Quantum Yield Of Triplet Formation Research Articles

TADF‐Emitting Nanoparticles for Application as Probes in Time‐Resolved Imaging and 1O2 Photosensitizers

AbstractOrganic Thermally Activated Delayed Fluorescence (TADF) molecules are luminescent compounds capable of harvesting energy from triplet states without using heavy metals. This process results in oxygen‐sensitive, long‐lived delayed emission, suitable for developing optical probes for time‐gated cell imaging, oxygen sensors, and singlet oxygen photosensitizers. Despite their potential, the use of TADF emitters in these applications is hindered by poor performance in polar media and the challenge of balancing high photoluminescence quantum yields, long emission lifetimes, and high triplet formation quantum yields. In this study, novel TADF emitters are developed based on 1,8‐naphthalimide (NI) dyes, and how encapsulation in polymeric nanoparticles can enhance their performance in aqueous media is demonstrated. The resulting nanomaterials exhibit high delayed‐to‐prompt emission ratios, long delayed fluorescence lifetimes, and exceptional applicability in time‐resolved imaging. The singlet oxygen photosensitization capabilities of the TADF nanomaterials in the photodegradation of phenol, exploring a Förster Resonance Energy Transfer (FRET) system that improved the efficiency, are also assessed. These findings highlight the promising potential of TADF nanomaterials for diverse applications, including photodegradation of pollutants, cellular imaging, and biosensing.

Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves.

Accumulation of carotenoid (Car) triplet states was investigated by singlet-triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4μs length at half-height and 525-565 or 410-490nm spectral band, maximum intensity 2mol quanta m-2s-1, flash photon dose up to 10μmolm-2 or 4-10 PSII excitations. Superimposed upon the non-photochemically unquenched Fmd state, fluorescence was strongly quenched near the flash maximum (minimum yield Fe), but returned to the Fmd level after 30-50μs. The fraction of PSII containing a 3Car in equilibrium with singlet excitation was calculated as Te = (Fmd-Fe)/Fmd. Light dependence of Te was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9μs, but it was strongly shortened by the presence of O2. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 Fmd. The results show that in the Fmd state a light-adapted charge-separated PSIIL state is formed (Sipka et al., The Plant Cell 33:1286-1302, 2021) in which Pheo-P680+ radical pair formation is hindered, and excitation is terminated in the antenna by 3Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSIIL state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without 3Car formation.

Ultrafast spectroscopy reveals singlet fission, ionization and excimer formation in perylene film

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications.

Calixarenes as High Temperature Matrices for Thermally Activated Delayed Fluorescence: C70 in Dihomooxacalix[4]arene.

Thermally activated delayed fluorescence (TADF) of 12C70 and 13C70 was observed up to 140 °C in a p-tert-butyldihomooxacalix[4]arene solid matrix, a temperature range significantly higher than that of previous TADF quantitative studies. An effective singlet–triplet energy gap of 29 kJ/mol and triplet formation quantum yields of 0.97 and 0.99 were measured for 12C70 and 13C70, respectively. The photophysical properties of the two fullerenes in this new matrix are comparable to those obtained in polystyrene at a lower temperature range. Calixarenes are proposed to be suitable matrices for high temperature TADF studies and applications.

1,3-Diphenylisobenzofuran: a Model Chromophore for Singlet Fission.

In this review we first provide an introductory description of the singlet fission phenomenon and then describe the ground and electronically excited states of the parent 1,3-diphenylisobenzofuran chromophore (1) and about a dozen of its derivatives. A discussion of singlet fission in thin polycrystalline layers of these materials follows. The highest quantum yield of triplet formation by singlet fission, 200% at 80K, is found in one of the two known crystal modification of the parent. In the other modification and in many derivatives, excimer formation competes successfully and triplet yields are low. A description of solution photophysics of covalent dimers is described in the next section. Triplet yields are very low, but interesting phenomena are uncovered. One is an observation of a separated-charges (charge-transfer) intermediate in highly polar solvents. The other is an observation of excitation isomerism in both singlet and triplet states, where in one isomer the excitation is delocalized over both halves of the covalent dimer, whereas in the other it is localized on one of the halves. In the last section we present the operation of a simple device illustrating the use of triplets generated by singlet fission for charge separation.

Broad-Band N(∧)N Pt(II) Bisacetylide Visible Light Harvesting Complex with Heteroleptic Bodipy Acetylide Ligands.

Pt(II) dbbpy bisacetylide (dbbpy = 4,4'-di(tert-butyl)-2,2'-bipyridine) complex (Pt-1) with two different Bodipy ligands was prepared with the goal to attain broad-band visible light absorbing, efficient funneling of the photoexcitation energy (via resonance energy transfer, RET) to the energy acceptor and high triplet formation quantum yields. Construction of the above-mentioned molecular structural motif is challenging because two different arylacetylide ligands are incorporated in the complex; normally two homoleptic acetylide ligands were used for this kind of N(∧)N Pt(II) complexes. A reference complex with trans bis(tributylphosphine) Pt(II) bisacetylide protocol (Pt-4) was prepared for comparison of the photophysical properties. The two different Bodipy ligands in Pt-1 and Pt-4 constitute singlet/triplet energy donor/acceptor, as a result the harvested photoexcitation energy can be funneled to the triplet state confined on one of the two Bodipy ligands. The photophysical properties of the complexes were studied with steady state UV-vis absorption and luminescence spectroscopies, femto- and nanosecond transient absorption spectroscopies, cyclic voltammetry, as well as DFT/TDDFT calculations. Fluorescence/phosphorescence dual emission were observed for the complex. The ultrafast intramolecular singlet/triplet energy transfer in Pt-1 was confirmed by the transient absorption spectroscopy (kFRET = 2.6 × 10(11) s(-1), ΦFRET = 87.1%) followed by an intersystem crossing (kISC = 1.9 × 10(10) s(-1)), and the triplet state lifetime (τT) is 54.1 μs. The reference complex Pt-4 shows drastically different kinetics with kFRET = 6.9 × 10(10) s(-1), ΦFRET = 81.0%, kISC = 5.83 × 10(9) s(-1), and τT = 147.9 μs. Different singlet oxygen ((1)O2) quantum yields (ΦΔ = 75% and 70%) and triplet state quantum yields (ΦT = 91% and 69%, respectively) were observed for complexes Pt-1 and Pt-4.

Temperature dependence of the phosphorescence and of the thermally activated delayed fluorescence of 12C70 and 13C70 in amorphous polymer matrices. Is a second triplet involved?

The phosphorescence and thermally activated delayed fluorescence (TADF) lifetimes of 12C70 and 13C70 in two different glassy hydrocarbon polymers, one aliphatic (cyclic polyolefin) and one aromatic (polystyrene), were measured between −200 and 100 ºC. The temperature dependence of the lifetimes is equally well described by a three-state mechanism (ground state, S0, and two excited states in thermal equilibrium, T1 and S1, the lifetime of T1 being temperature dependent) and by a four-state mechanism (ground state, S0, and three excited states in thermal equilibrium, T1, T2 and S1, all with temperature-independent lifetimes). The estimated S1–T1 and T2–T1 energy gaps (four-state mechanism) are in good agreement with spectroscopic measurements. These and the determined quantum yield of triplet formation, 0.997 ± 0.001, are found to be essentially independent of the polymer matrix and of the isotopic composition of the fullerene. On the other hand, the lifetimes of both T1 and T2 (four-state mechanism) are weakly dependent on the polymer matrix but strongly vary with the fullerene isotopic composition, nearly doubling when going from 12C70 to 13C70. A parameter useful for the characterization of TADF, the on-set temperature T0, is also introduced.

Primary Photochemical Properties of Difloxacin in Neutral Aqueous Solution

The photochemical properties of difloxacin (DFX) were investigated in neutral aqueous solution. DFX aqueous solution showed intense absorption with one peak at 273 nm (molar absorption coefficient epsilon=33000 dm(3).mol(-1).cm(-1)) and two other peaks at 323 and 335 nm (epsilon=15500 dm(3).mol(-l).cm(-1)) with the same molar absorption coefficient. Both the absorption and emission properties of DFX were pH-dependent. The acid dissociation constant (pK(a)) for the protonation equilibria of the ground state (5.9 and 9.8) were determined spectroscopically. DFX fluoresces weakly, and its maximum quantum yield for fluorescence emission was 0.06 at pH 3. Laser flash photolysis and pulse radiolysis studies were carried out to characterize the transient species of DFX aqueous solution. Triplet-triplet absorption reached a maximum at 620 nm with a molar absorption coefficient of 7900 dm(3).mol(-1).cm(-1). The energy transfer method was used to estimate the triplet energy of DFX, which was 263.5 kJ.mol(-1). The quantum yield of triplet formation was determined to be 0.21. Furthermore, DFX showed monophotonic photoionization with a quantum yield of 0.02. Pulse radiolysis indicated that DFX could react with e(aq)(-); and .OH, and the bimolecular rate constants for these reactions were 1.72x10(10) and 1.0x10(10) dm(3).mol(-1).s(-1), respectively. It is expected that this research may be helpful in determining the phototoxicity mechanism of DFX.

Triplet–triplet exciton dynamics in single-walled carbon nanotubes

Semiconducting single-walled carbon nanotubes (SWNTs) are considered as building blocks for novel optoelectronic and photonic devices. Energy transport, dissipation and nonlinear optical properties of such devices depend critically on the dynamics of singlet and triplet excitons. However, little is known about triplet excitons in SWNTs despite their important role in photovoltaic, photoelectric and other applications. We present pump–probe and spin-sensitive photoluminescence studies of semiconducting SWNTs that allow the determination of the quantum yield of triplet formation (5 ± 2%), the triplet lifetime (30 ± 10 µs) and the triplet exciton size (0.65 nm). Triplet–triplet annihilation is also found to induce delayed fluorescence. The power-law decay of pump–probe and time-resolved photoluminescence signals is characteristic of diffusion-limited annihilation in one-dimensional systems and allows an estimation of the triplet diffusion constant of 0.1 cm2 s−1. This work suggests that exciton annihilation in SWNTs is reduced by one-dimensional confinement of diffusive exciton motion. Little is known about triplet excitons in semiconducting single-walled nanotubes, despite their importance in various applications. The pump–probe and spin-sensitive photoluminescence of such nanotubes is studied, and the quantum yield of triplet formation, triplet lifetime and triplet exciton size are found to be 5 ± 2%, 30 ± 10 µs and 0.65 nm, respectively.

Photosensitized Generation of Singlet Oxygen from Re(I) Complexes: A Photophysical Study Using LIOAS and Luminescence Techniques

Quantum yields and efficiencies of (1)O2 ((1)Δg) production along with photophysical properties for a number of Re(I) complexes in acetonitrile solutions are reported. Two different classes of Re(I) complexes, L(S)-CO2-Re(CO)3(bpy) (L(S) = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)3L (X = CF3SO3, py; L = bpy, phen), were probed as photosensitizers for (1)O2 ((1)Δg) production in air-saturated acetonitrile solutions. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of (1)O2 ((1)Δg) is either a metal-to-ligand charge transfer ((3)MLCT) or a ligand centered ((3)LC) state. With L(S)-CO2-Re(CO)3(bpy) complexes, (1)O2 ((1)Δg) is produced by oxygen quenching of (3)LC states of anthracene and pyrene with high quantum yields (ΦΔ between 0.8 and 1.0), while the complexes bearing the ligands L(S) = 2-anthraquinone, 2-pyrazine, and 2-naphthalene did not yield (1)O2. XRe(CO)3L complexes generate (1)O2 ((1)Δg) mainly by oxygen quenching of their (3)MLCT luminescence with an efficiency of (1)O2 ((1)Δg) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)3L complexes' emission by molecular oxygen range between 1 × 10(9) and 2 × 10(9) M(-1) s(-1), and they are all ≤ (1/9)kd, in good agreement with the predominance of the singlet channel in the mechanism of (1)O2 ((1)Δg) generation using these Re(I) complexes as photosensitizers. All the experimental singlet oxygen efficiencies are consistent with calorimetric and luminescence data for the studied complexes. With L(S)-CO2-Re(CO)3(bpy) complexes, calorimetric experiments were utilized in the calculation of the quantum yields of triplet formation; namely φT = 0.76 and 0.83 for the triplet states of anthracene and pyrene, respectively.

Silicon tetrabenzotriaza corrole and silicon phthalocyanine: Synthesis, photophysics and singlet oxygen generation

Synthesis, photophysical and photochemical measurements were carried out on silicon tetrabenzotriaza corrole. The results were compared with that of phosphorous tetrabenzotriaza corrole and silicon phthalocyanine. UV–vis absorption spectra, fluorescence emission spectra, fluorescence quantum yields and lifetimes, triplet–triplet absorption spectra, triplet formation quantum yields and lifetimes, and singlet delta oxygen quantum yields were measured in DMF. Silicon tetrabenzotriaza corrole exhibits quite different spectral and photophysical properties from both silicon phthalocyanine and phosphorous tetrabenzotriaza corrole. Silicon tetrabenzotriaza corrole features a longest fluorescence lifetime of 5.84 ns, but a lowest fluorescence quantum yield of 0.24. The quantum yields of the triplet state formation and sensitized generation of singlet delta oxygen by silicon tetrabenzotriaza are 0.72 and 0.61, respectively, which are significantly higher than those of silicon phthalocyanine and phosphorous tetrabenzotriaza corrole. Silicon tetrabenzotriaza corrole also showed a long triplet lifetime of 143 μs. Silicon tetrabenzotriaza corrole is therefore a very efficient photosensitizer for singlet oxygen generation.

Singlet-Triplet States Interaction Regions in DNA/RNA Nucleobase Hypersurfaces.

The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet crossing region directly connecting the lowest singlet and triplet ππ* excited states. Adenine, thymine, and uracil display additional higher-energy crossing regions related to the presence of low-lying singlet and a triplet nπ* state. These funnels are absent in guanine and cytosine, which have the bright (1)ππ* state lower in energy and less accessible nπ* states. Knowledge of the location and accessibility of these regions, in which the singlet-triplet interaction is related to large spin-orbit coupling elements, may help to understand experimental evidence such as the wavelength dependence measured for the triplet formation quantum yield in nucleobases and the prevalence of adenine and thymine components in the phosphorescence spectra of DNA.

Tetrabrominated Lead Naphthalocyanine for Optical Power Limiting

The complex 2,(3)-tetrabromo-3,(2)-tetra[(3,5-di-tert-butyl)phenyloxy]-naphthalocyaninato lead [Br(4)(tBu(2)C(6)H(3)O)(4)NcPb, 1] has been prepared and its optical limiting properties for ns light pulses have been measured. Complex 1 behaves as a reverse saturable absorber within the spectral range 440-720 nm with a limiting threshold of 0.1 J cm(-2) at 532 nm. The lifetime of the absorbing triplet excited state has been evaluated as 3.8 x 10(-7) s and the quantum yield of triplet formation has been measured as 0.07 in toluene. The nonlinear optical transmission properties of complex 1 have also been determined in Plexiglas [naphthalocyanine content: 5.0 x 10(-4) M (0.1% by weight)]. A reversible nonlinear absorption was again observed for a fluence above 0.4 J cm(-2), but through different excited-state dynamics. This may be rationalized in terms of aggregation of the molecule in the polymer matrix.

Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin†

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (epsilon=36800 and 17000 dm3 mol(-1) cm(-1), respectively). Both the absorption and emission properties of SFX are pH-dependent; pKa values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet-triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000+/-1000 dm3 mol(-1) cm(-1). The quantum yield of triplet formation has been determined to be 0.35+/-0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (3A*) was found to react with phosphate buffer to form triplet 3B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.

Photophysical properties of a photocytotoxic fluorinated chlorin conjugated to four β-cyclodextrins

A meso-tetrakis(pentafluorophenyl)-chlorin with the reduced pyrrole ring linked to an isoxazolidine ring (FC) has been conjugated to four beta-cyclodextrins (CDFC). The CDFC exhibits excellent water solubility and is a potent photosensitizer towards proliferating NCTC 2544 human keratinocytes. The study by conventional steady state absorption and fluorescence spectroscopies and by time-resolved femto- and nanosecond laser flash spectroscopies suggests that in ethanol and pH 7 buffer the beta-cyclodextrins embed the highly hydrophobic tetrakis(pentafluorophenyl)-chlorin macrocycle and strongly interact with the chlorin rings in the singlet and triplet manifolds. In these solvents, femtosecond spectroscopy suggests that the conjugate undergoes a rapid relaxation in the upper excited singlet states induced by photochemical and/or conformation change(s) at a rate of about 5 ps(-1) to fluorescent states whose lifetime is approximately 8 ns. This interaction is destroyed upon addition of Triton X100 to buffer. Both FC and CDFC strongly fluoresce (Phi(F) approximately 0.5) in micelles. Similar behavior is observed at the triplet level. In ethanol and water, the initial transient triplet state absorbance decays within 1-3 mus yielding a longer lived triplet with spectral properties indistinguishable from that of original difference absorbance spectra. The determination of the molar absorbance in the 440-460 nm region ( approximately 35 000 M(-1) cm(-1)) leads to an estimate of approximately 0.2 for the triplet formation quantum yield of FC in toluene and of FC and CDFC in Triton X100 micelles. Quenching of the CDFC triplets by dioxygen in buffer produces (1)O(2) in a good yield consistent with the effective photocytotoxicity of the chlorin-cyclodextrins conjugate towards cultured NCTC 2544 human keratinocytes. By contrast, FC which aggregates in buffer produces little if any (1)O(2).

Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: Application to the fullerenes

In efficient thermally activated delayed fluorescence (TADF) the excited chromophore alternates randomly between the singlet and triplet manifolds a large number of times before emission occurs. In this work, the average number of cycles n is obtained and is shown to have a simple experimental meaning: n+1 is the intensification factor of the prompt fluorescence intensity, owing to the occurrence of TADF. A new method of data analysis for the determination of the quantum yield of triplet formation, combining steady-state and time-resolved data in a single plot, is also presented. Application of the theoretical results to the TADF of [70]fullerenes shows a general good agreement between different methods of fluorescence analysis and allows the determination of several photophysical parameters.

Primary Photophysical Properties of Ofloxacin ¶

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 ± 500 dm3 mol−1 cm−1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 ± 0.2 × 1010 dm3 mol−1 s−1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 ± 300 dm 3 mol−1 cm−1) and at 530 nm. Triplet–triplet absorption has a maximum at 610 nm with an extinction coefficient of 11 000 ± 1500 dm 3 mol−1 cm−1. The quantum yield of triplet formation has been determined to be 0.33 ± 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and ≤0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.

Femtosecond ground state recovery: Measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc) and Al(pyr)2(tpfc-Br8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD AX, AY, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.

A Molecular Thermometer Based on the Delayed Fluorescence of C70 Dispersed in a Polystyrene Film

A new optical molecular thermometer, based on the thermally activated delayed fluorescence of C70 dispersed in a polystyrene film, was developed. In the presence of oxygen, the fluorescence intensity of the C70 film is essentially temperature independent in a wide range. In the absence of oxygen, however, the fluorescence intensity markedly increases with temperature. At room temperature (25 degrees C), and after degassing the sample, the fluorescence intensity of C70 increases 22 times, while at 100 degrees C the fluorescence intensity is increased by 70 times. With our system, the very weak fluorescence of C70 (phi(F) congruent with 5 x 10(-4), in toluene) can be increased up to 91 times (up to an estimated maximum value phi(F) = 0.046). The estimate value of the singlet-triplet gap (29 kJ mol(-1)) and the fluorescence lifetime (0.63 ns) of the C70 in film are in agreement with the values reported in the literature for C70 in solution. The values of the phosphorescence lifetime at room temperature (23 ms) and the quantum yield of triplet formation (0.989) were also determined. The system is completely reversible with respect to heating-cooling cycles.