Lowest Excited Triplet State Research Articles

Neutral iridium(iii) complexes bearing BODIPY-substituted N-heterocyclic carbene (NHC) ligands: synthesis, photophysics, in vitro theranostic photodynamic therapy, and antimicrobial activity.

The synthesis, photophysics, and photobiological activities of a series of novel neutral heteroleptic cyclometalated iridium(iii) complexes incorporating boron dipyrromethene (BODIPY) substituted N-heterocyclic carbene (NHC) ligands (Ir1-Ir5) are reported. The effect of the substitution position of BODIPY on the NHC ligands, either on C4 of the phenyl ring (Ir1-Ir3) or C5 of the benzimidazole unit (Ir4 and Ir5), and its linker type (single or triple bond) on the photophysical properties was studied. Ir1-Ir5 exhibited BODIPY-localized intense 1IL (intraligand transition)/1MLCT (metal-to-ligand charge transfer) absorption at 530-543 nm and 1,3IL/1,3CT (charge transfer) emission at 582-610 nm. The nanosecond transient absorption results revealed that the lowest triplet excited states of these complexes were the BODIPY-localized 3π,π* states. Complexes Ir4 and Ir5 exhibited blue-shifted 1IL absorption and 1,3IL/1,3CT emission bands compared to the corresponding absorption and emission bands in complexes Ir1 and Ir3. However, replacing the methyl substituents on N3 of benzimidazole in complexes Ir1 and Ir4 with oligoether substituents in Ir3 and Ir5, respectively, did not impact the energies of the low-energy absorption and emission bands in the corresponding complexes. Water-soluble complexes Ir3 and Ir5 have been explored as photosensitizers for in vitro photodynamic therapy (PDT) effects toward human SKMEL28 melanoma cells. Ir3 showed no dark cytotoxicity (EC50 > 300 μM) but good photocytotoxic activity (9.66 ± 0.28 μM), whereas Ir5 exhibited a higher dark cytotoxicity (20.2 ± 1.26 μM) and excellent photocytotoxicity (0.15 ± 0.01 μM). The phototherapeutic indices with visible light (400-700 nm) activation were >31 for Ir3 and 135 for Ir5. Ir3 and Ir5 displayed 1O2 quantum yields of 38% and 22% in CH3CN, respectively, upon 450 nm excitation. Ir5 was more effective at generating reactive oxygen species (ROS) in vitro. Ir5 was also active against Staphylococcus aureus upon visible light activation, with a phototherapeutic index of >15 and EC50 value of 6.67 μM. These photobiological activities demonstrated that these neutral Ir(iii) complexes are promising in vitro PDT reagents, and substitution at C5 on the benzimidazole group of the NHC ligand was superior to C4 substitution on the phenyl ring.

Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors.

Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor-acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.

A theoretical study on spectroscopic properties and quantum yields of chiral-at-metal cyclometalated Pt(II) complexes

A new kind of chiral-at-metal Pt(II) complexes have been explored with theoretical methods. These chiral complexes are synthesized with achiral chelate ligands and the platinum(II) ions serve as the stereo center. To explore the intersystem crossing (ISC) and radiative decay process, we investigated the absorption and phosphorescence property, spin-orbit coupling (SOC) matrix element, major intersystem crossing channel and phosphorescent transition rate. For the temperature-dependent nonradiative decay process, the metal-centered excited state (3MC), the transition state (TS), and the minimum energy crossing point (MECP) were investigated with theoretical calculation. The way of temperature-independent nonradiative decay which has weak influence on nonradiative decay rate was also compared through the calculation of reorganization energy (λ) between the ground state (S0) and the lowest triplet excited state(T1).

Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes.

Three Pt(II) complexes, Pt(czpyOczpy), Pt(czpyOczpy-Me), and Pt(czpyOczpy-OMe), are designed to elucidate the inherent relationship between electronically excited-state and photo- and electroluminescent properties. These complexes showed a blue-shifted phosphorescence with a narrowing spectral profile, which are interrelated with the variation of T1 states from the 3MLCT, hybridized 3(MLCT/LC) to 3LC transition. This change is ascribed to the destabilization of LUMO energy levels on the pyridinyl site, leading to more electron distribution on the carbazolide unit in T1. Moreover, the solution-processed device of Pt(czpyOczpy-OMe), featuring a 3LC transition, shows the best color purity of blue light. Compared to the device of Pt(czpyOczpy) with 3MLCT character, the device of Pt(czpyOczpy-Me) with hybridized 3(MLCT/LC) exhibits improved color purity and external quantum efficiency (10.2%) at a luminance of 1000 cd/m2. Therefore, this work gives a mechanistic interpretation of the phosphorescent properties of tetradentate Pt(II) complexes derived from the manageable lowest triplet excited states.

The low-lying electronic states of HfO2− and HfO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The low-lying doublet states of the HfO2− anion and low-lying triplet excited states of the HfO2 molecule were studied using the SAC-CI method. The vertical excitation spectrum of HfO2− was calculated. Four valence-bound states (X2A1, 12B1, 22A1, and 12B2(II)) and one dipole-bound state (2A1(DB)) of HfO2− were found. The vertical electron detachment energies (VDEs) from the ground electronic state of HfO2− to the triplet excited states of HfO2 were obtained. These VDEs were compared with previous photoelectron spectra of HfO2−, and possible candidate states for the unassigned bands were found. The equilibrium geometries of the valence-bound states of HfO2− and some selected triplet excited states of HfO2 were optimized. The adiabatic relative energies of these electronic states were obtained. Density functional theory (BPW91) was also applied to compute some electronic states of HfO2− and HfO2 for comparison.

A New Class of Homoleptic and Heteroleptic Bis(terpyridine) Iridium(III) Complexes with Strong Photodynamic Therapy Effects.

Six homo- or heteroleptic tricationic Ir(R1-tpy)(R2-tpy)3+ complexes (Ir1-Ir6, R1/R2 = Ph, 4'-N(CH3)2Ph, pyren-1-yl, or 4'-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}Ph, tpy = 2,2';6',2"-terpyridine) were synthesized and tested for photodynamic therapy (PDT) effects. The ground- and excited-state characteristics of these complexes were studied systematically via spectroscopic methods and quantum chemistry calculations. All complexes possessed intraligand charge transfer (1ILCT) / metal-to-ligand charge transfer (1MLCT) dominated transition(s) in their low-energy absorption bands, which red-shifted with the increased electron-releasing strength of the R1/R2 substituent. Five of the complexes exhibited ligand-centered 3 π,π*/3ILCT/3MLCT emission. With a stronger electron-releasing R1/R2 substituent, the degree of charge transfer contribution increased, leading to a decrease of the emission quantum yield. When the 4'-N(CH3)2Ph substituent was introduced on both tpy ligands, the emission of Ir3 was completely quenched. Our study on the transient absorption of these complexes demonstrated that they all possessed broadband triplet excited-state absorption in the 400-800 nm region. Pyrenyl substitution of one or more tpy ligands, as in Ir4 and Ir5, increased the lifetimes of the lowest triplet excited state and the singlet oxygen (1O2) production efficiencies. Ir1-Ir5 were nontoxic toward SK-MEL-28 cells, with photocytotoxicities that varied from 0.18 to 153 µM. Among them, Ir4 had the highest 1O2 quantum yield (0.81) in cell-free conditions, showing the largest photocytotoxicity against SK-MEL-28 cells for Ir(III) PSs to date, and was the most efficient generator of reactive oxygen species (ROS) in vitro. Ir4 possessed a very large phototherapeutic index (PI = dark EC50 / light EC50) of >1657, the largest reported for an Ir(III) complex photosensitizer upon broadband visible light (400-700 nm) activation. Ir4 also exhibited a very strong PDT effect toward MCF-7 breast cancer cells and its xenograft tumor model. Upon 450-nm light activation, Ir4 dramatically inhibited the xenograft tumor growth and exhibited negligible side effects upon PDT treatment.

Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes.

Three Ir(C∧N)2(acac)-type and one Ir(C1∧N)(C2∧N)(acac)-type coumarin-based cyclometalated Ir(III) complex isomers (IrC5, IrC7, IrC7-A, and IrC8) have been obtained using three coumarin-based isomers of 2-phenylpyridine (ppy)-type cyclometalating ligands (L-C5, L-C7, and L-C8). Two coordination isomers emerging as principal products (IrC7 and IrC7-A) are obtained in the synthesis of corresponding coumarin-based cyclometalated Ir(III) complexes because of two different coordination sites in ligand L-C7 to form a C-Ir bond. To the best of our knowledge, there are no such isomers reported to date. Interestingly, a broad range of phosphorescent color tuning from green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been realized through variation of the pyridyl substitution positions on the fused phenyl ring of the coumarin skeleton. In addition, based on natural transition orbital (NTO) analyses, features of the lowest triplet excited states (T1) from these coumarin-based cyclometalated Ir(III) complex isomers can be tuned easily by these ligand isomers as well. IrC5, IrC7, and IrC7-A show prevailing 3MLCT character associated with their T1 states which emit the phosphorescent signals, while the T1 state of IrC8 exhibits the dominant ligand-centered π-π* transition feature. Importantly, owing to the strong rigidity of the coumarin skeleton, all the coumarin-based cyclometalated Ir(III) complex isomers can show high phosphorescent quantum yields Φp (ca. 0.4-1). Together with the improved electron-injection/electron-transport (EI/ET) ability, all the phosphorescent emitters display impressive electroluminescence (EL) performance. The device based on IrC8 gives the highest EL efficiencies of external quantum efficiency (ηext) 22.7%, current efficiency (ηL) 79.7 cd A-1, and power efficiency (ηP) 58.2 lm W-1, representing the most state-of-the-art EL ability ever achieved by coumarin-based phosphorescent emitters. All these encouraging data definitely suggest the great potential of the coumarin skeleton in both easy tuning of the photophysical properties of ppy-type Ir(III) phosphorescent complexes and developing high-performance phosphorescent emitters.

Roles of Localized Electronic Structures Caused by π Degeneracy Due to Highly Symmetric Heavy Atom-Free Conjugated Molecular Crystals Leading to Efficient Persistent Room-Temperature Phosphorescence.

Conjugated molecular crystals with persistent room‐temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C6H5)4), tetraphenylsilane (Si(C6H5)4), and tetraphenylgermane (Ge(C6H5)4) showing blue‐green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge‐transfer characteristics between high‐order singlet excited states (Sm) and the ground state (S0). The combination of the moderate charge‐transfer characteristics of the Sm–S0 transition and local‐excited state characteristics between the lowest excited triplet state and S0 accelerates the phosphorescence rate independent of the vibration‐based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C6H5)4 crystals.

Theoretical insight into the photophysical properties of five phosphorescent heteroleptic iridium(iii) complexes bearing oxadiazol-substituted amide ligands.

A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra and the phosphorescent emission properties of five heteroleptic iridium(iii) complexes have been performed to obtain a better understanding of structure-property relationships. The key aim was to investigate the effect of π-conjugation on the photophysical properties of these studied complexes. The lowest energy absorption wavelengths are located at 461 nm for 1, 418 nm for 2, 422 nm for 3, 426 nm for 4 and 417 nm for 5. The lowest energy emissions of complexes 1-5 are localized at 572, 463, 583, 707 and 553 nm, respectively, simulated in acetonitrile medium at M052X level. These findings could be useful to provide good guidance for the further design of new potential phosphorescent organic light-emitting diode (OLED) materials.

TADF activation by solvent freezing: The role of nonradiative triplet decay and spin-orbit coupling in carbazole benzonitrile derivatives

Thermally activated delayed fluorescence (TADF) materials have attracted considerable attentions as a new kind of emitters in organic light-emitting diodes. While it is requisite to minimize an energy difference between the lowest excited triplet state (T1) and lowest excited singlet state (S1), so-called ΔEST, a deeper understanding of the emission mechanism is desirable to clarify the comprehensive molecular design. In this paper, we present that the TADF ability and efficiency of (rare-)metal free organic molecules are surely influenced by both the nonradiative decay of T1 and spin-orbit coupling. By investigating a temperature dependent photoluminescence of carbazole benzonitrile derivatives in toluene solutions using a newly developed liquid nitrogen cryostat, we demonstrate the activation of TADF by solvent freezing for room temperature-TADF inactive molecules. Transient photoluminescence measurements of the frozen samples show a significant increase of a lifetime of T1, probing the suppression of nonradiative decay path of T1. A magnitude of the TADF activation by the solvent freezing is closely related to the degree of spin-orbit coupling of the molecules. The present results emphasize the importance of suppression of nonradiative decay of T1 and an increase of spin-orbit coupling together with reducing ΔEST to achieve a high TADF emission efficiency.

Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules.

Thermally activated delayed fluorescence (TADF) has shown great potential as a mechanism for harvesting low-lying triplet excited states in organic molecules and is therefore of great interest in the context of organic electronics, especially organic light emitting diodes (OLEDs). Herein we study the mechanism for triplet harvesting in triquinolonobenzene (TQB), which instead of relying upon the well-established donor-acceptor (D-A) scheme uses excited-state intramolecular proton transfer (ESIPT). We demonstrate that upon photoexcitation into the lowest singlet excited state the proton is transferred within 20 fs, suggesting it plays little role in triplet harvesting, which occurs on the nano- to microsecond time scale. However, TQB exhibits multiple low-lying triplet states that are strongly coupled along this proton transfer coordinate. The majority of these states favor the structure prior to proton transfer (TQB-TA) and this means that the proton transfer dynamics (3TQB-TA → 1TQB-TB) plays a crucial role in triplet harvesting. This mechanism yields an energy gap in good agreement with that reported experimentally and is consistent with previous photophysical characterization. Finally, a discussion upon extending this understanding into a device context is also presented.

Ultrahighly-efficient and pure deep-blue thermally activated delayed fluorescence organic light-emitting devices based on dimethylacridine/thioxanthene-S,S-dioxide

A novel deep-blue emissive dopant material, 2,7-bis(9,9-dimethylacridine-10(9H)-yl)-9,9-diphenyl-9H-thioxanthene 10-,10-dioxide (DMA-ThX), is synthesized for use in highly-efficient thermally activated delayed fluorescence organic light-emitting devices (TADF OLEDs). The molecular design of DMA-ThX retains the overlap between the lowest singlet excited state (S1) and the lowest triplet excited state (T1) because of its twisted molecular structure. From photoluminescence emission measurements at low temperature, the difference between the S1 and the T1 energy levels of DMA-ThX is found to be 0.07 eV. Furthermore, the external quantum efficiency of the deep-blue-emitting TADF OLEDs with DMA-ThX-doped bis[2-(diphenylphosphino)phenyl] ether oxide is 18.4%, which is significant, and the Commission Internationale de l'Eclairage coordinates are (0.14, 0.14), indicative of very pure deep-blue emission.

Evidence of low-energy singlet excited states in the spin- 12 polyhedral clusters {Mo72V30} and {W72V30} with strongly frustrated kagome networks

Magnetization, specific heat, and electron spin resonance (ESR) measurements are carried out to clarify the low-energy excitations for the S = 1/2 polyhedral clusters {Mo$_{72}$V$_{30}$} and {W$_{72}$V$_{30}$}. The clusters provide unique model systems of Kagom\'e network on a quasi-sphere. The linear field variation of magnetization at low temperatures indicates that the ground state is singlet for both clusters. The temperature and the magnetic field dependence of specific heat shows a distinct difference between two clusters with differing structural symmetries. In {W$_{72}$V$_{30}$} with I$_h$ symmetry, the existence of the several tens of low-energy singlet excited states below the lowest triplet excited state is revealed. The specific heat of the slightly distorted {Mo$_{72}$V$_{30}$} with D$_{5h}$ symmetry leads to a drastic decrease of the singlet contribution, which is consistent with the partial lifting of the frustration and the decrease of degenerated low-energy states. The singlet excitation existence is confirmed further by the temperature dependence of the ESR spectra. Comprehensive experimental studies have demonstrated unique low-energy excitations of the spherical Kagom\'e networks and their sensitivity to the cluster symmetry.

Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet-Singlet Intersystem Crossing.

Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S1 and T2 and rather large T2 -T1 energy gap, which are expected to impede the internal conversion (IC) from T2 to T1 and facilitate the reverse intersystem crossing from the high-lying triplet state (hRISC). Through sensitizing the T2 state with ketones, the existence of the hRISC process with an ns-scale delayed lifetime is confirmed. Benefiting from this fast triplet-singlet conversion, the nondoped device based on this "hot exciton" material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll-off and CIE coordinates of (0.15, 0.13). These results reveal that the "hot exciton" path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure-blue and deep-blue fluorescent organic light-emitting devices.

Heteroleptic Ir(III)N6 Complexes with Long-Lived Triplet Excited States and in Vitro Photobiological Activities.

A series of cationic heteroleptic iridium(III) complexes bearing tris-diimine ligands [Ir(phen)2(R-phen)]3+ (R-phen = phenanthroline (1), 3,8-diphenylphenanthroline (2), 3,8-dipyrenylphenanthroline (3), 3-phenylphenanthroline (4), 3-pyrenylphenanthroline (5), and 3,8-diphenylethynylphenanthroline (6)) were synthesized and characterized. These complexes possessed phen ligand-localized 1π,π* transitions below 300 nm, and charge transfer (1CT) and/or 1π,π* transitions between 300 and 520 nm. In 1, 2, 4, and 6, the low-energy bands were mixed 1CT/1π,π*. However, the increased π-donating ability of the pyrenyl substituent(s) in 3 and 5 split the low-energy bands into a pyrene-based 1π,π* transition at 300-380 nm and an intraligand charge transfer (1ILCT) transition at 380-520 nm. All complexes were emissive at room temperature in CH3CN, but the parentage of the emitting state varied depending on the R substituent(s). Complex 1 exhibited predominantly phen ligand-localized 3π,π* emission mixed with metal-to-ligand charge transfer (3MLCT) character, whereas the emission of 2, 4, and 6 was predominantly from the excited-state with 3π,π*/3ILCT/3MLCT character. The emission from 3 and 5 was dominated by pyrene-based 3π,π* states mixed with 3ILCT character. The different natures of the lowest triplet excited states were also reflected by the different spectral features and lifetimes of the triplet transient absorption of these complexes. Complexes 3 and 5 had singlet oxygen quantum yields as high as 81 and 72%, respectively. Both gave submicromolar phototoxicities toward cancer cells (SK-MEL-28 human melanoma) and bacteria ( S. aureus and S. mutans) with visible-light activation (and marginal to no photobiological activity with red light). Their visible-light phototherapeutic indices (PIs) toward SK-MEL-28 cells were 248 for 3 and >435 for 5; PIs were lower in bacteria (≤62) because of their inherent antimicrobial activities. Both complexes were shown to produce substantial amounts of intracellular reactive oxygen species (ROS), which may account for their photobiological activities.

Triplet Tuning: A Novel Family of Non-Empirical Exchange-Correlation Functionals.

In the framework of density functional theory (DFT), the lowest triplet excited state (T1) can be evaluated using multiple formulations, the most straightforward of which are unrestricted density functional theory (UDFT) and time-dependent density functional theory (TDDFT). Assuming the exact exchange-correlation (XC) functional is applied, UDFT and TDDFT provide identical energies for T1 ( ET), which is also a constraint that we require our XC functionals to obey. However, this condition is not satisfied by most of the popular XC functionals, leading to inaccurate predictions of low-lying, spectroscopically and photochemically important excited states, such as T1 and the lowest singlet excited state (S1). Inspired by the optimal tuning strategy for frontier orbital energies [T. Stein, L. Kronik, and R. Baer, J. Am. Chem. Soc. 2009, 131, 2818], we proposed a novel and nonempirical prescription of constructing an XC functional in which the agreement between UDFT and TDDFT in ET is strictly enforced. Referred to as "triplet tuning", our procedure allows us to formulate the XC functional on a case-by-case basis, using the molecular structure as the exclusive input, without fitting to any experimental data. The first triplet tuned XC functional, TT-ωPBEh, is formulated as a long-range-corrected (LRC) hybrid of Perdew-Burke-Ernzerhof (PBE) and Hartree-Fock (HF) functionals [M. A. Rohrdanz, K. M. Martins, and J. M. Herbert, J. Chem. Phys. 2009, 130, 054112] and tested on four sets of large organic molecules. Compared to existing functionals, TT-ωPBEh manages to provide more accurate predictions for key spectroscopic and photochemical observables, including but not limited to ET, the optical band gap ( ES), the singlet-triplet gap (Δ EST), and the vertical ionization potential ( I⊥), as it adjusts the effective electron-hole interactions to arrive at the correct excitation energies. This promising triplet tuning scheme can be applied to a broad range of systems that were notorious in DFT for being extremely challenging.

A DFT/TDDFT study on the mechanisms of direct and indirect photodegradation of tetrabromobisphenol A in water

Tetrabromobisphenol A (TBBPA) is the most widely used commercial brominated flame retardant. However, the mechanisms underlying the photodegradation of TBBPA remain unclear. Here we use density functional theory and time-dependent density functional theory to examine the photodegradation of the two species of TBBPA in water: TBBPA (neutral form) and TBBPA− (anionic form). The study includes direct photodegradation and indirect photodegradation of TBBPA with ·OH and 1O2. The results of the calculations indicate that indirect photodegradation of TBBPA and TBBPA− with ·OH occurs via OH-addition and Br-substitution. All of the OH-addition and Br-substitution pathways are exothermic. Indirect photodegradation of TBBPA and TBBPA− by 1O2 proceeds via H abstraction by 1O2.Ea was higher for H abstraction of TBBPA than H abstraction of TBBPA−. The mechanisms for the direct photodegradation of TBBPA and TBBPA− include debromination, C1C7/C7C13 cleavage, and cyclization. CBr cleavage was observed in the optimized geometries of TBBPA and TBBPA− at the lowest excited triplet state. However, high Ea values and an endothermic nature indicated that C1C7/C7C13 cleavage and cyclization reactions were not the main pathways. OH-adducts, Br-substitution products, H-abstraction (by 1O2) products, and debromination products were the main products of photodegradation of TBBPA. These findings provide useful information for risk assessment and pollution control of brominated flame retardants.

The low-lying triplet electronic excited states of TiO2 and ZrO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The vertical excitation energies and electron detachment energies to the low-lying triplet excited states of TiO2 and ZrO2 were calculated at the SAC-CI/aug-cc-pVTZ levels. The vertical electron detachment energies to the triplet excited states of TiO2 were compared with previous photoelectron spectra. Possible candidate states for the unassigned bands in the previous photoelectron spectrum were found. Several triplet excited states of TiO2 and ZrO2 were optimized at the SAC-CI/aug-cc-pVDZ and SAC-CI/def2-SVPD levels, respectively. The adiabatic excitation energies and electron detachment energies to the selected triplet states were obtained by single-point calculations at the SAC-CI/def2-TZVPPD level. Density functional theory (BPW91/aug-cc-pVTZ) was also applied to study some of the triplet states for comparison. The calculated electron detachment energies and optimized geometries of the triplet excited states should be useful for studying the photoelectron spectra of TiO2− and ZrO2−.

Photochemistry of SO2 at the Air-Water Interface: A Source of OH and HOSO Radicals.

The photochemistry of sulfur dioxide in the near UV-vis energy range has been studied in aqueous environments. The combination of previously reported experimental measurements with accurate quantum chemical calculations achieved in this work reveals that the process represents an important source of OH radicals in the troposphere. It implicates the reaction of the lowest triplet excited state of SO2 with a water molecule. When the process occurs in the gas-phase, photochemical OH production is only significant under high humidity conditions and high SO2 concentrations as those measured in polluted urban areas. However, the OH production rate increases by several orders of magnitude when the process takes place at the surface of water droplets. The present study indicates therefore that the atmospheric importance of sulfur dioxide goes beyond its well-known role as acid rain and aerosol formation precursor.

Intrinsic Analysis of Radiative and Room-Temperature Nonradiative Processes Based on Triplet State Intramolecular Vibrations of Heavy Atom-Free Conjugated Molecules toward Efficient Persistent Room-Temperature Phosphorescence.

The radiative rate ( kp) of the lowest triplet excited state (T1) and the nonradiative rate based on intramolecular vibrations at room temperature [ knr(RT)] from T1 for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T1. Donor substitution did not strongly influence knr(RT) but greatly enhanced kp. The knr(RT) values were comparable between donor-substituted molecules and nonsubstituted molecules, which we explain by similar vibrational spin-orbit coupling (SOC) related to the transition from T1 to the ground state (S0). We attribute the enhancement of kp induced by donor substitution to the appearance of a large SOC between high-order singlet excited states (Sm) and T1 together with the large transition dipole moments of the Sm-S0 transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals.