved photocatalytic performances. This work creates an innovative route for constructing Z-scheme photocatalytic systems with great photocatalytic activities for H2 production.

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base.

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

Red Absorbing Cyclometalated Ir(III) Diimine Photosensitizers Competent for Hydrogen Photocatalysis.

Two new cyclometalated Ir(III) diimine complexes were used as photosensitizers for homogeneous hydrogen evolution reaction (HER). These complexes were characterized by electrochemistry, ultraviolet-visible absorption, time-resolved and steady-state photoluminescence spectroscopy as well as by theoretical methods. The metal-ligand-to-ligand charge transfer character of their lowest excited state was shown to be competent for efficient H2 photoproduction in the presence of [Co(dmgH)2(py)Cl] as the hydrogen evolution catalyst, triethanolamine as the sacrificial electron donor, and HBF4 as the proton source. Under optimized experimental conditions, both complexes displayed HER over a period of more than 90 h, with turnover numbers reaching up to 11,650, 10,600, and 174 molH2 molPS-1 under blue-, green-, and red-light irradiation, respectively. Both complexes showed higher stability and efficiency vs HER than most of the previously described systems of the same kind.

Role of Oxygen States in the Low Valence Nickelate La4Ni3O8

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La$_{4}$Ni$_{3}$O$_{8}$ and a prototypical cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $\Delta$. Nevertheless, the transition-metal-oxygen hopping is larger in La$_{4}$Ni$_{3}$O$_{8}$ than in La$_{2-x}$Sr$_{x}$CuO$_{4}$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La$_{4}$Ni$_{3}$O$_{8}$ despite its larger $\Delta$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

Synthesis, photophysical, electrochemical and computational studies of novel 2-aminoimidazolones with D-π-A framework

Two novel 2-aminoimidazolone derivatives based on donor-linker-acceptor (D-π-A) architecture (imidazoLUMS1 and imidazoLUMS2) have been synthesized and characterized by standard spectroscopic techniques. The donor building block in these compounds comprises of 4-benzylidene-2-(piperidin-1-yl)-1H-imidazol-5(4H)-one and 4-(thiophen-2-ylmethylene)-2-((3,4,5-trimethoxyphenyl)amino)-1H-imidazol-5(4H)-one respectively, while cyanoacetic acid was used as the electron acceptor linked to donor through a π-conjugated spacer. Synthetic methodology involved our multicomponent reaction following by Suzuki coupling and, Knoevenagel condensation reactions. ImidazoLUMS1 (λmax = 436 nm) and imidazoLUMS2 (λmax of 482 nm) showed a bathochromic shift of 58 nm and 104 nm with respect to their parent molecule 1 (λmax = 378 nm). The observed stoke shifts (Δλ = 134 nm and Δλ = 112 nm) and band gap values 2.40 eV and 2.26 eV support thermodynamic feasibility for intramolecular charge transfer character from donor to acceptor moiety. DFT studies, performed on GAUSSIAN 09, for the charge properties show their suitability for optoelectronic devices and an excellent correlation between experimental and computational data.

Dimethyl malonate based organic compounds bearing different aromatic substituents: Synthesis, photophysics and application in anti-blue light lenses

Five new organic conjugated molecules (TM1–TM5) based on the electron-withdrawing unit (dimethyl malonate) were synthesized and studied. Among them, TM1–TM4 exhibited intense absorption bands from ultraviolet to blue region, which were mainly arisen from the π–π* transitions, mixed with intramolecular charge transfer (ICT) characters. The absorption bands in the region of 360–450 nm for TM5 might be assigned to ICT transitions, mixed with the π–π* feature. Meanwhile, the increasing electron-donating ability of the substituents caused a bathochromic shift of emission. The different electron-donating substituents in the molecular structure make a slight difference in oxidation potential. Furthermore, there is almost no mass loss up to 200 °C for these compounds and all of them exhibit the satisfactory photostability. In addition, the anti-blue light performance of these compounds doped in optical films was studied and TM5 exhibited the best anti-blue light performance. These photophysical, photochemical and electrochemical properties indicated that organic conjugated small molecules containing dimethyl malonate unit are excellent optical functional materials for anti-blue light applications.

Revealing the mechanisms of mercury adsorption on metal-doped kaolinite(001) surfaces by first principles

Natural kaolinite exhibit high affinity for heavy metals while the interaction mechanisms in the presence of heteroatoms remain largely elusive, which are tackled by first principles. In this paper, three common dopants (Mg, Ca, Fe) were employed to construct metal-doped kaolinite(001) (K(001)) surfaces. We found that Mg-doped K(001) was the most stable surface in terms of thermal stability and structural analysis, consistent with the pervasive isomorphic substitution in kaolinite minerals. The interaction of mercury with Mg-doped K(001) surface was investigated in the form of predominant top-site and bridge-site models. The effects of chloride on the interaction were also studied. The results demonstrated that the strongest adsorption occured in the present of dopants and the absence of chloride. The electronic properties revealed a significant charge transfer (up to 1.28 electrons) and chemisorption character at the interfaces when dopants were introduced, which could be ascribed to the overlapping of Hg-5d and Os-2p (surface O) orbitals in the range of −7.5 eV to +0.5 eV. Additionally, the chloride had a profoundly adverse influence on mercury adsorption due to the upward shift of Hg-6s and Hg-6p orbitals. The studies are beneficial to understand the interaction mechanisms of natural minerals toward environmental pollutants in actual applications.

Thermally Activated Delayed Fluorescence Mechanism of a Bicyclic "Carbene-Metal-Amide" Copper Compound: DFT/MRCI Studies and Roles of Excited-State Structure Relaxation.

Herein we investigated the luminescence mechanism of one "carbene-metal-amide" copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S1 state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S0, S1, and T1) model is sufficient to describe the TADF mechanism, and the T2 state should play a negligible role. The calculated S1-T1 energy gap of 0.10 eV and proper spin-orbit couplings facilitate the reverse intersystem crossing (rISC) from T1 to S1. At 298 K, the rISC rate of T1 → S1 (∼106 s-1) is more than 3 orders of magnitude larger than the T1 phosphorescence rate (∼103 s-1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101 s-1). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck-Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important.

Oligotriarylamine-Extended Organoboranes with Tunable Electron-Donating Strength by Changing the Number of Donor Units.

Triarylborane (Ar3B) and triarylamine (Ar3N) have been widely employed to construct electronically different donor-acceptor (D-A) systems. Herein, we describe a series of A-D-A-type luminescent organoboranes L-B2Nn (n = 1, 3, 5) that show an increased number of Ar3N units as electron donors and two terminal Ar3B as acceptors. When the Ar3N moieties were extended from one to five units, their electron-donating strength was gradually enhanced and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps could also be tuned, which was further reflected in the red-shifted emissions from blue (λem = 458 nm) to orange (λem = 595 nm) with a decrease in Egap(elect) from 3.19 to 2.61 eV. L-B2N5 showed a huge Stokes shift (∼14 057 cm-1) and a considerably bright emission with an enhanced solid-state quantum efficiency (ΦS = 98%) compared with the other members. L-B2N3 and L-B2N5 exhibited aggregation-induced emissions (AIEs), and an apparent solvatochromic shift was also observed in the emission spectra as the solvent was changed from hexane to tetrahydrofuran (THF) (430 → 595 nm). In addition, the donor-acceptor charge-transfer character in these organoboranes caused a thermally responsive emission over a broad range.

Photophysical properties and sensing mechanism of fluorescent coumarin–chalcone hybrid for biothiols: A theoretical study

AbstractThe photophysical properties and sensing mechanism of a novel coumarin–chalcone hybrid fluorescent probe (CCH) for biothiols were investigated by density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods. The theoretical calculations well reproduced the experimental spectra and clarified the experimental observed fluorescence “on–off” switching. Potential energy surface (PES) results revealed that the excited‐state intramolecular proton transfer (ESIPT) process is thermodynamically and kinetically viable for CCH, with the keto form more stable than the enol form. ESIPT process and the significant changes of the orbital energies should be responsible for the red fluorescence of CCH. The results revealed that the lowest lying transition of probe CCH corresponds to electronic transition between HOMO and LUMO with charge transfer (CT) character. The addition of cysteine to CCH breaks the conjugation and the intramolecular CT. A nonclassical photoinduced electron transfer (PET) process should contribute to the fluorescence quenching of CCH‐Cys. Thermodynamic and kinetic calculations on the recognition reaction predicted a moderate Gibbs free energy barrier, which proves the possibility of the Michael addition reaction and induces the rapid response of the probe CCH for cysteine.

Effects of the Structure and Temperature on the Nature of Excitons in the Mo0.6W0.4S2 Alloy.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4S2 compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton–phonon interactions. The exciton–phonon interactions in the alloy are closer to those for MoS2 than those for WS2. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.

Construction of heteroaryl-bridged NIR AIEgens for specific imaging of lipid droplets and its application in photodynamic therapy

As a kind of subcellular organelle, lipid droplets (LDs) play a critical role in the body’s normal metabolism. LDs have gained increasing attention as a fluorescent photodynamic target site. Near-infrared (NIR) organic light-emitting luminescent materials, with aggregation-induced emission (AIE)-active feature, preeminent LD-imaging ability, and effective reactive oxygen species (ROS) production property, have been widely used for photodynamic therapy (PDT) in diagnostic therapeutics, but its application remains challenging. In the present work, three novel NIR organic compounds with AIE-active feature, namely, TPET-Is, TPET-Fu, and TPEF-Is, were developed and synthesized. These heteroaryl-bridged molecules possess a donor–donor–π–acceptor structure and strong intramolecular charge transfer character. These AIEgens are capable of high-fidelity LD imaging in living cells (Pearson’s coefficient values: 0.94, 0.96, 0.97) due to their biocompatibility, good photostability, and strong lipophilicity (LogP values: 9.39, 7.89, 8.03), respectively. Moreover, they can be also applied in bright imaging the LDs of oil-rich plant tissues, such as those of sunflower seeds. The respective AIEgens TPET-Fu of these compounds can also produce ROS in the condition of white light to effectively kill live Hela cells. The present study thus provides a potential strategy through heteroaryl-bridged molecular engineering for LD-targeted imaging and PDT application.

Optoelectronic Current through Unbiased Monolayer Amorphous Carbon Nanojunctions.

Monolayer amorphous carbon (MAC) is a recently synthesized disordered 2D carbon material. An ensemble of MAC nanofragments contains diverse manifestations of lattice disorder, and because of disorder the key unifying characteristic of this ensemble is poor electronic conductance. Curiously, our computational analysis of the electronic properties of MAC nanofragments revealed an additional commonality: a robust presence of charge-transfer character for electronic transitions from the occupied to virtual orbitals. This charge-transfer property suggests possible applications in optoelectronics. In this Letter, we demonstrate computationally that a laser pulse induces directional electronic currents in unbiased MAC nanojunctions and discuss the effects of pulse intensity on the magnitude of electron transfer.

Impact of Charge-Resonance Excitations on CT-Mediated J-Type Aggregation in Singlet and Triplet Exciton States of Perylene Di-Imide Aggregates: A TDDFT Investigation

The modulation of intermolecular interactions upon aggregation induces changes in excited state properties of organic molecules that can be detrimental for some optoelectronic applications but can be exploited for others. The time-dependent density functional theory (TDDFT) is a cost-effective approach to determining the exciton states of molecular aggregates, and it has been shown to provide reliable results when coupled with the appropriate choice of the functional. Here we apply a general procedure to analyze the aggregates’ exciton states derived from TDDFT calculations in terms of diabatic states chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space. We apply the approach to study energy profiles, interstate couplings, and the charge-transfer character of singlet and triplet exciton states of perylene di-imide aggregates (PDI). We focus on the intermolecular displacement along the longitudinal translation coordinate, which mimics different amounts of slip-stacking observed in PDI crystals. The analysis, in terms of symmetry-adapted Frenkel excitations (FE) and charge-resonance (CR) states and their interactions, discloses how the interchange of the H/J character for small longitudinal shifts, previously reported for singlet exciton states, also occurs for triplet excitons.

Synthesis of selective heteroatomic collectors for the improved separation of sulfide minerals

The separation of sulfide minerals has always been a difficult problem in the field of flotation due to their often-similar affinity to the collectors. It is therefore crucial to improve the selectivity of collectors and to clarify the mechanism of separation. In this study, chelating collectors based on heterocyclic thioketone framework, namely benzo[d]oxazole-2(3H)-thione (BOT), 1H-benzo[d]imidazole-2(3H)-thione (BIT) and 5-(butylthio)-1,3,4-thiadiazole-2-thiol (CSC-1), were synthesized. An industry standard collector ethyl xanthate (EX) was used as a reference to evaluate the flotation performances of the synthesized collectors. The flotation results show that the trend of selectivity is BIT ≈ BOT > CSC-1 > EX. Zeta potential measurements confirm the above trend. These results are further rationalized by theoretical investigations, namely the adsorption free energy, electrostatic potential energy surface (EPS), total atomic dipole moment corrected Hirshfeld (ADCH) charge along with the HOMO-LUMO topology modelled by DFT calculation. By introducing concepts that are capable of describing the ionic character (the total ADCH charge of the coordination sites (T-ADCH-C)) and the charge transfer character of coordination (the degree of electron transfer (DET)), a qualitative model can be derived to demonstrate the theoretical rational of selectivity. Strong ionic collectors (EX) with a higher DET have a stronger collecting ability and in turn, a worse selectivity. Non-ionic collectors (BOT and BIT) with lower DET, on the contrary, display a better selectivity and a weaker collecting ability. Weak ionic collectors (CSC-1) are at the intermediate level. This work not only reveals the potential of using thioketones as selective sulfide collectors, but also provides a selectivity model based on the bonding and charge transfer character that can guide further development of more selective sulfide collectors.

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements.

The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H2 evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure–reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet–triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp2)–H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H2 and contra-thermodynamic isomerization.