Triplet Exciton Band Research Articles

Raman spectra of terephthalic acid crystals in the temperature range 5 K–300 K

Raman spectra of terephthalic acid crystals were taken in the temperature range 5K–300K. The temperature dependence of the vibrational frequency of the O⋯O hydrogen bond is found to contain information on the mechanism of the proton motion along the bond (hopping and/or tunneling). Onset temperatures of both tunneling and ordering (termination of the hopping) process are determined. Triplet exciton bands observed in the high-wavenumber spectral range also exhibit the relation with proton motions between oxygen atoms. The energy spectra of molecular chains of terephthalic acid, the proton potential energy along the bond, and the probabilities of tunneling as a function of the hydrogen bond lengths are calculated.

Exciton Excitations in a One-Dimensional Band Insulator withHubbard Interactions

We study the properties of exciton excitations in theHubbard model with alternating potential and dimerized hoppingterms at half-filling. With increasing Coulomb repulsion, we findthat a spin triplet exciton band develops below the band gap. At acertain critical interaction, the excitation gap of this excitonband vanishes and a new phase with a dimerized ground stateemerges. The value of this critical Coulomb interaction isdetermined perturbatively.

Optical and ODMR spectroscopy of an induced triplet X-trap in 1,4-dibromobenzene single crystals

Single crystals of p-dibromobenzene (DBB) are doped with perdeuterated p-dichlorobenzene, whose triplet energy is slightly above the triplet exciton band of DBB. This gives rise to the formation of DBB X-traps. Optical and ODMR spectroscopy provide evidence that these X-traps are spatially extended. From coherent ODMR experiments we conclude that these X-traps contain more than two molecules forming an energy sink for triplet excitation. The exchange rate for triplet energy between molecules within the sink is at 1.5 K as low as 3 × 10 3 s −1.

Photophysics of single-crystal [Pt(cis-bis(thiophene-quinoline)) 2]: Optical properties in external magnetic fields

The influences of temperature (2≤ T≤295 K) and homogeneous magnetic fields (0≤ H≤6 T) on the optical absorption and emission spectra of single-crystal [Pt(thq) 2] are reported. Analysis of the spectra shows that the emission at low temperatures originates predominantly from several types of traps located energetically below the lowest triplet exciton band. When a magnetic field is raised from H=0 to 6 T, beside the zero-field emission line of each trap an additional line appears, which is red-shifted by Δ ▪≈5 cm −1. This result is explained by a magnetic-field-induced mixing of the two lowest excited triplet components of each trap.

Spectroscopic studies of cyclo-metallated Pt(lI) complexes: optical absorption and emission and the structure of single crystal [Pt(bpm)(CN) 2]·H 2O (bpm = 2,2′-bipyrimidine)

The polarized optical absorption and emission spectra of single crystal [Pt(bpm)(CN) 2]·H 2O (bpm =2,2′-bipyrimidine) have been measured as functions of temperature (1.9 K⩽ T⩽295 K) and of homogeneous magnetic fields (0 ⩽ H⩽6 T). Analysis of the spectra shows that the inter-complex coupling is weak and that the emission at low temperatures originates mainly from two types of traps located energetically by Δv̄ ∼40 and ∼45 cm −1, respectively, below the triplet exciton band. The spectroscopic properties are discussed on the basis of the crystallographic data for [Pt(bpm)(CN) 2]·H 2O: monoclinic, space group P2 1/ c, a = 7.076(1), b = 8.999(2), c = 18.127(2) )Å, β = 99.84(1)°, V = 1137.29 Å 3, Z = 4, D x = 2.47 g cm −3. Anisotropic refinement (2743 reflexes I>; 2σ( I), 163 variables) resulted in R = 0.066, R w = 0.037, S = 0.78.

Polarized Optical Absorption and Emission of Pt (II)Complexes:Single Crystal [Pt(o-phen)(en)]Cl2 · 2H2O

Abstract The influence of temperature (1.9 ≦ T ≦ 295 K) on the polarized optical absorption and emission spectra of single crystal [Pt(o-phen)(en)]Cl2 · 2H2O is reported. Analysis of the spectra shows that the emission at low temperatures originates from the states of three types of traps located energetically by Δῡ ≲ 5 cm-1, 43 cm-1, and 194 cm-1, respectively, below the triplet exciton band.

Optical excitation of a three-dimensional triplet energy funnel

Triplet excitation spectroscopy at 4.2 K of X-traps induced in para-dibromobenzene (DBB) by para-xylene guest molecules reveals that several DBB host states split off from the triplet exciton band. These states from an energy funnel with a depth of 122 cm -1 for DBB triplet energy transport. The relative trap energies within the funnel suggest a three-dimensional structure.

On the number of effective dimensions of exchange interactions in the triplet state of naphthalene

Recent spectra of the triplet exciton band of naphthalene, due to Doberer and Port (1984), thought to imply effective three-dimensional exchange coupling, are explained within the usual two-dimensional model. This model describes adequately both the substructure of the Davydov bands and their frequency shift, when account is taken of shallow traps present in the crystals.

The dynamical properties of triplet excitations in the 1 : 1 CT crystal anthracene/TCNB at 1·2–4·2 K

We have studied the dynamical properties of triplet excitations in neat A/TCNB upon laser flash excitation in the T 0 ← S 0 triplet exciton band using E.S.E. techniques. The observed signals correspond with various traps amongst which a rapid communication exists at temperatures as low as 1·2 K. Our observations exclude the possibility that the observed E.S.E. signals correspond with pairs of exciton states nor that they can be interpreted as superposition states of non-equivalent molecules.

Triplet exciton in a non-reactive bis-triphenyllead (or tin) diacetylene crystal. A spectroscopic study

The lowest triplet exciton state of crystals formed by ( C 6 H 5) 3− X− C ≡ C− C ≡ C− X−( C 6 H 5) 3 where X is Pb or Sn and an equimolecular amount of CH 2Cl 2 solvent molecules, is studied through the analysis of absorption and of phosphorescence emission and excitation spectra at low temperatures. In the lead compound, the origins of absorption and emission are identical, showing the dominance of free exciton emission and the absence of crystal lattice relaxation in the excited state; the splitting is accounted for by symmetry considerations in the different crystalline phases; the triplet exciton band is several cm -1 wide, although the nearest diacetylene-diacetylene distance is about 10 Å. There is evidence for triplet exciton motion, through the detection of trap (presumably X-trap) emission in the tin compound, and through the analysis of the excitation spectrum lineshape, showing the effect of exciton-exciton interaction in the lead compound.

Triplet-state dimers in isotopically mixed phenazine crystals

High-resolution optical spectroscopy and ESR are applied to the guest triplet (T 1) states in isotopically mixed phenazine crystals at helium temperatures. The T 1 ← S 0 0-0 transitions in P- h g in P- d g and P- d g in P- h g are investigated using laser-excitation spectroscopy and phosphorescence emission spectroscopy. The P- h g guest aggregate spectra at 1.6 K are studied as a function of the guest concentration up to 5%. Due to the high resolution achieved monomers and three dimer lines are spectraly resolved. The quantitative analysis allows a more detailed assignment of the triplet exciton band structure of phenazine. ESR of the guest triplet state of P- h g in P- d g is investigated at helium temperature for guest concentrations between 0.5 and 5%. ESR of the AB guest dimer is observed only at c G = 2% and only at Q-band but not at X-band frequencies. The explanation of the experimental findings is based on the model of the coherent dimer excited state.

Anomalous temperature-dependent phosphorescence of Cu porphin in anthracenea)

The temperature dependence of the phosphorescence spectrum of copper porphin in anthracene has been studied from room temperature down to 8 K and the 4.2 K spectrum has been obtained. A 23.4 cm−1 crystal field splitting of the orbital components of the TQ state was observed. The 0–0 transition appears abruptly at approximately 25 K and increases rapidly in intensity as the temperature is lowered. A model involving energy transfer from the copper porphin to the triplet exciton band of anthracene gives a satisfactory explanation for the anomalous temperature dependence of the copper porphin phosphorescence intensity.

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. High resolution excitation of the two Davydov components of the N- d8 host crystal

Direct excitation spectra of the triplet exciton band of deuterated naphthalene are presented. The high resolution spectra (0.1 cm −1) of the two Davydov components show a substructure that we attribute to the presence in the crystal of natural traps and local strains. A model based on the calculation of the eigenstates of a finite plane of molecules is provided for lineshape simulations. In this work the artificial edge effects introduced by the finite size of the model crystal are eliminated by appropriate boundary conditions. Our method allows us to introduce a random distribution of traps, with variable energies, located inside and outside the exciton band, thus featuring a real crystal. Comparison between calculations and experimental results accounts for the salient effects of the traps on the spectroscopic properties of a mixed crystal.

High-resolution measurements of the triplet t 1 davydov components in naphthalene N- h8 AND N- d8

Absorption lines of the first singlet-triplet 0,0 transition in naphthalene crystals are investigated at 1.6 K using high-resolution excitation spectroscopy with cw dye lasers. The observed substructure of the upper Davydov component is explained quantitatively both for N- h 8 and N- d 8. It originates from impurity Icvels of isotopes in natural abundance, which are not amalgamated in the triplet exciton band.

Triplet states in isotopically mixed anthracene crystals: High resolution optical spectroscopy

The triplet O,O transitions of guest and host in isotopically mixed anthracene crystals of various compositions (A- h 10, 13C-monosubstituted A- h 10, A- d 1 h 9, A- d 2 h g in A- d 10 and A- d 10 in A- h 10) have been investigated using high resolution laser excitation spectroscopy. The guest aggregate spectra have been studied in polarized light as a function of guest concentration up to 15%. The analyses allow us to identify the monomer, dimer and trimer lines. From the dimer splittings the dominant resonance pair interactions are dedu The comparison of different mixed crystal systems with guest levels below and above the host exciton band reveals that quasiresonance and superexchange corrections are of minor importance. The experimental resonance pair interactions are used to calculate the triplet exciton band structure of anthracen and the observed guest polarization behaviour is interpreted quantitatively by the Rashba effect. Finally, the lower Davydov component of the host is s and broadened with increasing guest concentration. The shift is discussed using a theoretical model of Lifshitz.

Phosphorescence- and delayed-fluorescence-detected magnetic resonance at zero-field on mobile and localized triplet states in anthracene

Optically-detected magnetic resonance (ODMR)-experiments at zero field were performed on the lowest triplet state of anthracene single crystals monitoring the delayed fluorescence (DF) as well as the phosphorescence (P) at 1.2 K. At low RF-power levels the P-ODMR and DF-ODMR signals are almost identical and can be attributed to at leat two X-traps with the following ZFS-parameters: D1 = 0.06967(3), |E1| = 0.00793(3), D2 = 0.06940(3) and |E2| = 0.00808(3). At RF-levels exceeding the threshold of the trap signals by about 5 orders of magnitude, DF-ODMR signals from excitons are observed. Their ZFS-parameters show clearly, that the motion is mainly between translationally non-equivalent oriented anthracene molecules [D* = −0.004256(1) cm −1 and E* = 0.03311(1) cm −1]. ODMR-linewidth analysis for optical excitation directly into one of the two Davydov components of the first triplet exciton band indicate, that radiationless interbranch transitions between both components can populate k ≠ 0 states in the lower band with band energies higher than kT. The thermalization time within this band can be estimated to be about 10 −7 s.

Exciton dynamics: direct observation of K → K' scattering in a one-dimensional triplet exciton system by electron spin echo spectroscopy

Direct excitation with a pulsed laser into the O—O band of the T o ← S o absorption of a single crystal of 1,2.4,5-tetra-chlorobenzene creates a large excess population in the K ≈ O region of the triplet exciton band. This non-thermal distribution and subsequent evolution towards equilibrium is measured by spin-echo spectroscopy of one of the triplet exciton zero-field transitions.

Exciton scattering and the dephasing of ESR and optical absorption lines: The 1,2,4,5-tetrachlorobenzene triplet exciton problem

The relationship between exciton scattering induced dephasing of the ESR and the optical transitions of triplet exciton bands is examined by properly including exchange in the line shape formalism. First, a simple heuristic model demonstrates that while the optical dephasing time can be associated with the exciton scattering rate, the ESR dephasing time is more closely associated with the distribution of final states in the scattering process. The full exchange problem is then examined and a formal solution is obtained for the low power steady state ESR spectrum. Application to the 1,2,4,5-tetrachlorobenzene triplet exciton system shows that the exciton ESR spectrum with its associated 1 μsec dephasing time can be reproduced using a 20 psec exciton scattering rate obtained from the optical dephasing time. Exciton scattering occurs to a small region of k space (∼5% of the band) surrounding the initial k state. These results resolve the apparent conflict between the optical and ESR spectra in this system and indicate that exciton transport, while not strictly coherent, is nonetheess predominantly wavelike in character.

The exciton—phonon interaction for triplet exciton bands of organic solids: An experimental and theoretical study

Very high resolution optical data on the temperature dependences of the Davydov component absorption profiles and polarization dependent one-phonon structures associated with the lowest triplet exciton band of anthracene are presented along with a theoretical framework for their interpretation. The interpretation of the one-particle exciton—phonon transitions involving both one-phonon creation and annihilation (cold and hot phonon transitions) is entirely consistent with the analysis of the T-dependent dephasing of the lowest zero-phonon Davydov transition in terms of two mechanisms: delocalized exciton—delocalized phonon scattering operative for the low frequency (< 30 cm −) phonons which undergo no lattice distortion and: Raman like phonon scattering operative for the higher frequency phonons which do. It is the latter which leads to the identical T 2 linewidth dependences of the two Davydov components in the high T limit. The former scattering is dominant at the lowest temperatures. In addition, the, marked and polarization dependent mirror symmetry breakdown between the hot and cold one-particle transitions can be nicely understood in terms of interferences occurring between the Condon and phononic (from the dependence of the pure exciton transition dipoles on phonon coordinate displacement) contributions to the one-particle transition dipoles. It is argued that our findings for anthracene should prove useful for triplet exciton bands in other organic solids.

Interband transitions in molecular crystals

Absorptions from the triplet exciton bands in benzophenone, benzil and benzoin have been observed in crystals excited by 337 nm light from a N2 laser. In benzophenone and benzil, the crystal absorptions are much wider than those of the isolated molecules, showing that considerable mixing between the upper triplet exciton states and singlet and CT states is taking place. The phosphorescence spectrum of benzophenone crystals at high density excitation shows a (0, 0) band which is much weaker relative to the (0, 1) band than at low density excitation : this has been attributed to a high concentration of phonons in the crystal. The high excitation density also enabled the triplet-triplet exciton annihilation rates for benzophenone to be measured : γTT (295 K) = 1 × 10-18 m3 s-1 and γTT (77 K) = 6 × 10-9 m3s-1.