Photoexcited triplet state and singlet oxygen generation of quinine, an antimalarial drug.

Energy transfer from the lowest excited triplet (T1) state of quinine (QN) to ground-state molecular oxygen produces singlet oxygen. In aqueous solutions, a neutral form QN, a singly protonated cation QNH+ and doubly protonated cation QNH22+ are present according to their pKa values. To the best of our knowledge, the pH dependence of QN-photosensitized singlet oxygen generation has not been reported. In the present study, the quantum yields of photosensitized singlet oxygen generation (ΦΔ) by QN, QNH+ and QNH22+ have been determined through the measurements of time-resolved near-IR phosphorescence. ΦΔ decreases in the following order: ΦΔ (QNH+) > ΦΔ (QNH22+) > ΦΔ (QN). The nature of the T1 states of QN, QNH+ and QNH22+ has been studied through the measurements of transient absorption, phosphorescence and EPR by changing the pH of the medium. This is the first report of EPR for the T1 state of QN. The photoexcited T1 state of 6-methoxyquinoline (6-MeOQL), a closely related component, has been studied for comparison. The observed zero-field splitting parameters, phosphorescence spectra and triplet lifetimes suggest that the nature of the T1 state of QN can be regarded as a locally excited 3ππ*state within 6-MeOQL. The two unpaired electrons localize mainly on 6-MeOQL. The nature of the T1 state of QN scarcely changes when the quinuclidine nitrogen site is protonated. Applying the Förster cycle to the T1 states of QN and its protonated cations, it was found that QNH+ becomes more basic when excited to its T1 state.

Detecting inattentiveness caused by mind-wandering during a driving task: A behavioral study.

This study aims to investigate whether behavioral variability and participants' self-ratings can be used to detect mind-wandering while driving and to examine their effects on braking performance during a driving task. We created a novel driving task and added a sustained attention response task (SART). We examined the effects of mind-wandering on braking performance and whether mind-wandering could be detected from SART response variability. The within-subjects results showed that self-reports of inattentiveness during driving correlated significantly with SART response variability. Multiple regression analysis with brake reaction time as the dependent variable revealed a significant relationship between self-reports of inattentiveness and mind-wandering. However, there were no other consistent linear associations between mind-wandering and SART response variability. Our results not only suggest that inattentiveness to driving caused by mind-wandering impairs braking performance but also emphasize the importance and difficulty of detecting this state from behavioral data alone.

Suppression of menthyl anthranilate (UV-A sunscreen)-sensitized singlet oxygen generation by Trolox and α-tocopherol.

Menthyl anthranilate (MA, tradename meradimate) is a UV-A absorber. The interactions of ground-state molecular oxygen with the long-lived triplet state of MA produce singlet oxygen through energy transfer. The quantum yield of singlet oxygen generation is 0.12 in air-saturated ethanol. Kinetic traces of the near-IR phosphorescence of singlet oxygen generated by MA-photosensitization have been measured in the absence and presence of Trolox (a water-soluble analogue of vitamin E and a quencher of singlet oxygen) and α-tocopherol (vitamin E, a natural antioxidant) in ethanol. Fluorescence and transient absorption measurements suggest that Trolox and α-tocopherol quench the lowest excited singlet and triplet states of MA. As a result, Trolox and α-tocopherol suppress MA-photosensitized singlet oxygen generation. Not only the quenching of singlet oxygen but also the suppression of singlet oxygen generation is the mechanism of antioxidant properties of Trolox and α-tocopherol for MA. The ability of α-tocopherol to suppress the MA-photosensitized singlet oxygen generation in isododecane, used as a solvent for an oil-soluble UV absorber, is close to that in ethanol. Suppression of sunscreen-photosensitized singlet oxygen generation is an important method for the formulation of safe cosmetic sunscreens.

A novel characteristic of salicylate UV absorbers: suppression of diethylhexyl 2,6-naphthalate (Corapan TQ)-photosensitized singlet oxygen generation.

2-Ethylhexyl salicylate (EHS, octyl salicylate) and homomenthyl salicylate (HMS, homosalate) are UV-B absorbers used in cosmetic sunscreens. Diethylhexyl 2,6-naphthalate (DEHN, tradename Corapan TQ) used as a mildly effective photostabilizer for labile butylmethoxydibenzoylmethane, the most widely used UV-A absorber, is an efficient singlet oxygen photosensitizer with a high quantum yield of singlet oxygen generation ΦΔ = 0.44 in ethanol. The effects of EHS, HMS, l-ascorbic acid (AA, vitamin C), 3-O-ethyl-l-ascorbic acid (3-EtAA) and Trolox (TX, a water-soluble analogue of vitamin E) on DEHN-photosensitized singlet oxygen generation have been studied through measurements of time-resolved near-infrared phosphorescence in ethanol. These compounds suppress DEHN-photosensitized singlet oxygen generation. 40% of ΦΔ is decreased by adding 32 mmol dm-3 EHS or HMS. ΦΔ is decreased to a half by adding 3.0 mmol dm-3 TX. Fluorescence and transient absorption measurements suggest that the observed suppression is due to the quenching of the excited singlet state of DEHN by EHS, HMS, AA and 3-EtAA. These compounds do not quench the excited triplet state of DEHN. On the other hand, TX quenches both the excited singlet and triplet states of DEHN. It was shown that these UV-B absorbers have a novel characteristic as a suppressor of photosensitized singlet oxygen generation. Suppressing singlet oxygen generation is an important method for the prevention of skin damage. To the best of our knowledge, this is the first report on the suppression of photosensitized singlet oxygen generation by UV absorbing molecules.

Suppression of riboflavin-sensitized singlet oxygen generation by l-ascorbic acid, 3-O-ethyl-l-ascorbic acid and Trolox.

Riboflavin (RF), a water-soluble vitamin B2, is an endogenous singlet oxygen photosensitizer in human skin and eye. Time profiles of the near-infrared phosphorescence of singlet oxygen generated by RF have been measured in the absence and presence of l-ascorbic acid (AA, vitamin C), 3-O-ethyl-l-ascorbic acid (3-EtAA) and Trolox (TX, a water-soluble analogue of vitamin E) in phosphate buffer (pH 6.8). These substances suppress the RF-photosensitized singlet oxygen generation. For example, the quantum yield of singlet oxygen generation is decreased to a third by adding 0.4 mmol dm-3 AA or TX (the concentration of dissolved oxygen in air-saturated water is 0.27 mmol dm-3). AA and TX are more efficient suppressors of RF-photosensitized singlet oxygen generation than 3-EtAA. The bimolecular rate constants for quenching of the excited singlet and triplet states of RF by AA, 3-EtAA and TX have been determined through measurements of fluorescence and transient absorption. These measurements suggest that the observed suppression is due to the quenching of the excited singlet and triplet states of RF by AA, 3-EtAA and TX. The bimolecular rate constants for quenching of singlet oxygen by AA, 3-EtAA and TX were determined to be 1.8 × 108, 0.27 × 108, and 4.4 × 108 mol-1 dm3 s-1, respectively.

Photophysical properties of diethylhexyl 2,6-naphthalate (Corapan TQ), a photostabilizer for sunscreens.

Diethylhexyl 2,6-naphthalate (DEHN, tradename Corapan TQ) is used as a photostabilizer for a widely-used UV-A absorber, butylmethoxydibenzoylmethane (BMDBM). The photophysical properties of DEHN have been studied through measurements of the transient absorption, fluorescence, phosphorescence and EPR spectra in ethanol. The energy level of the lowest excited singlet state of DEHN is higher than that of BMDBM (enol form). The singlet-singlet energy transfer from BMDBM to DEHN is energetically unfavourable. The energy level of the lowest excited triplet (T1) state of DEHN is lower than that of BMDBM. It is probable that the mechanism of photostabilization of BMDBM by DEHN is the triplet-triplet energy transfer from BMDBM to DEHN. The observed phosphorescence spectrum, T1 lifetime and zero-field splitting parameters indicate that the T1 state of DEHN can be regarded mainly as a locally excited 3ππ* state within naphthalene. The sum of the quantum yields of fluorescence of DEHN (0.59 ± 0.09) and singlet oxygen generation by DEHN (0.44 ± 0.04) is almost unity in ethanol. About 60% of the UV energy absorbed by DEHN is released as fluorescence, while about 40% is used to produce the excited triplet state. The photoexcited states of 2-naphthoic acid have been studied for comparison.

Photophysical properties of hexyl diethylaminohydroxybenzoylbenzoate (Uvinul A Plus), a UV-A absorber.

Hexyl diethylaminohydroxybenzoylbenzoate (DHHB, Uvinul A Plus) is a photostable UV-A absorber. The photophysical properties of DHHB have been studied by obtaining the transient absorption, total emission, phosphorescence and electron paramagnetic resonance spectra. DHHB exhibits an intense phosphorescence in a hydrogen-bonding solvent (e.g., ethanol) at 77 K, whereas it is weakly phosphorescent in a non-hydrogen-bonding solvent (e.g., 3-methylpentane). The triplet-triplet absorption and EPR spectra for the lowest excited triplet state of DHHB were observed in ethanol, while they were not observed in 3-methylpentane. These results are explained by the proposal that in the benzophenone derivatives possessing an intramolecular hydrogen bond, intramolecular proton transfer is an efficient mechanism of the very fast radiationless decay from the excited singlet state. The energy level of the lowest excited triplet state of DHHB is higher than those of the most widely used UV-B absorbers, octyl methoxycinnamate (OMC) and octocrylene (OCR). DHHB may act as a triplet energy donor for OMC and OCR in the mixtures of UV-A and UV-B absorbers. The bimolecular rate constant for the quenching of singlet oxygen by DHHB was determined by measuring the near-IR phosphorescence of singlet oxygen. The photophysical properties of diethylaminohydroxybenzoylbenzoic acid (DHBA) have been studied for comparison. It is a closely related building block to assist in interpreting the observed data.

Optical and electron paramagnetic resonance studies of the excited triplet states of UV-B absorbers: 2-ethylhexyl salicylate and homomenthyl salicylate.

The energy levels and lifetimes of the lowest excited triplet (T1) states of UV-B absorbers, 2-ethylhexyl salicylate (EHS) and homomenthyl salicylate (HMS), and their deprotonated anions (EHS(-) and HMS(-)) were determined through measurements of phosphorescence and electron paramagnetic resonance (EPR) spectra in rigid solutions at 77 K. The observed T1 energies of EHS and HMS are higher than those of butylmethoxydibenzoylmethane, the most widely used UV-A absorber, and octyl methoxycinnamate, the most widely used UV-B absorber. The T1 states of EHS, HMS, EHS(-) and HMS(-) were assigned to almost pure (3)ππ* state from the observed T1 lifetimes and zero-field splitting parameters. EHS and HMS with an intramolecular hydrogen bond show a photoinduced phosphorescence enhancement in ethanol at 77 K. The EPR signals of the T1 states of EHS and HMS also increase in intensity with UV-irradiation time (photoinduced EPR enhancement). The T1 lifetimes of EHS and HMS at room temperature were determined through triplet-triplet absorption measurements in ethanol. The quantum yields of singlet oxygen production by EHS and HMS were determined by using time-resolved near-IR phosphorescence.

Photoexcited triplet states of UV-B absorbers: ethylhexyl triazone and diethylhexylbutamido triazone.

The excited states of UV-B absorbers, ethylhexyl triazone (EHT) and diethylhexylbutamido triazone (DBT), have been studied through measurements of UV absorption, fluorescence, phosphorescence, triplet-triplet absorption and electron paramagnetic resonance spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states and quantum yields of fluorescence and phosphorescence of EHT and DBT were determined. In ethanol at 77 K, the deactivation process of EHT and DBT is predominantly fluorescence, however, a significant portion of the S1 molecules undergoes intersystem crossing to the T1 state. The observed phosphorescence spectra, T1 lifetimes and zero-field splitting parameters suggest that the T1 state of EHT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid, while the T1 state of DBT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid or p-amino-N-methylbenzamide. The quantum yields of singlet oxygen generation by EHT and DBT were determined by time-resolved near-IR phosphorescence measurements in ethanol at room temperature. EHT and DBT did not exhibit significantly antioxidative properties by quenching singlet oxygen, in contrast to the study by Lhiaubet-Vallet et al.

Photoexcited states of UV absorbers, benzophenone derivatives.

The UV absorption, phosphorescence and phosphorescence-excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet-triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2',4,4'-tetrahydroxybenzophenone (BP-2), 2-hydroxy-4-methoxybenzophenone (BP-3), 2,2'-dihydroxy-4,4'-dimethoxybenzophenone (BP-6), 5-chloro-2-hydroxybenzophenone (BP-7) and 2-hydroxy-4-n-octyloxybenzophenone (BP-12). The energy levels and lifetimes of the lowest excited triplet (T1 ) states of these BP derivatives were determined from the first peak of phosphorescence. The time-resolved near-IR emission spectrum of singlet oxygen generated by photosensitization with BP-7 was observed in acetonitrile at room temperature. BP-2, BP-3, BP-6 and BP-12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T1 states of 2-hydroxy-5-methylbenzophenone, 4-methoxybenzophenone and 2,4'-dimethoxybenzophenone have been studied for comparison.

Triplet-triplet energy transfer from a UV-A absorber butylmethoxydibenzoylmethane to UV-B absorbers.

The phosphorescence decay of a UV-A absorber, 4-tert-butyl-4'-methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV-B absorbers, 2-ethylhexyl 4-methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T1) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet-triplet energy transfer occurs from BMDBM to OMC and OCR. The T1 state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T1 state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy-donor phosphorescence decay measurements can be applied to the study of the triplet-triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet-triplet annihilation was observed in the BMDBM-OMC and BMDBM-OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.

Optical and time-resolved electron paramagnetic resonance studies of the excited states of a UV-B absorber (4-methylbenzylidene)camphor.

The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet-triplet (T-T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T(1)) state of MBC were determined. The energy level of the T(1) state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T-T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S(1)) molecules undergoes intersystem crossing to the T(1) state and the deactivation process from the T(1) state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)ππ* character in the T(1) state. The zero-field splitting parameters in the T(1) state are D = 0.0901 cm(-1) and E = -0.0498 cm(-1). The sublevel preferentially populated by intersystem crossing is T(y) (y close to in-plane short axis and to the C═O direction).

Excited states of menthyl anthranilate: a UV-A absorber.

The photophysical properties of menthyl anthranilate (MA), a UV-A absorber, have been studied through measurements of UV absorption, fluorescence, triplet-triplet absorption and time-resolved thermal lens in ethanol at room temperature and/or 77 K. The phosphorescence and time-resolved electron paramagnetic resonance spectra have also been observed in ethanol at 77 K. The energy levels of the lowest excited singlet (S(1)) state and triplet (T(1)) state were determined. The quantum yields of fluorescence, phosphorescence and S(1) → T(1) intersystem crossing (ISC) were also determined. From the data on the lifetime and quantum yield, the rate constants of the radiative decay, internal conversion and ISC of the excited states of MA were determined. The exceptionally high ISC quantum yield of MA shows that the deactivation processes of the S(1) state of MA are different from those of most UV absorbers such as cinnamate, salicylate, 2-hydroxybenzophenone, benzotriazole and dibenzoylmethane derivatives, where the internal conversion rates of the S(1) states are much faster than the ISC rates. The observed T(1) lifetime and zero-field splitting parameters suggest that the T(1) state of MA possesses almost pure (3)ππ* character.

Photoexcited singlet and triplet states of a UV absorber ethylhexyl methoxycrylene.

The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states of EHMCR were determined. The energy levels of the S1 and T1 states of EHMCR are much lower than those of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The energy levels of the S1 and T1 states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR B(min) signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S1 molecules undergoes intersystem crossing to the T1 state, and the deactivation process from the T1 state is predominantly radiationless. The photostability of EHMCR arises from the (3)ππ* character in the T1 state. The zero-field splitting (ZFS) parameter in the T1 state is D** = 0.113 cm(-1).

Photophysical properties of dioctyl 4-methoxybenzylidenemalonate: UV-B absorber.

Dioctyl 4-methoxybenzylidenemalonate (DOMBM) is an effective stabilizer for photolabile 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM). DOMBM is superior to the most widely used UV-B absorber, octyl methoxycinnamate (OMC), at preserving its UV-B absorbance in the presence of BMDBM. The energy levels of the lowest excited singlet (S(1)) and triplet (T(1)) states of DOMBM were determined by measuring fluorescence and phosphorescence spectra in ethanol at 77 K. The energy level of the T(1) state of DOMBM is lower than that of BMDBM and is higher than that of OMC. The triplet-triplet energy transfer from BMDBM to DOMBM was demonstrated by measuring the electron paramagnetic resonance (EPR) and time-resolved phosphorescence spectra. The phosphorescence and EPR signals of DOMBM are too weak to be observed through the direct excitation in ethanol at 77 K. The phosphorescence spectrum was observed by using external heavy atom effects of ethyl iodide. The EPR spectrum was observed by using benzophenone as a triplet sensitizer. The fluorescence quantum yield of DOMBM is small in ethanol at room temperature. Only the fast component, due to the heat released from the excited state of DOMBM, was observed in the time-resolved thermal lensing experiments at room temperature. These results show that the quantum yield of the S(1) → T(1) intersystem crossing is negligible and the deactivation process is predominantly internal conversion to the ground state for DOMBM.

Photoexcited triplet states of new UV absorbers, cinnamic acid 2-methylphenyl esters.

Phosphorescence spectra of nonphosphorescent or very weakly phosphorescent new UV absorbers, 2-methylphenyl cinnamate (MePC), 2-methylphenyl 4-methoxycinnamate (MePMC) and 2-methylphenyl 4-ethoxycinnamate (MePEC) have been observed by using external heavy atom effects of ethyl iodide in ethanol at 77 K. The lowest excited triplet (T(1)) energies of these new UV absorbers are lower than those of a widely used UV-A absorber, 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM), in both keto and enol forms. The intermolecular triplet-triplet energy transfer from photolabile BM-DBM to MePMC was observed by measuring the time-resolved phosphorescence spectra. Electron paramagnetic resonance spectra have been observed for the T(1) states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T(1) lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T(1) states of these new UV absorbers posses mainly (3)ππ* character. The deactivation processes of the lowest excited singlet (S(1)) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S(1) state of MePC is internal conversion to the G state. The molar absorption coefficients of MePMC and MePEC in the UV-A and UV-B regions are larger than that of most widely used UV-B absorber, octyl methoxycinnamate.

Photoinduced diffusive mass transfer in o-Cl-HABI amorphous thin films.

The first demonstration of photoinduced surface relief grating formation using amorphous thin films composed of a radical dissociative photochromic compound, 2,2'-di(ortho-chlorophenyl)-4,4',5,5'-tetraphenylbiimidazole, show that mass transfer occurred from the UV-light-irradiated area to the shaded area by patterned light irradiation (365 nm).

Optical and electron paramagnetic resonance studies of the excited states of 4-tert-butyl-4'-methoxydibenzoylmethane and 4-tert-butyl-4'-methoxydibenzoylpropane.

The excited states of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM) and 4-tert-butyl-4'-methoxydibenzoylpropane (BM-DBP), the model of the keto form of BM-DBM, have been studied through measurements of UV absorption, fluorescence, phosphorescence, and electron paramagnetic resonance spectra in ethanol at 77 K. The energy levels and lifetimes of the lowest excited triplet (T(1)) states of BM-DBP, dibenzoylpropane (DBP), the model of the keto form of dibenzoylmethane (DBM), and the keto and enol forms of BM-DBM and DBM were determined. The energy level of the T(1) state of the keto form is much higher than that of the enol form in BM-DBM. The effect of tautomerization on the T(1) lifetime is small in DBM but large in BM-DBM. The methoxy and tert-butyl groups play an important role in lengthening the T(1) lifetimes of BM-DBP and the keto form of BM-DBM. The nature of the T(1) states of the keto and enol forms of BM-DBM can be explained in terms of the mixing of the (3)npi* and (3)pipi* states. The observed energy levels and the lifetimes of the T(1) states of the UV absorbers provide a useful suggestion for designing more photostable UV absorbers.

A novel transgenic chimaeric mouse system for the rapid functional evaluation of genes encoding secreted proteins.

A major challenge of the post-genomic era is the functional characterization of anonymous open reading frames (ORFs) identified by the Human Genome Project. In this context, there is a strong requirement for the development of technologies that enhance our ability to analyze gene functions at the level of the whole organism. Here, we describe a rapid and efficient procedure to generate transgenic chimaeric mice that continuously secrete a foreign protein into the systemic circulation. The transgene units were inserted into the genomic site adjacent to the endogenous immunoglobulin (Ig) kappa locus by homologous recombination, using a modified mouse embryonic stem (ES) cell line that exhibits a high frequency of homologous recombination at the Igkappa region. The resultant ES clones were injected into embryos derived from a B-cell-deficient host strain, thus producing chimaerism-independent, B-cell-specific transgene expression. This feature of the system eliminates the time-consuming breeding typically implemented in standard transgenic strategies and allows for evaluating the effect of ectopic transgene expression directly in the resulting chimaeric mice. To demonstrate the utility of this system we showed high-level protein expression in the sera and severe phenotypes in human EPO (hEPO) and murine thrombopoietin (mTPO) transgenic chimaeras.

Time-resolved electron paramagnetic resonance of the lowest excited triplet state of phenazinium cation.

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T(1)) states of phenazine (Phz) and its singly protonated cation (phenazinium) in sulfuric acid-ethanol mixtures at 77K. The single protonation appears to have little effect on the anisotropic sublevel populating rates of the T(1) state of phenazine. However, the zero-field splitting (ZFS) parameter D decreases on the protonation, reflecting the increase of delocalization of the two unpaired electrons. The sublevel preferentially populated by intersystem crossing (ISC) is T(y) in both phenazine and phenazinium (the y-axis is parallel to the in-plane long axis). From the analysis of the observed anisotropy in the ISC rates and the semi empirical molecular orbital calculations of the ZFS parameters, we concluded that the T(1) state of phenazinium is the (3)A(1)(pipi*) state.