Unit Quantum Yield Research Articles

Chiral sulfoximine-based TADF emitter for circularly polarized luminescence and highly efficient OLEDs

This work describes the first example of circularly polarized thermally activated delayed fluorescent (CP-TADF) materials containing heteroatomic stereocenter. A pair of central chiral enantiomers, namely (R)-FAC-SIC and (S)-FAC-SIC, were assembled by sulfoximine-based acceptor and acridan-based donor in a highly twisted manner. The assymetric sulfur atom served as a chiral source, enabling the emitters with distinct circular dichroism (CD) and circularly polarized luminescence (CPL) signals. Benefited from the close-lying excited-state energy levels and multiple intra-/intermolecular interactions in film state, the enantiomers possessed highly efficient TADF emission with almost unit quantum yield (ФPL). Consequently, the corresponding device showed excellent maximum external quantum efficiency (EQEmax) of 28.5 %, current efficiency (CEmax) of 85.6 cd/A, and power efficiency (PEmax) of 84.0 lm W−1. We believe the introduction of heteroatom-based central chirality would significantly broaden the design scope of high-performance CP-TADF emitters.

Liquid Nitrogen Passivation for Deep-Blue Perovskite Quantum Dots with Nearly Unit Quantum Yield

Liquid Nitrogen Passivation for Deep-Blue Perovskite Quantum Dots with Nearly Unit Quantum Yield

The Photoactive Excited State of the B12-Based Photoreceptor CarH.

We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.

Controlling the excited-state dynamics of low band gap, near-infrared absorbers via proquinoidal unit electronic structural modulation.

While the influence of proquinoidal character upon the linear absorption spectrum of low optical bandgap π-conjugated polymers and molecules is well understood, its impact upon excited-state relaxation pathways and dynamics remains obscure. We report the syntheses, electronic structural properties, and excited-state dynamics of a series of model highly conjugated near-infrared (NIR)-absorbing chromophores based on a (porphinato)metal(ii)-proquinoidal spacer-(porphinato)metal(ii) (PM-Sp-PM) structural motif. A combination of excited-state dynamical studies and time-dependent density functional theory calculations: (i) points to the cardinal role that excited-state configuration interaction (CI) plays in determining the magnitudes of S1 → S0 radiative (kr), S1 → T1 intersystem crossing (kISC), and S1 → S0 internal conversion (kIC) rate constants in these PM-Sp-PM chromophores, and (ii) suggests that a primary determinant of CI magnitude derives from the energetic alignment of the PM and Sp fragment LUMOs (ΔEL). These insights not only enable steering of excited-state relaxation dynamics of high oscillator strength NIR absorbers to realize either substantial fluorescence or long-lived triplets (τT1 > μs) generated at unit quantum yield (ΦISC = 100%), but also crafting of those having counter-intuitive properties: for example, while (porphinato)platinum compounds are well known to generate non-emissive triplet states (ΦISC = 100%) upon optical excitation at ambient temperature, diminishing the extent of excited-state CI in these systems realizes long-wavelength absorbing heavy-metal fluorophores. This work highlights approaches to: (i) modulate low-lying singlet excited-state lifetime over the picosecond-to-nanosecond time domain, (ii) achieve NIR fluorescence with quantum yields up to 25%, (iii) tune the magnitude of S1-T1 ISC rate constant from 109 to 1012 s-1 and (iv) realize T1-state lifetimes that range from ∼0.1 to several μs, for these model PM-Sp-PM chromophores, and renders new insights to evolve bespoke photophysical properties for low optical bandgap π-conjugated polymers and molecules based on proquinoidal conjugation motifs.

Toward the Design of Photoresponsive Conditional Antivitamins B12: A Transient Absorption Study of an Arylcobalamin and an Alkynylcobalamin.

Cobalamins are of widespread importance in biology. Both of the cofactors essential for human metabolism, the organocobalamins coenzyme B12 and methylcobalamin, are highly photolabile, as are other alkylcobalamins. The alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and the arylcobalamin 4-ethylphenylcobalamin (EtPhCbl) with "atypical" Co-C-bonds to unsaturated carbons, were recently designed as metabolically inert cobalamins, classified as "antivitamins B12". The further development of an ideal light-activated or "conditional" antivitamin B12 would require it to be readily converted by light into an active B12 vitamin form. Very photolabile "antivitamins B12" would also represent particularly useful scaffolds for therapeutic light-activated reagents. Here, the photoactive arylcobalamin EtPhCbl and the remarkably photostable alkynylcobalamin PhEtyCbl are examined using femtosecond to picosecond UV-visible transient absorption spectroscopy. PhEtyCbl undergoes internal conversion to the ground state with near unit quantum yield on a time scale < 100 ps and an activation energy of 12.6 ± 1.4 kJ/mol. The arylcobalamin EtPhCbl forms an excited state with a ca. 247 ps lifetime. This excited state branches between internal conversion to the ground state and formation of a long-lived base-off species with a quantum yield of ∼9%. Anaerobic steady state photolysis of "light-sensitive" EtPhCbl results in the formation of cob(II)alamin, but only with quantum yield <1%. Hence, our studies suggest that suitably modified arylcobalamins may be a rational basis for the design of photoresponsive "antivitamins B12".

Energy cascades, excited state dynamics, and photochemistry in cob(III)alamins and ferric porphyrins.

Porphyrins and the related chlorins and corrins contain a cyclic tetrapyrrole with the ability to coordinate an active metal center and to perform a variety of functions exploiting the oxidation state, reactivity, and axial ligation of the metal center. These compounds are used in optically activated applications ranging from light harvesting and energy conversion to medical therapeutics and photodynamic therapy to molecular electronics, spintronics, optoelectronic thin films, and optomagnetics. Cobalt containing corrin rings extend the range of applications through photolytic cleavage of a unique axial carbon-cobalt bond, permitting spatiotemporal control of drug delivery. The photochemistry and photophysics of cyclic tetrapyrroles are controlled by electronic relaxation dynamics including internal conversion and intersystem crossing. Typically the electronic excitation cascades through ring centered ππ* states, ligand to metal charge transfer (LMCT) states, metal to ligand charge transfer (MLCT) states, and metal centered states. Ultrafast transient absorption spectroscopy provides a powerful tool for the investigation of the electronic state dynamics in metal containing tetrapyrroles. The UV-visible spectrum is sensitive to the oxidation state, electronic configuration, spin state, and axial ligation of the central metal atom. Ultrashort broadband white light probes spanning the range from 270 to 800 nm, combined with tunable excitation pulses, permit the detailed unravelling of the time scales involved in the electronic energy cascade. State-of-the-art theoretical calculations provide additional insight required for precise assignment of the states. In this Account, we focus on recent ultrafast transient absorption studies of ferric porphyrins and corrin containing cob(III)alamins elucidating the electronic states responsible for ultrafast energy cascades, excited state dynamics, and the resulting photoreactivity or photostability of these compounds. Iron tetraphenyl porphyrin chloride (Fe((III))TPPCl) exhibits picosecond decay to a metal centered d → d* (4)T state. This state decays on a ca. 16 ps time scale in room temperature solution but persists for much longer in a cryogenic glass. The photoreactivity of the (4)T state may lead to novel future applications for these compounds. In contrast, the nonplanar cob(III)alamins contain two axial ligands to the central cobalt atom. The upper axial ligand can be an alkyl group as in the two biologically active coenzymes or a nonalkyl ligand such as -CN in cyanocobalamin (vitamin B12) or -OH in hydroxocobalamin. The electronic structure, energy cascade, and bond cleavage of these compounds is sensitive to the details of the axial ligand. Nonalkylcobalamins exhibit ultrafast internal conversion to a low-lying state of metal to ligand or ligand to metal charge transfer character. The compounds are generally photostable with ground state recovery complete on a time scale of 2-7 ps in room temperature aqueous solution. Alkylcobalamins exhibit ultrafast internal conversion to an S1 state of d/π → π* character. Most compounds undergo bond cleavage from this state with near unit quantum yield within ∼100 ps. Recent theoretical calculations provide a potential energy surface accounting for these observations. Conformation dependent mixing of the corrin π and cobalt d orbitals plays a significant role in the observed photochemistry and photophysics.

Ultrafast Excited-State Dynamics and Photolysis in Base-Off B12 Coenzymes and Analogues: Absence of the trans-Nitrogenous Ligand Opens a Channel for Rapid Nonradiative Decay

Ultrafast transient absorption spectroscopy was used to investigate the photochemistry of adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), and n-propylcobalamin (PrCbl) at pH 2 where the axial nitrogenous ligand is replaced by a water molecule. The evolution of the difference spectrum reveals the internal conversion process and spectral characteristics of the S(1) excited state. The photolysis yield in the base-off cobalamins is controlled by competition between internal conversion and bond homolysis. This is in direct contrast to the process in most base-on alkylcobalamins where primary photolysis occurs with near unit quantum yield and the photolysis yield is controlled by competition between diffusive separation of the radical pair and geminate recombination. The absence of the axial nitrogenous ligand in the base-off cobalamins modifies the electronic structure and opens a channel for fast nonradiative decay. This channel competes effectively with the channel for bond dissociation, dropping the quantum yield for primary radical pair formation from unity in base-on PrCbl and AdoCbl to 0.2 ± 0.1 and 0.12 ± 0.06 in base-off PrCbl and AdoCbl, respectively. The photolysis of base-off MeCbl is similar to that of base-off AdoCbl and PrCbl with competition between rapid nonradiative decay leading to ground state recovery and formation of a radical pair following bond homolysis.

Excitation of Highly Conjugated (Porphinato)palladium(II) and (Porphinato)platinum(II) Oligomers Produces Long-Lived, Triplet States at Unit Quantum Yield That Absorb Strongly over Broad Spectral Domains of the NIR

Transient dynamical studies of bis[(5,5'-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II)]ethyne (PPd(2)), 5,15-bis{[(5'-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II)]ethynyl}(10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II) (PPd(3)), bis[(5,5'-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)platinum(II)]ethyne (PPt(2)), and 5,15-bis{[(5'-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)platinum(II)]ethynyl}(10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)platinum(II) (PPt(3)) show that the electronically excited triplet states of these highly conjugated supermolecular chromophores can be produced at unit quantum yield via fast S(1) → T(1) intersystem crossing dynamics (τ(isc): 5.2-49.4 ps). These species manifest high oscillator strength T(1) → T(n) transitions over broad NIR spectral windows. The facts that (i) the electronically excited triplet lifetimes of these PPd(n) and PPt(n) chromophores are long, ranging from 5 to 50 μs, and (ii) the ground and electronically excited absorptive manifolds of these multipigment ensembles can be extensively modulated over broad spectral domains indicate that these structures define a new precedent for conjugated materials featuring low-lying π-π* electronically excited states for NIR optical limiting and related long-wavelength nonlinear optical (NLO) applications.

Closed Nanocontainer Enables Thioketones to Phosphoresce at Room Temperature in Aqueous Solution

Thiocarbonyl compounds possess unusual photophysical properties: they fluoresce from S(2), phosphoresce from T(1) only at extremely low concentrations in solution at room temperature, have unit quantum yield of intersystem crossing from S(1) to T(1), undergo self-quenching at diffusion-controlled rates, and are quenched by ground-state oxygen leading to self-destruction. In this article, we are concerned with finding a new method to observe phosphorescence from thioketones at room temperature in aqueous solution at high concentrations. To achieve this goal, one needs to find ways to eliminate diffusion-limited self-quenching and oxygen quenching. We present here a general strategy that has allowed us to record phopshorescence from a number of thioketones in aqueous solution at room temperature. The method involves encapsulation of thioketone molecules within a "closed nanocontainer" made up of two cavitand molecules known by its trivial name as octa acid. In these supramolecular complexes, despite two thiocarbonyl compounds being present in close proximity, no self-quenching occurs within the confined space due to curtailment of their rotational freedom. Although phosphorescence could also be observed when these thioketones are included in open containers, such as cucurbiturils and cyclodextrines, the closed container made up of octa acid is found to be the best medium to observe phosphorescence from thioketones whose excited state chemistry is essentially controlled by self-quenching.

Photodissociation dynamics of ethyl ethynyl ether: A new ketenyl radical precursor

The work presented here investigates the dynamics of the photodissociation of ethyl ethynyl ether at 193.3 nm with photofragment translational spectroscopy and laser-induced fluorescence. The data from two crossed laser-molecular beam apparatuses, one with vacuum ultraviolet photoionization detection and one with electron bombardment detection, showed that only cleavage of the C–O bond to form a C2HO radical and a C2H5 (ethyl) radical occurs. We observed neither cleavage of the other C–O bond nor molecular elimination to form C2H4+CH2CO (ketene). The C2HO radical is formed in two distinct product channels, with 37% of the radicals formed from a channel with recoil kinetic energies extending from about 10 to 70 kcal/mole and the other 63% formed from a channel with lower average recoil energies ranging from 0 to 40 kcal/mole. The measurements using photoionization detection reveal that the C2HO radical formed in the higher recoil kinetic-energy channel has a larger ionization cross section for photon energies between 10.3 and 11.3 eV than the radical formed in the lower recoil kinetic-energy channel, and that the transition to the ion is more vertical. The radicals formed in the higher recoil kinetic-energy channel could be either X̃(2A″) or Ã(2A′) state ketenyl (HCCO) product and the shape of the recoil kinetic-energy distribution fitting this data does not vary with ionization energy between 10.3 and 11.3 eV. The C2HO formed in the channel with the lower kinetic-energy release is likely the spin forbidden ã(4A″) state of the ketenyl radical, reached through intersystem crossing. The B̃ state of ketenyl is energetically inaccessible. We also consider the possibility that the lower kinetic-energy channel forms two other C2HO isomers, the CCOH (hydroxyethynyl) radical or the cyclic oxiryl radical. Signal from laser-induced fluorescence of the HCCO photofragment was detected at the electronic origin and the 510 band. The fluorescence signal peaks after a 20 μs delay, indicating that HCCO is formed with a significant amount of internal energy and then subsequently relaxes to the lowest vibrational level of the ground electronic state. The data show that the photodissociation of ethyl ethynyl ether produces C2HO with unit quantum yield, establishing it as the first clean photolytic precursor of the ketenyl radical, a key species in combustion reactions.

Measurements of quantum yields of bromine atoms in the photolysis of bromoform from 266 to 324 nm

The quantum yield for the formation of bromine atoms in the photolysis of bromoform, CHBr3, has been measured between 266 and 324 nm. For 303 to 306 nm the quantum yields are unity within the experimental uncertainty of the measurements. At longer wavelengths, where the bromoform cross sections decrease rapidly, an apparent trend to slightly lower quantum yields is probably the result of systematic and random errors or incorrect CHBr3 absorption cross sections. Support for a unit quantum yield for all wavelengths longer than 300 nm comes from the recent theoretical calculations of Peterson and Francisco. At 266 nm the bromine atom quantum yield is 0.76 (±0.03), indicating that at least one additional dissociation channel becomes important at shorter wavelengths. For modeling of the troposphere, it is recommended that a quantum yield of unity be used for wavelengths of 300 nm and longer.

Measurement of Relative Product Yields from the Photolysis of Dichlorine Monoxide (Cl2O)

The Cl and ClO quantum yields from the photodissociation of Cl2O have been measured at 320, 360, 400, and 440 nm with the use of chemical ionization mass spectrometry (CIMS). All yields were measured at a pressure of 50 Torr and were normalized to a unit quantum yield at 255 nm. There is no evidence that either the Cl or ClO quantum yields deviate significantly from unity over the range of wavelengths employed. Comparison with other recent work on Cl2O photolysis is made, and implications with respect to the understanding of the dissociation dynamics involved are discussed.

Photoinduced electron transfer from enol silyl ethers to quinone. Part 2. Direct observation of ion-pair dynamics by time-resolved spectroscopy

Time-resolved spectroscopy in two time domains (encompassing picoseconds and nonseconds/microseconds) allows all the reactive intermediates involved in the photooxidation of enol silyl ethers (ESE) by chloranil (CA) to be identified, and each step in their temporal evolution to α-enone (E) and adduct (A) to be delineated. Thus, the radical–ion pair [ESE˙+, CA˙–] is the common intermediate formed in unit quantum yield via the highly efficient quenching of triplet chloranil by enol silyl ethers in both dichloromethane and acetonitrile. In the context of the Fuoss–Winstein formulation, the first-formed [ESE˙+, CA˙–] is a contact ion-pair, which in a non-polar solvent, such as dichloromethane, predominantly suffers an initial ion-pair collapse (internal return) by β-proton transfer, and the resultant radical pair ultimately leads to α-enone (E). The contact ion-pair formed in the polar solvent undergoes diffusive separation, and the free ion ESE˙+ suffers desilylation by acetonitrile and ultimately leads to adduct (A). Added electrolyte to dichloromethane solutions modulates the ion-pair behaviour via the ‘special salt effect’ to divert the enone pathway to adduct formation.

Deactivation of excited xanthene dye dimers

Photoacoustic calorimetry was used to investigate the deactivation of electronically excited dimers of rhodamine B and eosin Y in water. Temperature dependence experiments were used to evaluate a possible photodissociative deactivation path, which should be accompanied by chemical volume changes. The quantitative evaluation demonstrates that photodissociation plays no role. Radiationless deactivation occurs for both dimers with unit quantum yield. Since the entire absorbed photon energy is released very rapidly as heat, deactivation occurs by internal conversion but not by intersystem crossing.

Achievement of incident photon to electric current conversion yields exceeding 80% in the spectral sensitization of titanium dioxide by coumarin

We have discovered that the laser dye coumarin 343 sensitizes the visible light induced injection of electrons in the conduction band of TiO 2 films with a very high yield. The incident photon to current conversion efficiency (IPCE) exceeds 70% in the entire wavelength domain between 400 and 500 nm. It is as high as 83.5% around the absorption maximum of the dye, i.e. 440 nm. These unprecedentedly high current yields arise from efficient light collection by the dye attached to the surface of the TiO 2 film and practically unit quantum yield for charge injection. Factors contributing to the unusually high visible light harvesting efficiency of the dye-coated TiO 2 films, which is related to their specific surface texture, are scrutinized.

Photochemical Ring Slippage of Bis(pentafluorophenyl)titanocene: Reaction kinetics and matrix isolation of the primary photoproduct

AbstractLaser flash photolysis and matrix‐isolation techniques were applied to elucidate the photochemistry of the orange complex bis(pentafluorophenyl)titanocene ( = bis(η5‐cyclopentadienyl)bis(pentafluorophenyl)titanium‐ (IV), Cp2TiIV{C6F5}2, Cp = η5‐C5H5, 1) which is used as a polymerization photo‐initiator. The primary photo‐reaction of 1 is the formation of a highly reactive blue isomer X with unit quantum yield. In carefully dried and degassed benzene solution, the photoisomer X rearranges to starting material 1 with a first‐order reaction, k = 5 · 103 s−1 at room temperature. X is highly reactive towards H2O, MeOH, acetone, MeCN, MeNO2, butane‐1,4‐diyl diacrylate, 2,2,6,6‐tetramethylpiperidine N‐oxide, CO, O2, and N2; absolute bimolecular rate constants range from 106 to 109 M−1 · s−1. The primary photoisomerization 1→X is tentatively ascribed to a cyclopentadienyl ring slippage from η5 to a lower hapticity, a process that opens up coordinative unsaturation.

Selective vibronic excitation of singlet oxygen–furan reactions in cryogenic matrices

The reactions of 2,5-dimethylfuran (DMF), 2-methylfuran (MF), and furan with molecular oxygen to form endoperoxides have been induced in Ar and O2 matrices at 12 K with selective vibronic excitation of O2 using near infrared light. Reaction was induced through excitation of the 1Δg(v′=0, v′=1) and 1Σg+(v′=0, v′=1) O2 states near 8000 and 13 100 cm−1, respectively, as well as by the (1Δg, 1Δg) simultaneous transitions of (O2)2 at 15 900 and 17 300 cm−1. The 1Δg ← 3Σg− vibronic progression in solid O2 was recorded by FTIR spectroscopy, whereas members of the 1Σg+ ← 3Σg− and (1Δg, 1Δg) ← (3Σg−, 3Σg−) progressions in Ar matrices were located by ‘‘reaction excitation’’ spectroscopy. The DMF+O2 reaction is a single photon process, apparently with unit quantum yield, for all vibronic levels excited. For MF+O2(1Δg, v′=0) and furan +O2(1Δg, v′=0), quantum yields were high, ∼0.6 and 0.4, respectively, but the reservoirs of reactive pairs were 10–20 times smaller than for DMF+O2. The furan+O2 reaction rate showed an 18O2 isotope effect 0.78±0.15, which can be interpreted in terms of quantum mechanical tunneling on the lowest singlet hypersurface. The IR matrix spectra of DMF endoperoxide and its peroxidic 18O counterpart are reported, and a partial vibrational assignment for this intermediate is given. The endoperoxide vibration that absorbs at 815.7 cm−1 is dominated by the bridge O–O stretching motion.

Decay dynamics of aromatic thione triplet states in fluid solution

Phosphorescence emission and excitation spectra, quantum yields, and lifetimes of several aromatic thiones have been measured in fluid solution at room temperature. Excitation to any of the excited electronic states accessible in the near-uv and visible regions of the spectrum results in population of the lowest triplet state with near unit quantum yield. Triplets of relatively inflexible thiones decay non-radiatively with rate constants of the order of 105 s−1 whereas thiones capable of higher amplitude flexion or group rotation exhibit non-radiative decay constants ≥ 107 s−1 at infinite dilution in aprotic solvents. Self-quenching occurs at diffusion-controlled rates.

Photochemical modulation spectroscopy of oxygen atom reactions with olefins

We used photochemical modulation spectroscopy to study the primary radical products of oxygen atom—olefin reactions by their optical absorption. In contrast with earlier work done by mass spectrometric sampling we found that the “vinoxy” radical CH 2CHO is an important primary product with ethylene and monosubstituted ethylenes. We characterized two electronic transitions of this radical with origins at 347.7 and 1249 nm and estimated the yields of CH 2CHO and HCO in the oxygen atom reactions by comparison with reference reactions of unit quantum yield. Two sources of CH 2CHO formation were identified: a pressure-independent pathway and a pressure-dependent mechanism which complements the stabilization of epoxides and aldehydes. Mercury-photo-sensitized decomposition of the corresponding epoxides also produces large yields of CH 2CHO. Our observations of pressure-independent fragmentation forming CH 2CHO link together early observations of a similar mechanism by Cvetanovic and recent low pressure flow and molecular beam experiments by several groups. A mechanism which accounts for the different characteristics of the pressure-independent and pressure-dependent pathways is discussed.

Kinetics and fusion laser potential for the terbium chloride–aluminum chloride vapor complex

Fluorescence responses of the Tb3+ : 5D3 and 5D4 metastable levels are presented for the terbium chloride–aluminum chloride vapor complex excited by pulsed KrF laser radiation. Assuming unit quantum yield, excited-state densities as high as 3×1016 cm−3 (10 J/l) are inferred for the experimental conditions and maintained for times ≳10 μs.