Excited State Relaxation in Cyanine Dyes: A Remarkably Efficient Reverse Intersystem Crossing from Upper Triplet Levels

Shortwave infrared polymethine dyes for bioimaging: ultrafast relaxation dynamics and excited-state decay pathways

Excited-state relaxation in two prototypical shortwave infrared (SWIR) polymethine dyes developed for bioimaging, heptamethine chromenylium Chrom7 and flavylium Flav7, is studied by means of femtosecond transient absorption with broadband ultraviolet-to-SWIR probing complemented by steady-state and time-resolved fluorescence and phosphorescence measurements. The relaxation processes of the dyes in dichloromethane are resolved with sub-100 fs temporal resolution using SWIR, near-IR, and visible photoexcitation. Different population members of the ground-state inhomogeneous ensemble are found to equilibrate via skeletal deformation changes with time constants of 90 fs and either 230 fs (Chrom7) and 350 fs (Flav7) followed by slower evolution matching the 1-ps timescale of diffusive solvation dynamics. Molecules excited into high-lying singlet electronic states (Sn) by visible excitation repopulate with time constants of 400 fs (Chrom7) and 450 fs (Flav7) the corresponding first excited singlet S1 states, which decay within several hundreds of picoseconds in dichloromethane and chloroform solvents. Vibrational relaxation in S1 for both Chrom7 and Flav7 in dichloromethane occurs with time constants of 350 and 800 fs for excess of vibrational energy of ∼1000 and 10 000 cm-1 deposited by near-IR and visible excitation, respectively. Two competing non-radiative processes are present in S1: temperature-independent internal conversion, and thermally-activated twisting about a carbon-carbon bond of the conjugated chain, which is substantial at room temperature but essentially nonreactive, producing traces of isomer product. Intersystem crossing in S1, and thus the triplet quantum yield, is minor. The importance of absorption bands from the excited S1 state in applications requiring high-intensity excitation conditions is discussed.

Near-infrared co-illumination of fluorescent proteins reduces photobleaching and phototoxicity

Here we present a method to reduce the photobleaching of fluorescent proteins and the associated phototoxicity. It exploits a photophysical process known as reverse intersystem crossing, which we induce by near-infrared co-illumination during fluorophore excitation. This dual illumination method reduces photobleaching effects 1.5-9.2-fold, can be easily implemented on commercial microscopes and is effective in eukaryotic and prokaryotic cells with a wide range of fluorescent proteins.

Robust luminogens as cutting-edge tools for efficient light emission in recent decades

Blue luminogens play a vital role in white lighting and potential metal-free fluorescent materials and their high-lying excited states contribute to harvesting triplet excitons in devices. However, in TADF-OLEDs (ΔEST < 0.1 eV), although T1 excitons transfer to S1via RISC with 100% IQE, the longer lifetime of blue TADF suffers from efficiency roll-off (RO). In this case, hybridized local and charge transfer (HLCT) materials have attracted significant interest in lighting owing to their 100% hot exciton harvesting and enhanced efficiency. Both academics and industrialists widely use the HLCT strategy to improve the efficiency of fluorescent organic light-emitting diodes (FOLEDs) by harvesting dark triplet excitons through the RISC process. Aggregation-induced emissive materials (AIEgens) possess tight packing in the aggregation state, and twisted AIEgens with HLCT behaviour have a shortened conjugation length, inducing blue emission and making them suitable candidates for OLED applications. TTA-OLEDs are used in commercial BOLEDs because of their moderate efficiency and reasonable operation lifetime. In this review, we discuss the devices based on TTA fluorophores, TADF fluorophores, HLCT fluorophores, AIEgens and HLCT-sensitized fluorophores (HLCT-SF), which break through the statistical limitations.

Recent progress in hot exciton materials for organic light-emitting diodes

According to Kasha's rule, high-lying excited states usually have little effect on fluorescence. However, in some molecular systems, the high-lying excited states partly or even mainly contribute to the photophysical properties, especially in the process of harvesting triplet excitons in organic electroluminescent devices. In the current review, we focus on a type of organic light-emitting diode (OLED) materials called "hot exciton" materials, which can effectively harness the non-radiative triplet excitons via reverse intersystem crossing (RISC) from high-lying triplet states to singlet states (T_n→ S_m; n≥ 2, m≥ 1). Since Ma and Yang proposed the hot exciton mechanism for OLED material design in 2012, there have been many reports aiming at the design and synthesis of novel hot exciton luminogens. Herein, we present a comprehensive review of the recent progress in hot exciton materials. The developments of the hot exciton mechanism are reviewed, the fundamental principles regarding molecular design are discussed, and representative reported hot exciton luminogens are summarized and analyzed, along with their structure-property relationships and OLED applications.

Triplet Shelving in Fluorescein and Its Derivatives Provides Delayed, Background-Free Fluorescence Detection

Fluorescence from the xanthene dyes rose bengal, erythrosine B, eosin Y, and fluorescein is modulated by reversibly optically populating and depopulating their long-lived triplet states through coillumination with a second, long-wavelength laser. Here, we show that repumping the S₁ state from the triplet generates strong, nanosecond-lived optically activated delayed fluorescence (OADF), microseconds to milliseconds after primary pulsed excitation. This time-delayed emission upon long-wavelength illumination generates fluorescence after triplet shelving and is a major contribution to fluorescence enhancement/modulation. The time-delayed and background-free OADF component is further increased using a >1 μs burst continuous wave excitation scheme to increase the steady-state triplet populations, yielding strong OADF even from strongly emissive fluorescein. Because emission is delayed long after the high-energy primary excitation, yellow-orange fluorescence is readily observed on zero background. As OADF generation depends on the triplet quantum yields and the reverse intersystem crossing rates, we directly probe the usually difficult-to-measure photophysics, create new zero-background detection schemes, and increase OADF through tailored excitation schemes, all improving sensitivity. The excellent match between experiments and simulations demonstrates the promise of these studies for OADF characterization, while enabling us to determine that OADF (in contrast to ground-state recovery and re-excitation) is the major component of fluorescence enhancement for xanthenes studied with triplet quantum yields exceeding 0.1.

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 Pt^II bisacetylide complex (Pt-1) and trans-bis(phosphine) Pt^II 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 (Φ_F = 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.

Photo-isomerization of the Cyanine Dye Alexa-Fluor 647 (AF-647) in the Context of dSTORM Super-Resolution Microscopy

Cyanine dyes, as used in super-resolution fluorescence microscopy, undergo light-induced "blinking", enabling localization of fluorophores with spatial resolution beyond the optical diffraction limit. Despite a plethora of studies, the molecular origins of this blinking are not well understood. Here, we examine the photophysical properties of a bio-conjugate cyanine dye (AF-647), used extensively in dSTORM imaging. In the absence of a potent sacrificial reductant, light-induced electron transfer and intermediates formed via the metastable, triplet excited state are considered unlikely to play a significant role in the blinking events. Instead, it is found that, under conditions appropriate to dSTORM microscopy, AF-647 undergoes reversible photo-induced isomerization to at least two long-lived dark species. These photo-isomers are characterized spectroscopically and their interconversion probed by computational means. The first-formed isomer is light sensitive and transforms to a longer-lived species in modest yield that could be involved in dSTORM related blinking. Permanent photobleaching of AF-647 occurs with very low quantum yield and is partially suppressed by the anaerobic redox buffer.

Optically Activated Delayed Fluorescence through Control of Cyanine Dye Photophysics

Merocyanine 540 fluorescence can be enhanced by optically depopulating dark photoisomer states to regenerate the fluorescence-generating manifold of the all-trans isomer. Here, we utilize a competing modulation route, long-wavelength coexcitation of the trans triplet population to not only modulate fluorescence through enhanced ground-state recovery but also generate optically activated delayed fluorescence (OADF) with longer-wavelength co-illumination. Such OADF (∼580 nm) is directly observed with pulsed fluorescence excitation at 532 nm, followed by long-wavelength (637 nm) continuous wave depopulation of the photogenerated triplet by repopulating the emissive S₁ state. Such reverse intersystem crossing (RISC) results in ns-lived fluorescence delayed by several microseconds after the initial primary excitation pulse and the prompt 1 ns-lived fluorescence that it induces. The dark state from which OADF is generated decays more rapidly with increased secondary laser intensity, as the optically induced RISC rate increases. This first OADF from organic dyes is observed, as the red secondary laser excites ∼580 nm, <1 ns-lived fluorescence from the previously optically prepared ∼1 μs-lived triplet state. This sequential two-photon, repumped fluorescence yields background-free collection with potential for new high-sensitivity fluorescence imaging schemes.

Exciton Gating and Triplet Deshelving in Single Dye Molecules Excited by Perovskite Nanocrystal FRET Antennae

The extraordinary absorption cross section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonance energy transfer (FRET). Here, we demonstrate that FAPbBr₃ perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single cyanine 3 molecules at a concentration 100-fold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules 100-fold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves 1.5-fold compared to emission collected directly from the nanocrystals because the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reverse intersystem crossing (RISC), translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of 3.

Nanolipid Formulations of Benzoporphyrin Derivative: Exploring the Dependence of Nanoconstruct Photophysics and Photochemistry on Their Therapeutic Index in Ovarian Cancer Cells

With the rapidly emerging designs and applications of light-activated, photodynamic therapy (PDT)-based nanoconstructs, photonanomedicines (PNMs), an unmet need exists to establish whether conventional methods of photochemical and photophysical characterization of photosensitizers are relevant for evaluating new PNMs in order to intelligently guide their design. As a model system, we build on the clinical formulation of benzoporphyrin derivative (BPD), Visudyne^® , by developing a panel of nanolipid formulations entrapping new lipidated chemical variants of BPD with differing chemical, photochemical and photophysical properties. These are 16:0 and 20:0 lysophosphocholine-BPD (16:0/20:0 BPD-PC), DSPE-PEG-BPD and BPD-cholesterol. We show that Visudyne^® was the most phototoxic formulation to OVCAR-5 cells, and the least effective was liposomal DSPE-PEG-BPD. However, these differences did not match their optical, photophysical and photochemical properties, as the static BPD quenching was highest in Visudyne, which also exhibited the lowest generation of singlet oxygen. Furthermore, we establish that OVCAR-5 cell phototoxicity also does not correlate with rates of photosensitizer photobleaching and fluorescence quantum yields in any nanolipid formulations. These findings warrant critical future studies into subcellular targets and molecular mechanisms of phototoxicity of photodynamic nanoconstructs, as more reliable prognostic surrogates for predicting efficacy to appropriately and intelligently guide their design.

Modulated emission from dark triplet excitons in aza-acene compounds: fluorescence versus phosphorescence

The enhanced charge-transfer interaction from DBP to TBP leads to higher energy triplet states, which reinforces reverse intersystem crossing.

Towards white-light generation through upconversion of low-power near-infrared photons

A broadband upconverted emission combined with a broadband infrared absorption takes place in films composed of the same infrared dye molecules.

Retrieving the Rate of Reverse Intersystem Crossing from Ultrafast Spectroscopy

Among various nonradiative photophysical processes related to energy migration in singlet-triplet coupled molecular systems, unlike such processes as internal conversion, intersystem crossing (ISC), and intramolecular vibrational relaxation that have been extensively addressed, the reverse ISC (rISC) is a unique one that apparently lacks sufficient interrogation probably owing to its intrinsically elusive nature. In particular, it still remains a nontrivial task to quantitatively describe the rISC pathway. Here we introduce a new, simple route to this end, just through explicit modeling and simulations on routinely available, experimental data from ultrafast transient absorption spectroscopy. We demonstrate on a proof-of-concept, rare-earth chelate molecular system that our approach, featuring spectral profile analysis together with wavelength-dependent global kinetics fitting, enables facile retrieval of the rISC rate from experimental data.

Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit

The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the "spin statistical limit" can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these T_N states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra- and interchain charge transfer states is critical in underpinning this mechanism.

Reverse intersystem crossing from upper triplet levels to excited singlet: a 'hot excition' path for organic light-emitting diodes

Since researches on the fate of highly excited triplet states demonstrated the existence of reverse intersystem crossing (RISC) from upper triplet levels to singlet manifold in naphthalene, quinoline, isoquinoline, etc. in the 1960s, this unique photophysical process was then found and identified in some other aromatic materials. However, the early investigations mainly focus on exploring the mechanism of this photophysical process; no incorporation of specific application was implemented. Until recently, our group innovatively used this 'sleeping' photophysical process to enhance the efficiency of fluorescent organic light-emitting diodes by simultaneously harvesting singlet and triplet excitons. Efforts are devoted to developing materials with high photoluminescence efficiency and effective RISC through appropriate molecular design in a series of donor-acceptor material systems. The experimental and theoretical results indicate that these materials exhibit hybridized local and charge-transfer excited state, which achieve a combination of the high radiation from local excited state and the high T(m)→S(n) (m≥2, n≥1) conversion along charge-transfer excited state. As expected, the devices exhibited favourable external quantum efficiency and low roll-off, and especially an exciton utilization efficiency exceeding the limit of 25%. Considering the significant progress made in organic light-emitting diodes with this photophysical process, we review the relevant mechanism and material systems, as well as our design principle in materials and device application.

Excited-state annihilation reduces power dependence of single-molecule FRET experiments

Singlet–singlet annihilation between FRET dyes is evident in nanosecond fluorescence cross-correlation measurements.

Photopolymerization inhibition dynamics for sub-diffraction direct laser writing lithography

Selective inhibition of the polymerization leads to sub-diffraction feature sizes in direct writing lithography-a principle based on the idea of stimulated emission depletion (STED) microscopy. However, the detailed understanding of the inhibition process is a key point to further enhance the resolution of the system. The authors present experiments focused on the time dynamics of the inhibition process, clarifying possible photophysical pathways.

Optically enhanced, near-IR, silver cluster emission altered by single base changes in the DNA template

Few-atom silver clusters harbored by DNA are promising fluorophores due to their high molecular brightness along with their long- and short-term photostability. Furthermore, their emission rate can be enhanced when co-illuminated with low-energy light that optically depopulates the fluorescence-limiting dark state. The photophysical basis for this effect is evaluated for two near-infrared-emitting clusters. Clusters emitting at ∼800 nm form with C(3)AC(3)AC(3)TC(3)A and C(3)AC(3)AC(3)GC(3)A, and both exhibit a trap state with λ(max) ∼ 840 nm and an absorption cross section of (5-6) × 10(-16) cm(2)/molecule that can be optically depopulated. Transient absorption spectra, complemented by fluorescence correlation spectroscopy studies, show that the dark state has an inherent lifetime of 3-4 μs and that absorption from this state is accompanied by photoinduced crossover back to the emissive manifold of states with an action cross section of ∼2 × 10(-18) cm(2)/molecule. Relative to C(3)AC(3)AC(3)TC(3)A, C(3)AC(3)AC(3)GC(3)A produces a longer-lived trap state and permits more facile passage back to the emissive manifold. With the C(3)AC(3)AC(3)AC(3)G template, a spectrally distinct cluster forms having emission at ∼900 nm, and its trap state has a ∼4-fold shorter lifetime. These studies of optically gated fluorescence bolster the critical role of the nucleobases in both the formation and excited state dynamics of these highly emissive metallic clusters.

Photoinduced millisecond switching kinetics in the GFPMut2 E222Q mutant

New probes for kinetic intracellular measurements in the millisecond range are desirable to monitor protein biochemical dynamics essential for catalysis, allosteric regulation, and signaling. Good candidates to this aim are the photoswitchable mutants of the green fluorescent protein, whose anionic fluorescence, primed by blue light, is markedly enhanced under an additional excitation at a shorter wavelength and relaxes within a few milliseconds. The aim of this report is to study how the brightness enhancement kinetics depends on the physical-chemical and spectroscopic parameters and to provide proof-of-concept experiments for the use of the fluorescence enhancement in conditions in which the protein diffusion is hindered and thereby photobleaching can be a limiting critical issue. Future, direct applications of photochromic mutants for modulated excitation imaging would in fact require such a detailed knowledge. We present here an extensive study of the photoswitching mechanism of the E222Q mutant of GFPMut2 (Mut2Q), pumped by visible 488 nm light and probed at 400-420 nm, as a function of pH, viscosity, temperature, and light intensity. In solution, two characteristic photoswitching times are found by means of modulated double beam fluorescence correlation spectroscopy in the 1-30 ms range, depending on the solution pH. The photoswitching kinetics is solved in terms of the eigenvalues and the eigenvectors of a specific energy diagram and used directly to fit the data, suggesting that the observed photoswitching amplitudes and kinetics are related to a single three-level transition loop. Finally, we give in vitro examples of the use of modulated excitation microscopy, based on fluorescence enhancement amplitude and kinetics detection, on Mut2Q protein samples immobilized in acrylamide gels.

Fluorescence lifetime properties of near-infrared cyanine dyes in relation to their structures

Structurally diverse near-infrared (NIR) absorbing polymethine dyes were prepared and their fluorescence lifetimes (FLT) were evaluated in relation to their structural features. Comparative FLT analysis based on the modification of methine chain length and heterocyclic system showed that indolium or benz[e]indolium heptamethine dyes exhibited longer FLT than the benz[c,d]indolium trimethine dye. Modification of heterocyclic system alone with an intact chain length showed that indolium-based heptamethine dyes showed approximately 30% longer FLT than the benz[e]indolium-based dyes. In general, the FLT of polymethine dyes increased from polar to non-polar solvents. In addition, correlation study between the theoretical and the experimental FLT for indocyanine green (ICG) suggests that the lack of structural rigidity for these cyanine dyes is primarily responsible for the loss of the excited state energy via non-radiative pathway.

Significant effects of substituents on substituted naphthalenes in the higher triplet excited state

Substituent effect on the lifetimes of a series of substituted naphthalenes (Np) in the higher triplet excited state (Tn) was studied with transient absorption measurements using the two-color two-laser flash photolysis technique. Lifetimes of Np(Tn) in cyclohexane solution were determined from the triplet energy transfer quenching by carbon tetrachloride to be 0.98-63 ps. The different lifetimes of Np(Tn) were explained by the energy gap law for the internal conversion from Np(Tn) to Np(T1), indicating that Np(Tn) quenched by carbon tetrachloride is assigned to Np(T2) with the longest lifetime among Np(Tn). The lifetime of Np(Tn) was correlative with the Hammett sigmap constant. Electronic characters of substituents showed a more significant influence on the energy of the T2 state than that of the T1 state.

Characterization of Photoinduced Isomerization and Intersystem Crossing of the Cyanine Dye Cy3

Several important photophysical properties of the cyanine dye Cy3 have been determined by laser flash photolysis. The triplet-state absorption and photoisomerization of Cy3 are distinguished by using the heavy-atom effects and oxygen-induced triplet --> triplet energy transfer. Furthermore, the triplet-state extinction coefficient and quantum yield of Cy3 are also measured via triplet-triplet energy-transfer method and comparative actinometry, respectively. It is found that the triplet --> triplet (T1-->Tn) absorptions of trans-Cy3 largely overlap the ground-state absorption of cis-Cy3. Unlike what occurred in Cy5, we have not observed the triplet-state T1-->Tn absorption of cis-Cy3 and the phosphorescence from triplet state of cis-Cy3 following a singlet excitation (S0-S1) of trans-Cy3, indicating the absence of a lowest cis-triplet state as an isomerization intermediate upon excitation in Cy3. The detailed spectra of Cy3 reported in this paper could help us interpret the complicated photophysics of cyanine dyes.

Spectral Identification of Specific Photophysics of Cy5 by Means of Ensemble and Single Molecule Measurements

The triplet-state characteristics of the Cy5 molecule related to trans-cis isomerization are investigated by means of ensemble and single molecule measurements. Cy5 has been used frequently in the past 10 years in single molecule spectroscopic applications, e.g., as a probe or fluorescence resonance energy transfer acceptor in large biomolecules. However, the unknown spectral properties of the triplet state and the lack of knowledge on the photoisomerization do not allow us to interpret precisely the unexpected single molecule behaviors. This limits the application of Cy5. The laser photolysis experiments demonstrate that the trans triplet state of Cy5 absorbs about 625 nm, the cis ground state absorbs about 690 nm, and the cis triplet state also absorbs about 690 nm. In other words, the T1-Tn absorptions largely overlap the ground-state absorptions for both trans and cis isomers, respectively. Furthermore, the observation of the cis triplet state indicates an important isomerization pathway from the trans-S1 state to the cis-T1 state upon excitation. The detailed spectra presented in this article let us clearly interpret the exact mechanisms responsible for several important and unexpected photophysical behaviors of single Cy5 molecules such as reverse intersystem crossing (RISC), the observation of dim states with a lower emission intensity and slightly red-shifted fluorescence, and unusual energy transfer from donor molecules to dark Cy5 molecules acting as acceptors in single molecule fluorescence resonance energy transfer (FRET) measurements. Spectral results show that the dim state in the single molecule fluorescence intensity time traces originated from cis-Cy5 because of a lower excitation rate, resulting from the red-shifted ground-state absorption of cis-Cy5 compared to that of the trans-Cy5.

Direct Observation of Delayed Fluorescence from a Remarkable Back-Isomerization in Cy5

The direct observations of delayed fluorescence and phosphorescence from the cyanine dye Cy5 are reported. The delayed fluorescence is generated from the S(1) state of trans-Cy5 through a reserve intersystem crossing from the cis-triplet state T(1) to the trans-singlet state S(1) via thermal activation. The lowest cis-triplet state is evidenced to be involved in the formation of the isomer. The back-isomerization from cis-triplet state to trans-singlet state crossing, a remarkably back-isomerization pathway that has not been reported before, plays a significant role in this unusual delayed fluorescence.

Intermolecular Electron Transfer from Naphthalene Derivatives in the Higher Triplet Excited States

Intermolecular electron transfer (ELT) from a series of naphthalene derivatives (NpD) in the higher triplet excited states (T(n)) to carbon tetrachloride (CCl(4)) in Ar-saturated acetonitrile was observed using the two-color two-laser flash photolysis method. The ELT efficiency depended on the driving force of ELT. Since the ELT from the T(n) state occurred competitively with the internal conversion (IC, T(n) --> T(1)) and the triplet energy transfer (ENT), the ELT became apparent only when sufficient free energy change of ELT was attained. On the other hand, ELT from the T(1) state was not observed, although ELT from the T(1) state with sufficiently long lifetime has a slightly exothermic driving force. The fast ELT from the T(n) state and lack of the reactivity of the T(1) state were explained well by the "sticky" dissociative electron-transfer model based on one-electron reductive attachment to CCl(4) leading to the C-Cl bond cleavage.

Reverse intersystem crossing in rose bengal. II. Fluence dependence of fluorescence following 532 nm laser excitation

A fluence-dependent fluorescence technique was used to observe reverse intersystem crossing from a certain higher-lying triplet state of rose bengal populated by a single pulse of 532 nm light. The quantum yield of reverse intersystem crossing from this state was determined to be 0.12+/-0.02 for rose bengal in phosphate-buffered saline. The importance of including molecular rotation effects in the analysis of fluorescence resulting from reverse intersystem crossing is discussed. Differences in the photochemical reactivity of upper triplet states in biological systems have been previously hypothesized to result from photophysical differences, particularly substantial differences in their reverse intersystem crossing yields. In this work this hypothesis is analyzed quantitatively, using numerical models of the population dynamics. These models suggest that reverse intersystem crossing alone cannot adequately explain the differences in biological response.