Triplet Absorption Spectra Research Articles

Effect of solvent on the excited-state photophysical properties of curcumin.

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Optical limiting and non linear optical properties of fullerene derivatives embedded in hybrid sol–gel glasses

Non linear optical properties of pyrrolidino-fullerenes in sol–gel matrix have been investigated for Optical Limiting (OL) applications. The fullerene functionalization increases the solubility in polar solvent and allows the preparation of solid materials via sol–gel method. It also extends the absorption of fullerenes to the red spectral region, where the maximum of the triplet to triplet absorption spectrum is located and, as a result, the Reverese Saturable Absorption (RSA) mechanism is enhanced. The use of the sol–gel method allows the preparation of samples with controlled thickness, with very good linear and non linear optical properties and with high damage fluences. This is useful to design and prepare a bottleneck structure, in order to optimize a laser protection device. The test of the non-linear optical properties of sol–gel samples containing fullerene derivatives has been made by optical limiting and Z-scan measurements and by comparing the experimental data with the calculated RSA behavior. Fullerene derivatives show good optical limiting performances in the red spectral region, better than fullerene. The pyrrolidino-fullerenes in sol–gel samples still show the same performances as solution samples. The comparison of the experimental data with the theoretical OL curves indicates that RSA is the predominant OL behavior, especially at 690 nm, but other different mechanisms, like non linear scattering, may contribute and are being considered in refining our theoretical model. The control of the propagation beam geometry is one of the critical point for OL measurements as well as for the optimal design of a protecting device.

Absorption, fluorescence and transient triplet–triplet absorption spectra of zinc tetramethylpyridylporphyrin in reverse micelles and microemulsions of aerosol OT–(AOT)

The photophysical properties of water soluble Zn(II) meso-tetrakis(N-methylpyridinium-4-yl)-porphyrin were studied in reverse micelles of the iso-octane–AOT–water system. Broadening of absorption and emission bands as well as shifts and increase of maxima intensities were observed with different values of W0 = [H2O]/[AOT], with a fixed AOT (0.1 M) concentration. These effects are generally good evidence for distinct probe locations inside the aggregates, but no clear evidence of equilibria (isosbestic and isoemissive points) was detected. The decomposition of the Soret absorption band into Gaussian functions showed the presence of two species, one near the well known blue Soret band in water (435 nm) and another more displaced to the red (442 nm). Transient triplet–triplet absorption spectra confirm the different locations of the porphyrin in the micelle interface. One of the species, which is located in a more hydrophobic region of the interface has a triplet decay constant around ten times slower than the other one located at the hydrophilic region of the interface with lower singlet energy and a fast triplet decay rate constant.

Photophysical and Electron Transfer Properties of C60(C6H5)5OH and C60(C6H4F)5OH: A Laser Flash Photolysis and Pulse Radiolysis Study

Photophysical properties of the excited singlet and triplet states of pentaphenylfulleren-2-ol (C60(C6H5)5OH, PPF) and penta-4-fluorophenylfulleren-2-ol [C60(C6H4F)5OH, PFPF], two derivatives of buckminsterfullerene, C60, have been investigated in benzene and benzonitrile solutions using two complementary transient absorption techniques, namely, laser flash photolysis and pulse radiolysis. Singlet−singlet and triplet−triplet absorption spectra have been characterized. Quantum yields for formation of the triplet states by intersystem crossing from the singlet states, and also the intrinsic lifetimes of the singlet and triplet states, have been determined. In addition to the intrinsic decay of the triplet state, the evidences have been obtained for the contribution of the other two processes, namely, the self-quenching by the ground state and the T−T annihilation reaction, toward the decay of the triplet state, and the rate constants for these processes have been determined. The formation of charge transfer (CT) complexes with two aromatic amines, diphenylamine (DPA) and triphenylamine (TPA), in benzonitrile and the electron transfer reactions in the excited singlet and CT complexes have been studied.

Triplet-State Properties and Singlet Oxygen Generation in a Homologous Series of Functionalized Fullerene Derivatives

The main properties of the triplet states and the yields of singlet oxygen generation for a series of methano[60]fullerene adducts with increasing numbers of substituents were determined. The triplet properties investigated are energies, triplet−triplet absorption spectra, extinction coefficients, and quantum yields. A strong correlation is observed between several triplet properties and the topology of the fullerene core. Successive fragmentation of the chromophore of the parent C60 results in higher triplet energies and lower triplet quantum yields. In addition, singlet oxygen quantum yields (φΔ) were determined for all adducts and decrease as the area of the conjugated fullerene core decreases.

Unusual triplet state relaxation in C60 oxide

Abstract The photophysics of C 60 O has been investigated. The calibrated UV–visible absorption spectrum, fluorescence quantum yield, and triplet–triplet absorption spectrum are compared to those of C 60 . Intrinsic triplet decay is anomalous, with typically weak temperature dependence below 180 K but unusual strong activation ( E A =20 kJ mol −1 ) above. At 295 K, the triplet lifetime is only 4.2 μs, or 34 times shorter than that of C 60 . It is proposed that the activated decay channel involves reversible cleavage of the C–C epoxide bond. C 60 O efficiently quenches triplet C 60 with a rate constant of 3.6×10 9 M −1 s −1 .

Photoexcitations of bridged quaterthiophene in solution

The suppression of inter-ring torsions in quaterthiophenes changes the deactivation path of the excited state leading to the reduction of non-radiative decay and increase of the photoluminescence quantum yield. Triplet transient absorption spectra are strongly affected by central inter-ring bridging, showing features which are tentatively related to different conformers.

Photophysical properties and photoinduced polymerisation activity of novel 1-chloro-4-oxy/acyloxythioxanthone initiators

The excited state characteristics of 13 novel 1-chloro-4-oxy/acyloxy derivatives of thioxanthone were determined using micro-, nano- and picosecond flash photolysis techniques. Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activities were measured using photocalorimetry. All the initiators exhibit high photopolymerisation activity except the 4-hydroxy model and 2-methyl-4-n-propoxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to polar solvents. This is consistent with close lying mixed triplet states of 3 ππ* and 3n π* character. Both the least active 4-hydroxy and 2-methyl-4-propoxy derivatives exhibit lower triplet energies suggesting the presence of a less active lower triplet 3 ππ* state. Triplet–triplet absorption spectra are obtained with all the thioxanthones in the range 600–680 nm with a marked blue shift from non-polar to polar solvents owing to stabilisation of the lowest triplet state by solvent reorganisation. In photoreductive solvents such as methyl alcohol and 2-propanol a longer lived species is observed absorbing in the region 400–500 nm associated with the formation of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol. The 4-hydroxy derivative gave only weak transient absorption and is consistent with its much lower initiation activity. In the presence of a tertiary amine no ketyl radical is observed. Triplet lifetimes increase with solvent polarity confirming the presence of mixed 3 ππ* and 3n π* states where vibronic coupling influences the rate of intersystem crossing to the ground S 0 state. Bimolecular triplet quenching rate constants indicate all the thioxanthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly with a range of tertiary amines, DABCO, triethylamine and methyldiethanolamine. The lower triplet quenching constants for the 2-methyl-4-n-propoxy and 2-hydroxy derivatives (an order of magnitude) indicates weaker interaction by the amine and is consistent with their lower photoinitiation activities. Low triplet quenching rates are also observed in the presence of monomer (methyl methacrylate). Bimolecular triplet quenching rates are also measured with naphthalene and are similar to those for benzophenone except the 2-methyl-4-n-propoxy and 4-hydroxy, derivatives which are an order of magnitude less. From this data triplet molar extinction coefficients are determined and found to be higher than that for benzophenone owing to the presence of mixed states. Again, the 2-methyl-4-n-propoxy and 4-hydroxy derivatives exhibited lower values as did the 4-benzoyloxy derivative. Relatively high quantum yields of intersystem crossing are observed (<0.6) but are all lower than that of benzophenone with the side chain substituents having no significant effect on the rate. The growth rate of the triplet state could be measured and triplet maxima accurately determined via picosecond flash photolysis. Within the growth time of the triplet state (6–18 ps) there is a concurrent formation of the ketyl radical except for the 4-acetyloxy and 4-acryloxy derivatives. The importance of this technique in gaining valuable information on the relative rates of concurrent excited state reactions for thioxanthones is discussed.

Photophysical properties of the fullerene C 60 core of a 6-arm polystyrene star

The photophysical properties of a new hexa-polystyrene adduct of C 60 were determined. This compound presented the characteristic features of a C 60 with multiple adducts in solution, i.e. a decrease of the triplet quantum yield and a blue shift of the triplet–triplet (T–T) absorption spectrum as compared to unlinked C 60. However the values of the quantum yield of singlet oxygen photosensitization and consequently of the triplet quantum yield were still relatively high, of the order of 0.65. This favorable triplet quantum yield, the high content of fullerene in (PS 1400) 6C 60, and the isotropic 3D structure apt to prevent close contact between fullerene cages might confer on this new compound good optical limiting properties.

High-Pressure Study on the Quenching Mechanism by Oxygen of the Lowest Triplet State in Solution

The mechanism for the quenching by oxygen of the lowest triplet state (T1) of 9-acetylanthracene(ACA) in five saturated hydrocarbon solvents at pressures up to 400 MPa was investigated. The quenching rate constants of the T1 state of ACA, kqT, decrease monotonically with increasing pressure, and the apparent activation volumes for kqT vary in the range 0.8−5.3 cm3/mol. It was also found that the plots of ln kqT against ln η, where η is solvent viscosity, show significant downward curvatures in all the solvents examined. From measurements by the time-resolved thermal lensing at 0.1 MPa, together with measurements of triplet−triplet absorption spectra as a function of pressure, the yields of the T1 state of ACA were found to be approximately unity in the experimental conditions examined. The quenching rate constants, kqS, by oxygen of the lowest excited singlet state (S1) of 9,10-dimethylanthracene (DMEA) whose van der Waals radius is nearly equal to that of ACA decrease strongly with increasing pressure, and the apparent activation volumes for kqS fall in the range of 9.4−14.9 cm3/mol. It was also found that the plots of ln kqS against −ln η are linear, with a slope of 0.59−0.71 depending on solvent. These results of kqS are consistent with our previous conclusion that the oxygen quenching of the S1 state of DMEA is diffusion controlled. The ratio, kqT/kqS, is approximately 1/9 in methylcyclohexane but is less than 1/9 in n-butane, n-pentane, n-hexane, and n-heptane at 0.1 MPa and 25 °C, and the ratio was found to increase over 1/9 with increasing pressure in all the solvents examined. By the bleaching method of DPBF, coupled with time-resolved luminescence measurements, the yields of singlet oxygen (1Δg) formed by the quenching of the T1 state of ACA, ΦΔ, were measured, and the values of ΦΔ were found to be approximately unity. These results were explained by a kinetic model in which the intersystem crossings between encounter complexes with different spin multiplicities are taken into account. From the analysis based on this model, the pressure dependence of kqT/kqS is discussed.

Excited-State Properties of Trichromophoric Dye Molecules

A new time-resolved laser-scanning-microscopy technique is used to measure the excited-state properties of two structural related rhodamine dyes and their 2,5-diphenyloxazole (PPO) derivates. The PPO antennas are separated by methylene bridges from the rhodamine chromophore. An efficient energy transfer from the excited PPO to the rhodamine chromophore is observed. Neither the fluorescence lifetime nor the fluorescence quantum yield of the rhodamine chromophore is affected by the PPO antennas. In contrast, the presence of the PPO antennas significantly influences the intersystem-crossing rate constant as well as the triplet -triplet and excited singlet absorption spectra of the rhodamine chromophore. This observation is clearly in contradiction to the simple model of separated individual chromophores, which is often used to describe multichromophoric dyes.

Proximity effects in the excited state ordering and photophysics of thienyl-pyridyl ketones

The triplet states of the six thienyl-pyridyl ketone isomers were investigated by stationary and time-resolved absorption and emission techniques. Phosphorescence was observed in both fluid solutions and frozen glasses. The intersystem crossing yields are unitary. In a rigid matrix (EPA) at low temperature (77 K), two types of phosphorescence emission were observed, depending on the position of the thienyl group. A short-lived one ( τ≈5 ms) was detected for the 3,n′-TPKs and assigned to n,π* transition; a longer-lived one ( τ≈100 ms) was observed for the 2,n′-TPKs and assigned to π,π* transition. In an EPI matrix at 77 K, the 2,n′-TPKs followed biexponential decay kinetics with a short-lived component ( τ≈5 ms) and a longer-lived component ( τ≈15-40 ms). From the emission excitation spectra, the spin-forbidden S 0→T 1 and S 0→T 2 transitions were detected. Spectra and decay kinetics of the transients produced by nanosecond laser flash photolysis and their reactivity in self-quenching processes indicate that the behavior of the 2,n′-TPKs is substantially different from that of the 3,n′-TPKs. The triplet state properties of these molecules are determined by the balance of the opposite effects of the thienyl (electron-donating) and pyridyl (electron-withdrawing) rings which cause the energies of the lowest n,π* and π,π* states to be very close. Consequently, for the 2,n′-TPKs, an excited state switching occurs going from the singlet (n,π*) to the triplet (π,π*) manifold. Due to the proximity of the lowest triplet states of different configuration, some unusual features were observed, such as room-temperature phosphorescence from molecules having the lowest π,π* triplet states and absorption spectra from T 1 and T 2 simultaneously populated under laser excitation.

Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.

Multistage Exit of Excited Xanthone from Micelles

The spectroscopic changes of xanthone associated with its exit from micelles following laser excitation occur in three distinct stages. These processes are accompanied by changes in kinetic behavior and spectra. The first stage of this spectral evolution is intersystem crossing (ISC), which is followed by spectral changes in the triplet−triplet absorption spectrum that reflect an increasingly polar environment as the probe moves from the micelle interior to the micellar surface and later to the aqueous bulk phase. In the case of sodium dodecyl sulfate micelles, these three processes occur with lifetimes of 55 ps (ISC), 3.2 ns, and 0.6 μs; the last two are assigned to diffusion toward the micelle−water interface and actual exit into the aqueous phase, respectively.

Photochemistry and Photophysics of 1-Azaxanthone in Organic Solvents

The photochemical and photophysical properties of 1-azaxanthone have been studied to determine its usefulness as a probe for polarity, reactivity, and/or radical pair dynamics in supramolecular systems. Contrary to the behavior of xanthone, a structurally related ketone, the triplet−triplet absorption spectrum of 1-azaxanthone does not present polarity-induced shifts in its maximum despite its n,π* character. Self-quenching is a remarkably fast process that occurs at nearly diffusion-controlled rates. Nevertheless, the reactivity of 1-azaxanthone toward hydrogen atom abstraction in polar media is by far superior compared to the reactivities of other aromatic ketones. This property results from the unaltered n,π* triplet character as a consequence of the presence of a pyridine ring and makes it a convenient probe for the study of radical pair reactions and guest/host interactions in highly polar microheterogeneous systems.

Cis−Trans Photoisomerization of the 1,6-Diphenyl-1,3,5-hexatrienes in the Triplet State. The Quantum Chain Mechanism and the Structure of the Triplet State

Fluorenone sensitization in solution interconverts all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) with its trans,cis,trans, cis,trans,trans, and cis,cis,trans (trace) isomers, tct-, ctt-, and cct-DPH, respectively. Photoisomerization quantum yields are reported for the three major isomers in degassed and air-saturated benzene. In degassed solutions the quantum yields are strongly concentration dependent due to quantum chain processes. The presence of air eliminates the quantum chain processes, as all DPH triplets are deactivated by oxygen. Triplet−triplet absorption spectra observed in the microsecond time scale starting from these three DPH isomers are identical. The concentration dependence of the DPH triplet lifetime is consistent with the concentration dependence of the isomerization quantum yields. The results indicate that in benzene DPH triplets exist as an equilibrium mixture of ttt, tct, and ctt isomers whose composition at 20 °C, 94% ttt, 5% tct, and 1% ctt is revealed by isomerization quantu...

Quantum-chemical study of the vibronic structure of the phosphorescence and T1→Tn absorption spectra of naphthalene

The vibronic structure of the phosphorescence spectrum of naphthalene is calculated from ab initio and semiempirical geometries and force-fields of the S0 and T1 states. An excellent agreement with the experiment is obtained. The final triplet state in the triplet→triplet absorption spectrum is identified as 3 3B3g state and the vibronic structure of the 1 3B1u→3 3B3g transition is found in a good agreement with the experimental spectrum. The wavefunction of the 3 3B3g state is dominated by the same electronic configurations as that of the lowest triplet of the same symmetry.

Laser flash photolysis studies on dynamic behaviour of triplet naphthalenophanes sensitized by triplet benzophenone

The formation and decay processes of triplet naphthalenophanes ( 3 NP*) sensitized by triplet benzophenone ( 3 BP*) have been investigated by means of laser photolysis techniques. It was found that the dynamic behaviour of 3 NP* markedly depends on their molecular conformations. For the cases of NP with tetramethylene chains (tNP), triplet energy transfer from 3 BP* occurred to produce triplet tNP ( 3 tNP*). The triplet–triplet (T–T) absorption spectrum of syn-tNP having full-overlapped naphthalene moieties (parallel form) consisted of local (430 nm) and excimer triplet (580 nm) absorption bands. For anti-tNP with partially overlapped naphthalene moieties (antiparallel form), only the local T–T absorption was observed. The efficiency (ϕ TET ) of triplet energy transfer was determined to be unity for syn-tNP while 0.76 for anti-tNP. The rate constants for the quenching of 3 BP* (k q ) and triplet energy transfer (k TET ) were determined. At high BP concentrations, it was shown that the decay processes of 3 tNP* mainly proceeded via a triplet exciplex between 3 tNP* and BP regardless of their molecular structures. For the cases of NP with a cyclobutane ring (cNP, tilted form), after triplet energy transfer from 3 BP*, the bond dissociation of the cyclobutane ring of 3 cNP* subsequently took place to give the corresponding triplet biradicals.

Laser Flash Photolysis Study of the Reaction Mechanism in the Photochromism of 1-(Acyloxy)-2-methoxyanthraquinones

Nanosecond laser flash photolysis has been used to study the primary processes in the photochromic reaction of five O-acylic derivatives of 1-hydroxy-2-methoxyanthraquinone with methyl (I), tolyl (II), phenyl (III), ethoxyl (IV), and diethylamino (V) groups in the migrating acyl. The triplet−triplet absorption spectra of the reactive triplet states of quinones IV and V were detected, and the rate constants of the primary photochemical step were measured. The temperature dependence of the rate constants of acyl migration in the triplet states of quinones IV and V was studied, and the Arrhenius parameters were determined. The rate constants of thermal acyl migration and their Arrhenius parameters were measured for compounds I−V. It was found that migrant nature significantly influences the activation energy of both thermal and photochemical reactions of acyl migration. The activation energy of thermal migration increases from 37.9 ± 0.2 kJ/mol for compound I up to 66.5 ± 0.7 kJ/mol for compound V. The photochemical process is characterized by considerably lower values of the activation energy (15.9 ± 0.8 and 26.0 ± 1.7 kJ/mol for compounds IV and V, respectively). It was confirmed in the present work that photochemical migration of acyl groups is an adiabatic process occurring on the triplet potential energy surface.

Theoretical study of the valence π→π* excited states of polyacenes: Benzene and naphthalene

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence π→π* excited states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence π active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low-lying valence π→π* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet–triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitation energy with an accuracy of about 0.29 eV. The covalent minus states and ionic plus states exhibit different behavior as far as the electron correlation is concerned. The ionic plus states are dominated by the single excitations but covalent minus states include a large fraction of doubly excited configurations. The covalent minus states always give lower energy than the corresponding ionic plus states. This is true for triplet states. The dynamic σ–π polarization effects introduced by perturbation theory are significant for the ionic plus states while those on covalent excited states are usually of the same order as in the covalent ground state. The enlargement of the active space of the reference functions represents a great improvement of the description of the ionic states. The present approach with the pairing properties has proved to be of great value in understanding and predicting the experimental data of the alternant hydrocarbons.