Triplet-triplet Absorption Bands Research Articles

Solvent effects on the structure of the triplet excited state of xanthone: a time-resolved resonance Raman study

Solvent effects, especially intermolecular hydrogen bonding, play a central role in the photophysics and photochemistry of aromatic ketones. To gain insight into the solute–solvent interactions and their implications for structure and reactivity, we studied xanthone (XT) in two different solvents of similar dipolarity: acetonitrile (ACN; aprotic) and methanol (MeOH; protic), using time-resolved resonance Raman (TR3) spectroscopy in conjunction with time-dependent density functional theory calculations. Raman excitation profiles of XT in ACN followed the triplet-triplet absorption band with a shoulder at the blue end, but for MeOH, they followed the triplet-triplet absorption band quite closely; therefore, we propose that the resonance enhancement of Raman peaks are from two states in ACN and from a single state in the MeOH solvent. Furthermore, a resonance Raman peak at 614 cm−1 (a2 symmetry) that appeared in ACN but not in the MeOH solvent has been identified as a vibronic active mode that could be involved in coupling the two lowest 13ππ* (13A1) and 13nπ* (13A2) excited states. This was further confirmed by depolarization ratio measurements of some of the representative TR3 peaks in ACN, which showed a depolarized intensity for the 614 cm−1 peak while the other peaks were polarized. Interestingly, we also observed blue shifting of some of the vibrational frequencies of XT in the 13ππ* state compared with the ground state with increasing solvent polarity. This anomalous blue shift casts doubt on the general use of the resonance canonical structure to explain the structure of the excited states. In summary, we propose that the different hydrogen bonding mechanisms exhibited by the two lowest triplet states of XT separate them further in energy and that this can contribute to its low reactivity towards H atom abstraction in protic solvents. Copyright © 2016 John Wiley & Sons, Ltd.

Properties of the Triplet State of Coumarin Substituted Compounds

The absorption spectra of the triplet excited state of coumarin sensitizers are investigated both theoretically and experimentally. The most intense triplet-triplet (TT) absorption bands are determined. The experimental spectra of the T-T absorption are compared with the theoretical T-T transitions. The phosphorescence spectra of five compounds are measured at a temperature of 77 K. The quantum phosphorescence yield is determined by the method of comparison with an etalon (8-methoxypsoralen). The phosphorescence lifetime is determined for the examined molecules at a temperature of 77 K. For 3,4-phenyl-4',5'-cyclohexylpsoralen, 4'-methyl-3,4-cycloheptylpsoralen, and 4'5'-dimethyl-3,4-cyclohexylpsoralen compounds, this time is equal to 1.1, 1.25, and 2.5 s, respectively. The main energy deactivation channel for all examined compounds is the phosphorescence. The positions of the lower excited triplet states, calculated by the quantum-chemical method of intermediate neglect of differential overlap with spectroscopic parameterization (INDO/S), are confirmed by the available experimental data.

Exciplexes in highly excited triplet states

A mechanism of energy transfer from highly excited triplet aromatic molecules has been developed, which involves a stage of formation of an exciplex between a highly excited energy-donor molecule and an unexcited energy-acceptor molecule. Interpretation of the experimental data on the shape and the intensity of triplet-triplet absorption bands and the energy transfer probability is presented. In this interpretation, the results of quantum-chemical calculations of the energies of highly excited triplet states of toluene and benzene molecules are used.

Triplet Energy Transfers in Electrostatic Host−Guest Assemblies of Unsaturated Organometallic Cluster Cations and Carboxylate-Containing Porphyrin Pigments

The unsaturated cyclic [M3(dppm)3(CO)](2+) clusters (M = Pt, Pd; dppm = Ph2PCH2PPh2; such as PF6(-) salt) exhibit a cavity formed by the six dppm-phenyl groups placed like a picket fence above the unsaturated triangular M3 dicationic center. Electrostatic interactions of the M(3+) units inside this cavity with the carboxylate anion RCO2(-) [R = tetraphenylporphyrinatozinc(II), ZnTPP; p-phenyltritolylporphyrinatozinc(II), ZnTTPP; p-phenyltritolylporphyrinatopalladium(II), PdTTPP] form dyads for through-space triplet energy transfers. The binding constants are on the order of 20,000 M(-1) in all six cases (298 K). The energy diagram built upon absorption and emission spectra at 298 and 77 K places the [Pt3(dppm)3(CO)](2+) and [Pd3(dppm)3(CO)](2+) as triplet energy donors, respectively, with respect to the ZnTPPCO2(-), ZnTTPPCO2(-), and PdTTPPCO2(-) pigments, which act as acceptors. Evidence for energy transfer is provided by the transient absorption spectra at 298 K, where triplet-triplet absorption bands of the metalloporphyrin chromophores are depicted at all time (at 298 K) with total absence of the charge-separated state in the nanosecond to microsecond time scale. Rates for energy transfer (ranging in the 10(4) s(-1) time scale) are extracted from the emission lifetimes of the [Pt3(dppm)3(CO)](2+) donor in the free chromophore and the host-guest assemblies. The emission intensity of [Pd3(dppm)3(CO)](2+) is too weak to measure its spectrum and emission lifetime in the presence of the strongly luminescent metalloporphyrin-containing materials. For the [Pd3(dppm)3(CO)](2+)...metalloporphyrin dyads, evidence for fluorescence and phosphorescence lifetime quenching of the porphyrin chromophore at 298 K is provided. These quenchings, exhibiting rates of 10(4) (triplet) and 10(8) s(-1) (singlet), are attributed to a photoinduced electron transfer from the metalloporphyrin to the cluster due to the low reduction potential.

Photoexcited State Properties of C60 Encapsulated in a Water-Soluble Calixarene

Abstract Properties of the excited states of C60 incorporated into a cationic homooxacalix[3]arene (C60/calixarene) in water have been investigated and compared with bare C60 in benzene. A new transient absorption band which appeared at about 545 nm was attributed to triplet-triplet (T-T) absorption band of C60/calixarene. The λmax of the T-T transition shifts to the shorter wavelength compared with that of bare C60 in benzene and the decay rate increases very much.

Study of the Heterocyclic-Substituted Platinum-1,2-Enedithiolate 3ILCT Excited States by Transient Absorption Spectroscopy

The photoluminescent 1,2-enedithiolate complexes, (dppe)Pt{S2C2(2-quinoxaline)(H)}, [L2Pt{S2C2(2-pyridinium)(H)}]+ where L2 = dppm and dppe, [L2Pt{S2C2(4-pyridinium)(H)}]+, [L2Pt{2C2(N-Methyl-4-pyridinium)(H)}]+ and [L2Pt{S2C2(CH2CH2-N-2-pyridinium)}]+ where L2 = dppm, dppe, and dppp are room temperature dual emitters where the emissions have thiolate to heterocycle π* intraligand charge transfer character (ILCT) singlet and triplet character. The pyridinium complexes have strong triplet-triplet absorption bands at approximately 400, 520 and 630 nm with a weaker band at 800 nm while (dppe)Pt{S2C2(2-quinoxaline)(H)} has strong triplet-triplet absorption bands at 385 and 550 nm with weaker bands at 610 and 805 nm. By fitting the decay of the transients to single exponential kinetics, the 3ILCT* lifetimes of the pyridinium complexes where determined to be 0.7 to 15.9 μs (DMSO) while the 3ILCT* lifetime of (dppe)Pt{S2C2(2-quinoxaline)(H)} was determined to be 2.8 μs (CH3CN). The transient absorption spectra of the complexes is affected by the appended heterocycle rather than the bulk of ancillary phosphine ligand or whether the heterocycle is protonated or alkylated.

Broadening of triplet-triplet absorption band in molecular structures with different spatial dimensionalities

Within the framework of a dipole approximation, the form of the light absorption band for exciton transitions between triplet zones in one-, two-, and three-dimensional periodic molecular structures is calculated. Allowance is made for exciton attenuation as a frequency-independent parameter. The analytical expressions obtained make it possible to analyze the band for molecular structures of different spatial dimensionalities as a function of the difference in bandwidths, attenuation, and lattice temperature.

Radiation chemical investigations on aqueous solutions of C60(OH)18

The ultraviolet-visible absorption spectrum of C60(OH)18 in water showed an absorption band with λmax = 215 nm and other characteristic absorption bands of C60 are not observed. The singlet-singlet and triplet-triplet absorption bands are not observed in the 400–900 nm region. It has low reactivity with e aq − and formed an absorption band with λmax = 580 nm. The hydroxyl radicals react with a bimolecular rate constant of 2.4×109 dm3 mol−1 s−1 and showed an absorption band at 540 nm.

Triplet excitations in sexithiophene films

Using photoinduced absorption and its version of optically detected magnetic resonance, we observe a vibronically split triplet-triplet absorption band in sexithiophene films. The observed vibration frequency is then used to determine the triplet manifold electronic susceptibility and the ground triplet manifold level 1 3 B u

Borohydride anion exchange resin stabilization of flash-lamp-pumped coumarin dye lasers

The operation of flash-lamp-pumped Coumarin 460 and 480 dye laser with ethanol solvent produced acetaldehyde; its production paralleled the increase in the optical density of the dye solution at the dye lasing wavelength and the decrease in the output intensity of the dye laser. The yield of acetaldehyde was shown to be higher with higher light intensity and with the presence of molecular oxygen. Acetaldehyde formation was further enhanced by simultaneous excitation of the singlet-singlet and triplet-triplet absorption bands of Coumarin 460. The formation of acetaldehyde and other material which interferes with stimulated emission is discussed in terms of upper triplet state energy transfer from the dye to the solvent. The concentrations of acetaldehyde and material absorbing at the lasing wavelength of the dye were controlled in real time by introducing a cartridge containing a borohydride anion exchange resin in the dye steam to reduce carbonyl functional groups to less absorbing alcohol functional groups as they were formed photochemically. The presence of the resin increased the initial output power of the dye laser and substantially improved its output stability.

Time-resolved absorption, circular dichroism, and emission of tRNA. Evidence that the photo-cross-linking of 4-thiouridine in tRNA occurs from the triplet state.

The time-resolved optical density (TROD) and time-resolved circular dichroism (TRCD) spectra of the lowest triplet state of 4-thiouridine (4t-Urd) in aqueous solutions of tRNA are reported. The TROD spectrum is consistent with the triplet state being primarily in the thione tautomer. The intersystem crossing yield to the triplet is 0.35 and 0.27 (+/- 10%), respectively, with and without 10(-2) Mg2+ added to the solution. Upon addition of increasing amounts of I- to solutions of tRNA, the initial triplet yield decreases, the rate of the observed triplet decay increases, and the quantum yield of internal photo-cross-linking decreases for the 4t-Urd chromophore. The results show quantitatively that the near-UV-induced photo-cross-linking reaction in tRNA occurs from the triplet state of 4t-Urd. From the TRCD spectrum the dissymmetry factor (delta epsilon/epsilon) of some of the triplet-triplet absorption bands is shown to be significantly larger than for any of the ground-state absorption bands. Two CD transitions are seen in the triplet-triplet spectrum which are obscured in the TROD spectrum by the strong ground-state bleaching signal near 335 nm. This shows that TRCD may be useful, in some cases, in locating electronic transitions that are not observed in TROD spectra.

Conformation-dependent excited state absorptions of 3,3″,5,5″-tetramethyl-para-terphenyl

The singlet-singlet and triplet-triplet excited state absorptions of 3,3″5,5″-tetramethyl-para-terphenyl (TM PTP) have been measured in the spectral range between 420 and 920 nm in two solvent mixtures as a function of the sample temperature between 293 and 93 K. For comparison the excited state absorptions of PTP have been calculated as functions of the twisting angle using a CNDO/S CI program. A level crossing in the S 1 state is computed and a new triplet-triplet absorption band has been measured at low temperatures but only in one of the two solvent mixtures.

Laser dye mixtures

Properties necessary to obtain efficient laser action from laser dyes are reviewed. It is concluded that the dye added (booster dye) to a laser dye (active dye) solution to increase its efficiency need not necessarily be a laser dye. This is illustrated by considering different spectral locations of a triplet-triplet absorption band present in the booster dye relative to the location of the laser action region (part of the fluorescence spectrum) of the active laser dye. Conditions required for booster dyes to form efficient dye mixtures are given. These observations are used to explain why specific well known laser dye mixtures show efficient, little, or no laser action at all.

ChemInform Abstract: THE TRIPLET‐TRIPLET ABSORPTION SPECTRA AND ELECTRONIC STRUCTURES OF SOME CYANO‐SUBSTITUTED BENZENES

Triplet-triplet (T–T) absorption bands were observed at 300, 297, and 320 nm for m- and p-dicyanobenzenes, and 1,3,5-tricyanobenzene, respectively. The nature of the observed spectra was investigated by calculating the wave functions of the triplet states and by analyzing them by the configuration analysis method. It was concluded that the T–T absorption bands of m- and p-dicyanobenzenes, and 1,3,5-tricyanobenzene correspond essentially to the transition from the locally (within the benzene ring) excited state to the intramolecular charge-transfer state.

ChemInform Abstract: THE TRIPLET STATES OF 1,3,5‐TRICYANOBENZENE COMPLEXES WITH METHYL SUBSTITUTED BENZENES

Triplet-triplet absorption, phosphorescence, and ESR spectra have been measured on the 1,3,5-tricyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. Four triplet-triplet absorption bands were commonly observed in the wave number region of 10000–30000 cm−1 for all the complexes. The observed phosphorescence and T-T absorption spectra could be understood systematically by considering the interaction among zero-order charge-transfer triplet states and zero-order locally (within the acceptor) excited triplet states. From an analysis of the zero-field splitting values obtained from the observed ESR spectra, the contribution of the zero-order charge-transfer triplet state to the lowest triplet state was determined to be 76,65,25, and 9% for the 1,3,5-tricyanobenzene complexes with hexamethylbenzene, durene, mesitylene, and toluene, respectively. From the consideration on the results obtained for various polycyanobenzene complexes, the phosphorescence lifetime was found to decrease rem...

The Triplet States of 1,3,5-Tricyanobenzene Complexes with Methyl Substituted Benzenes

Abstract Triplet-triplet absorption, phosphorescence, and ESR spectra have been measured on the 1,3,5-tricyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. Four triplet-triplet absorption bands were commonly observed in the wave number region of 10000–30000 cm−1 for all the complexes. The observed phosphorescence and T-T absorption spectra could be understood systematically by considering the interaction among zero-order charge-transfer triplet states and zero-order locally (within the acceptor) excited triplet states. From an analysis of the zero-field splitting values obtained from the observed ESR spectra, the contribution of the zero-order charge-transfer triplet state to the lowest triplet state was determined to be 76,65,25, and 9% for the 1,3,5-tricyanobenzene complexes with hexamethylbenzene, durene, mesitylene, and toluene, respectively. From the consideration on the results obtained for various polycyanobenzene complexes, the phosphorescence lifetime was found to decrease remarkably for the complexes with the charge-transfer character higher than 50% in their lowest triplet states.

Triplet-triplet absorption spectra of quinoline and isoquinoline

The difference between the singlet-singlet and the triplet-triplet absorption spectra of quinoline and isoquinoline has been measured from 45 000 to 15 000 cm −1, in a rigid hydrocarbon glass at 77 K, using a newly constructed computer controlled spectrograph. Both molecules show strong new triplet-triplet absorption bands in the ultraviolet region. Similar bands have recently been found in naphthalene and were assigned to transitions from the lowest triplet state to a triplet state which is doubly excited with respect to the closed shell ground state.

Stark effects and electric dipole coupling ofnπ* and ππ* states

The coupling of nπ* states amongst themselves or with ππ* states can be induced by static electric fields. The resulting linear or non-linear Stark effects expose hitherto unknown aspects of the properties of the n-orbitals. The matrix elements of the dipole operator are calculated for most of the important two-electron configurations that are expected to describe the low energy electronic states of azines. The case of s-triazine is treated in detail and results are given for pyridine, pyrazine, pyrimidine, pyridazine and s-tetrazine. On the basis of these calculations a number of predictions of measurable effects are made including estimates of the intensities of new infra-red triplet-triplet (and singlet-singlet) absorption bands of azines, and lifetimes of some electronic transitions in the infra-red that may be important in electronic relaxation in isolated molecules.

A study of some singlet triplet electronic states of pyrene

The polarized fluorescence spectra of pyrene pyrene- d 10 as guests in biphenyl and fluorene single crystal matrices at about 10°K are presented. An incomplete vibrational analysis is given in terms of known i.r. and Raman data. The spectrum is complex: As well as the normally allowed Franck-Condon and Herzberg-Teller vibrations, an exceptionally large number of modes of all symmetries appear in the spectra being induced by the crystal forces of the host material. An upper estimate for these interactions is 10 cm −1 probably arises mainly from repulsive contributions. In its first excited electronic state, pyrene is probably not situated at the inversion site of the biphenyl lattice. The mirror-image relationship between fluorescence and absorption, normally evident in the spectra of aromatic molecules, is lost for pyrene, and is probably due to wholesale changes primarily in the off-diagonal elements of the force constant matrix. The first transition is assigned 1B 2 u ← 1 A g and the second 1 B 1 u ← 1 A g , vibrations belonging to the first excited electronic state and having B 3 g total symmetry provide a vibronic interaction with the second electronic state. The more nearly degenerate the interacting states become the more intense is the coupling, A g modes of the first electronic state (built on false origins, the most important being a B 3 g fundamental at about 1564 cm −1 in pyrene- h 10) have about the same activity as their counterparts of the second electronic state in the energy region above the second origin; this activity of the A g , modes is independent of their Franck-Condon overlap with the vibrationless ground electronic state. A measurement of the absolute intensity of absorption indicates that the pyrene molecule causes a local expansion of the fluorene lattice in the ab plane. Triplet-triplet absorption bands at 18,400 23,850 cm −1 were directly measured in the fluorene lattice and were tentatively assigned 3 B 3 g ← 3 B 1 u (short-axis polarized) and 3 A g ← 3 B 1 u (long-axis polarized) respectively. This result agrees with recent photoselection measurements of polarization and calculations.

Triplet-triplet absorption and energy transfer from high triplet states

Abstract A simple method for determining of triplet-triplet extinction coefficients which is based on the non-linear effect in transitions between three levels of Jablonsky diagram has been suggested. Triplet concentrations were obtained from the expression N T st = C 0 (1 - τ r /τ d ), where τ r and τ d are the rise time and decay time of triplet concentrations respectively, C 0 is the ground state concentration of solute molecules. The triplet-triplet extinction coefficients, ϵ T , for 15 aromatic molecules have been measured including carbazole, diphenylamine, fluorene, benzidine, byphenyl, chrysene, coronene and some others. The results obtained have been used for evaluating the solute high triplet level energy escape efficiency β to solvent triplet level (solvent: glassy toluene at 77°K). The absorption of every hundred light quanta in triplet-triplet absorption band of aromatic solute results in 2–18 acts of big portion energy transfer into solvent. The observed differences in β values are tentatively explained in terms of distinct high triplet level lifetimes for various aromatic molecules.