Laser-Induced Luminescence of Flavinium Salts

Flavinium salts dissolved in an ethanolic glass exhibit blue fluorescence and orange-red phosphorescence upon excitation with a UV. line from an argon ion laser equipped with UV. optics. This arrangement enables the wavelength distribution and the time-dependence of the phosphorescence to be measured in a relatively short time. Four cationic flavins were investigated. In spite of the small difference in the chemical structure of the compounds studied, large differences in the spectral shapes and in the ratio of quantum efficiencies of phosphorescence and fluorescence became apparent. The phosphorescence lifetimes were of the same order of magnitude, which indicated a similar rate of depletion of the lowest excited triplet state for all four cations. However, the efficiency of triplet formation (intersystem crossing) is affected by slight structural modifications in the pyrimidine subnucleus of the flavinium salt. The results point to a possible role of vibronic spin-orbit coupling in the intersystem crossing.     

An analysis of heavy-atom perturbation of intersystem crossing as a mechanistic tool in photochemistry : Probing for singlet and triplet dominated reactions

A kinetic analysis of heavy-atom enhanced intersystem crossing is presented, with the object of using this perturbation method for determining whether the majority of a photoproduct originates from an excited singlet or an excited triplet precursor. Expressions are derived which relate the quantum efficiencies of product formation (φ_r) and S₁→T₁ intersystem crossing (φ_isc) with potential quenching or enhancement by the heavy-atom. These allow limits to be placed on the possible quenching or enhancement that may be observed in particular cases (see Fig. 4 and 6) and thereby make an interpretation of the data less ambiguous than has been suggested in recent discussions. A particularly useful observation is that for those molecules (for example, aromatics) having triplets which are relatively unaffected by heavy-atoms, quenching of a product by more than 50% demonstrates that a majority of this product must be derived from the excited singlet state.     

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S1 --> T1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S1 and T1, are of pi,pi* origin, whereas for their precursors S1 is n,pi*, and T1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S1 --> T1 yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed phiT value.     

Time resolved fluorescence in the picosecond region

Time resolved fluorescence studies of four organic molecules in solution show that, contrary to an earlier report, the relaxed fluorescent S₁ states are formed within 10 ps of excitation even when excitation is to higher states. Fluorescence lifetimes for fluorescein and its halogen substituted derivatives vary over a factor of 40 owing to enhanced intersystem crossing. Measured radiative lifetimes agree with those calculated from integrated absorption intensities and are relatively constant.     

External Heavy‐Atom Effect on the Prompt and Delayed Fluorescence of [70]Fullerenes

The influence of the external heavy‐atom effect (HAE) on the fluorescence properties of C₇₀ and a C₇₀ methano monoadduct is determined. For this purpose the photophysics of these [70]fullerenes is studied in polystyrene (PS) and in a related heavy‐atom polymer, poly(4‐bromostyrene) (PBS). In the absence of HAE (PS matrix) both fullerenes display a strong thermally activated delayed fluorescence (DF) that is more pronounced in the case of C₇₀. In the presence of HAE (PBS matrix) both prompt (PF) and DF intensities decrease significantly, the same happening to the delayed fluorescence lifetimes. The relative fluorescence intensities (DF intensity/PF intensity) for each fullerene are on the other hand surprisingly similar to the respective ones in PS for the full experimental temperature range. The HAE is responsible for a significant increase of the S₁→T₁ and S₁←T₁ intersystem crossing (ISC) rates, and of the T₁→S₀ radiative rate. In particular, the HAE on a S₁←T₁ ISC rate is reported here for the first time. The overall substantial insensitivity of relative fluorescence intensities to HAE is explained by a compensation effect: As the S₁→T₁ and S₁←T₁ ISC rates on the one hand, and the T₁→S₀ radiative rate on the other hand work in opposition with respect to DF, a near cancellation of effects occurs.     

Indium phthalocyanines with different axial ligands: a study of the influence of the structure on the photophysics and optical limiting properties

The photophysical properties of four axially substituted indium phthalocyanines, namely, 2,(3)-tetra- tert-butyl-phthalocyaninato indium chloride ( 1), 2,(3)-tetra-[(3,5-di- tert-butyl)-phenyloxy]-phthalocyaninato indium bromide ( 2), 2,(3)-tetra-[(3,5-di- tert-butyl)-phenyloxy]-phthalocyaninato indium iodide ( 3), and 2,3-octa-[(2-hexyl)-ethyloxy]-phthalocyaninato indium trifluoroacetate ( 4), have been investigated, and their optical limiting properties with nanosecond light pulses were evaluated. All complexes behave as reverse saturable absorbers in the range of 400-625 nm due to a triplet-triplet excited-state transition. Excited-state absorption cross sections and triplet state lifetimes are not significantly affected by the nature of the axial ligand. On the other hand, remarkable differences in the variation of nonlinear transmittance are observed for 1- 4 due to significantly different intersystem crossing rates. Heavier axial ligands in phthalocyanines 2 and 3 produce the largest variations of nonlinear transmission (heavy-atom effect). Complex 1 in polystyrene matrix shows reversible nonlinear absorption when incident fluence does not exceed 0.025 J cm (-2).     

Thermally assisted and heavy-atom assisted intersystem crossing in meso-substituted anthracenes

A single linear correlation between the activation energy of S₁—T intersystem crossing and the logarithm of the bimolecular fluorescence quenching rate constant by haloalkanes for meso-substituted anthracenes in fluid solutions is established. It is concluded that both the monomolecular and the heavy-atom-assisted deactivation of S₁ depend in the same way on the change of S₁ energy by substitution or solvent relative to the energy of the accepting triplet T_n. The anomalous increase of fluorescence of Br-substituted anthracenes in heavy-atom containing solvents is consistent with a widening of the S₁—T_n energy gap due to the changing solvent.     

Intersystem crossing processes in nonplanar aromatic heterocyclic molecules

A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin-orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin-orbit coupling matrix elements. We demonstrate that the magnitude of spin-orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment.     

Fullerene C60 derivatives as antimicrobial photodynamic agents

Functionalized fullerenes have shown interesting biomedical applications as potential phototherapeutic agents. The hydrophobic carbon sphere of fullerene C₆₀ can be substituted by cationic groups to obtain amphiphilic structures. These compounds absorb mainly UV light, but absorption in the visible region can be enhanced by anchoring light-harvesting antennas to the C₆₀ core. Upon photoexcitation, fullerenes act as spin converters by effective intersystem crossing. From this excited state, they can react with ground state molecular oxygen and other substrates to form reactive oxygen species. This process leads to the formation of singlet molecular oxygen by energy transfer or superoxide anion radical by electron transfer. Photodynamic inactivation experiments indicate that cationic fullerenes are highly effective photosensitizers with applications as broad-spectrum antimicrobial agents. In these structures, the hydrophobic character of C₆₀ improves membrane penetration, while the presence of positive charges increases the binding of the fullerene derivatives with microbial cells. Herein, we summarize the progress of antimicrobial photodynamic inactivation based on substituted fullerenes specially designed to improve the photodynamic activity.     

Benzonitrile and its van der waals complexes studied in a free jet. III. Enhancement of the intersystem crossing rate in the benzonitrile dimer and other complexes

By using the sensitized phosphorescence spectroscopy, the intensity of the phosphorescence has been recorded upon excitation of the benzonitrile dimer to the S₁ vibronic states in a free jet. The results indicate that the strong vibrational energy dependence of the fluorescence quantum yield, reported previously, is attributable to the increasing rate of intersystem crossing with increasing vibrational energy. Similar behavior is also observed in other van der Waals complexes of benzonitrile though the increase is less obvious. The enhancement of the intersystem crossing can be correlated with the state density of van der Waals modes in the S₁ electronic state. In case of the benzonitrile trimer and benzonitrile-Kr complex, intersystem crossing is found to be fully efficient even without vibrational excitation.     

Spectral and photophysical studies on cruciform oligothiophenes in solution and the solid state

The photophysical and spectroscopic properties of a new class of oligothiophene derivatives, designated as cruciform oligomers, have been investigated in solution (room and low temperature) and in the solid state (as thin films in Zeonex matrixes). The study comprises absorption, emission, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allow the determination of the rate constants for all decay processes. From these, several conclusions are drawn. First, in solution, the main deactivation channels for the compounds are the radiationless processes: S(1) --> S(0) internal conversion and S(1) --> T(1) intersystem crossing. Second, in general, in the solid state, the fluorescence quantum yields decrease relative to solution. A comparison is made with the analogous linear alpha-oligothiophenes, revealing a lower fluorescence quantum efficiency and, in contrast to the normal oligothiophenes, that internal conversion is an important channel for the deactivation of the singlet excited state. Replacement of thiophene by 1,4-phenylene units in the longer-sized cruciform oligomer increases the fluorescence efficiency. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (S(Delta) approximately 1) provides support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds.     

Radiative and radationless decay of exciplexes of 1,12-benzperylene

Many molecules in their excited states react with other species having suitable electron donor or acceptor properties to form complexes (exciplexes) stable only in the excited state. This letter reports a study of the modes of decay of a series of exciplexes in which the donor molecule (1,2-benzperylene) formed exciplexes with a series of dimethylaniline derivatives. By measuring the flourescence and intersystem crossing quantum yields, together with the fluorescence lifetimes of the exciplexes, it was possible to derive the rate constants for fluorescence, radationless decay to the ground state, and intersystem crossing. The first two decay processes were found to show a marked sensitivity to the exciplex energy, while the intersystem crossing rate constant was affected only by the presence of heavy atoms.     

Lifetimes of triplet formaldehyde

By direct optical excitation into the ³A₂ state of H₂CO (D₂CO) with a tunable laser, phosphorescence is observed and lifetimes are measured for this state. Very short lifetimes, apparently limited by intersystem crossing, are observed.     

Synthesis,Spectroscopy, Nonlinear Optics,and Theoretical Investigations of Thienylethynyl Octopoles with a Tunable Core

We have synthesized three new molecules that have three thienylethynyl arms substituting a central benzene core and different electron donor/acceptor groups in the three remaining phenyl positions. The absorption, fluorescence, phosphorescence, and transient triplet–triplet spectra are analyzed in the light of the electronic structure of the ground and excited states obtained from quantum‐chemical calculations. From the above, the relevant photophysical data (including quantum yields, lifetimes, and rate constants) could be derived. It was found that the major deactivation pathway is internal conversion, which competes with the fluorescence and intersystem crossing processes. For the three investigated compounds, we provide convincing theoretical support corroborating these findings and further conclusions based on the theoretical information obtained. These molecules are one of the very few cases in which the depolarization ratios, obtained from the NLO optical measurements, clearly reflect the octopolar configuration. Molecular hyperpolarizabilities have been measured and display a typical dependence on the donor–acceptor substitution pattern.     

Cluster Light-Emitting Diodes Containing Copper Iodine Cube with 100 % Exciton Utilization Using Host-Cluster Synergy

Clusters combine the advantages of organic molecules and inorganic nanomaterials, which are promising alternatives for optoelectronic applications. Nonetheless, recently emerged cluster light-emitting diodes require further excited state optimization of cluster emitters, especially to reduce population of the cluster-centered triplet quenching state (³CC). Here we report that redox-active ligands enhance reverse intersystem crossing (RISC) of Cu₄I₄ cluster for triplet-to-singlet conversion, and thermally activated delayed fluorescence (TADF) host can provide an external RISC channel. It indicates that the complementarity between TADF host and cluster in RISC transitions gives rise to 100 % triplet conversion efficiency and complete singlet exciton convergence, rendering 100-fold increased singlet radiation rate constant and tenfold decreased triplet non-radiation rate constant. We achieve a photoluminescence quantum yield of 99 % and a record external quantum efficiency of 29.4 %.     

Photophysical behaviour of azaphenanthrenes

Nitrogen position and internal heavy atom effects on the radiative and radiationless transitions from the lowest excited states of the isomeric azaphenanthrenes and some of their methyl, chlorine and bromine derivatives have been studied in E.P.A. solutions at 77 K. The nitrogen position affects the fluorescence and S₁-T₁ intersystem crossing rates more than the phosphorescence and T₁-S₀ intersystem crossing rates. Small differences in the behaviour of 9-azaphenanthrene are enhanced in non-hydroxylic solvents and at room temperature, and it is inferred that (n, π*) states play a more important role in the photophysical behaviour of this isomer. Halogen, substitution in all the isomers increases the phosphorescence rate, induces a smaller increase in the T₁-S₀ intersystem crossing rates and has a negligible effect on the fluorescence rate.     

PHOTOPHYSICAL PROPERTIES OF 3,3'-DIALKYLTHIACARBOCYANINE DYES IN HOMOGENEOUS SOLUTION

The photophysical properties of 3,3′-dialkylthiacarbocyanine iodides and chlorides were measured in various solvents. It was found that photoisomerization and fluorescence are the major contributors to the deactivation of the excited singlet state; intersystem crossing occurs with only a very low efficiency. In ethanol, a triplet yield of 0.004 and a singlet oxygen quantum yield of 0.002 were determined. The photophysical parameters of these dyes are not substantially influenced by the length of the alkyl chain or the size of the halide counterion. The substitution of an ethyl with an octadecyl-chain only slightly hinders photoisomerization, and the replacement of the chloride with an iodide reduces only marginally the fluorescence lifetimes and fluorescence quantum yields in chloroform. A significant external heavy-atom effect is observed using dibromoethane as a solvent: triplet and singlet oxygen yields increase7–10-fold, and the triplet lifetime decreases from 55 μs to 15 mUs.     

Fluoresence quenching of riboflavin in aqueous solution by methionin and cystein

The fluorescence quantum distributions, fluorescence quantum yields, and fluorescence lifetimes of riboflavin in methanol, DMSO, water, and aqueous solutions of the sulphur atom containing amino acids methionin and cystein have been determined. In methanol, DMSO, and water (pH=4–8) only dynamic fluorescence reduction due to intersystem crossing and internal conversion is observed. In aqueous methionin solutions of pH=5.25–9 a pH independent static and dynamic fluorescence quenching occurs probably due to riboflavin anion–methionin cation pair formation. In aqueous cystein solutions (pH range from 4.15 to 9) the fluorescence quenching increases with rising pH due to cystein thiolate formation. The cystein thiol form present at low pH does not react with neutral riboflavin. Cystein thiolate present at high pH seems to react with neutral riboflavin causing riboflavin deprotonation (anion formation) by cystein thiolate reduction to the cystein thiol form.     

A Synergistic Enhancement Strategy for Realizing Ultralong and Efficient Room-Temperature Phosphorescence

An enhancement strategy is realized for ultralong bright room-temperature phosphorescence (RTP), involving polymerization between phosphor monomers and acrylamide and host–guest complexation interaction between phosphors and cucurbit[6,7,8]urils (CB[6,7,8]). The non-phosphorescent monomers exhibit 2.46 s ultralong lifetime after copolymerizing with acrylamide. The improvement is due to the rich hydrogen bond and carbonyl within the polymers which promote intersystem crossing, suppress nonradiative relaxation and shield quenchers effectively. By tuning the ratio of chromophores, a series of phosphorescent copolymers with different lifetimes and quantum yields are prepared. The complexation of macrocyclic hosts CB[6,7,8] promote the RTP of polymers by blocking aggregation-caused quenching, and offsetting the losses of aforementioned interaction provided by polymer. Multiple lifetime-encoding for digit and character encryption are achieved by utilizing the difference of their lifetimes.     

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.