ENERGY TRANSFER IN PHENANTHRENE-DOPED NAPHTHALENE CRYSTALS

Abstract— Measurements at 77°K and room temperature are described on naphthalene crystals containing varying concentrations of phenanthrene. The lowest excited singlet and triplet states of phenanthrene iie between the singlet and triplet exciton bands of the host crystal. Triplet-triplet annihilation in the host was studied by absorbing radiation purely into the guest molecules; the absence of any phosphorescence or delayed naphthalene fluorescence allowed the radiationless processes out of the naphthalene triplet to be discussed. Singlet-singlet energy transfer was studied using radiation absorbed by the naphthalene. Two separate mechanisms of transfer were observed: (A) free exciton motion in the naphthalene band till encounter with a phenanthrene trap, the last step having an activation energy of 65 cm^-1, and (B) defect traps in the naphthalene transferring energy by a resonance mechanism to the phenanthrene; the average depth of the defect traps was calculated at 200 cm^-1.     

Fluorescence spectra of naphthalene-perdeuteronaphthalene mixed crystals - energy transfer via guest excitons

The fluorescence spectra of the mixed crystal system naphthalene in perdeuteronaphthalene at concentrations between 0.1 and 50% have been measured with high spectral resolution. Up to 10% the spectra are superpositions of the spectra of monomers, pairs, trimers and some higher aggregates. The relative intensities of these individual aggregate spectra given evidence for trap to trap energy transfer without thermal excitation into the host exciton band. The monomers as the most abundant traps form a dilute exciton band 50 cm^?1 below the host band. By thermal activation into this dilute band energy is transferred between the aggregate states. In the 50% crystal emission from a mixed guest-host exciton band without individual clusters is observed.     

Electronic energy trapping in a crystal due,to a dopant of higher excitation energy than the host

An excimer emitting crystal (9-cyanoanthracene) doped with a guest molecule (9-methoxyanthracene) having its first singlet level ca. 2000 cm^?1 above the host singlet exciton band exhibits efficient energy trapping as demonstrated by host sensitized, red-shifted emission and hetero-photodimerization. It is considered that the trapping is due to exciplex formation between host and guest molecules.     

ODMR and triplet state kinetics of a carbazolyl substituted diacetylene crystal

Carbazolyl substituted diacetylene (DCH) monomer crystals showing phosphorescence from four traps have been investigated by optically detected magnetic resonance (ODMR) at 1.2 K. These localized triplet states are attributed to the carbazolyl side groups. Their population and depopulation rate constants and zero field splitting parameters (0.097 <|D|< 0.1002 cm^?1; 0.0068 <|E|<0.0105 cm^?1) have been determined. The results suggest that the traps are disturbed substituents. The proposed interaction of the trap states with an exciton band at 23926 cm^?1 is supported by temperature dependent lifetime measurements.     

Fluorescence and energy transfer in polyvinyl carbazole

A model is proposed to explain the kinetics of fluorescence and energy transfer in thin films of polyvinyl carbazole based on the migration of monomer excitons with activator molecules, dimers, and excimer forming sites competing as traps for the exciton energy.     

Vibrational exciton cluster states,percolation and pairwise interactions: Benzene A2u band

Infrared spectra of isotopic mixed benzene crystals (C₆H₆/C₆D₆) were obtained at about boiling nitrogen temperature, with, emphasis on the umbrella mode (at about 700 cm^?1). These are interpreted in terms of guest cluster spectra (monomers, pairs, etc.) up to about 55% mole. Static percolation sets in at about 50%, resulting in an extended exciton band. The line positions, line intensities and the percolation concentration all agree quantitatively with an effective 2-dimensional square lattice exciton interaction topology. This means that very short range, nearest neighbor interchange equivalent, excitation exchange interactions dominate over the long range transition-dipole type terms even in this most intense, dipole allowed IR band. These results fully support our recent picture of the umbrella mode exciton band, in both solid and liquid benzene. Similarities between these solid and liquid exciton bands are pointed out. The existence of pseudolocalized states, in the middle of the energy band of the extended states, is illustrated.     

Photoelectron energy distribution measurements from benzene in rare gas matrices

Photoelectronic energy distributions (PED's) are presented for benzene molecules embedded in solid Ar, Kr and Xe rare gas matrices obtained with monochromatized synchrotron radiation for selective excitation in the range hν = 8 eV to 15 eV. For photon energies in the transparent region of the matrices direct emission from the occupied benzene initial states in the band gap of the matrix is observed. Energy transfer from the matrix exciton states to the benzene guest molecules takes place when the host exciton states are excited. The PED's show that energy of unrelaxed excitons is transferred and that transfer to initial states just at the ionization threshold is favoured.     

Singlet exciton energy transfer in tetracene-doped p-terphenyl single crystals

A picosecond laser system has been employed to study the kinetics of singlet exciton energy transfer in p-terphenyl single crystals doped with tetracene over the temperature range 77–300 K. First-order kinetics prevail and exciton migration from host to guest is controlled by thermal activation over a potential energy barrier.     

Comment on the applicability of Urbach's rule to molecular crystals

The extent to which the absorption profile below the first exciton band can be predicted without recourse to resonance transfer and strong exciton—phonon coupling is evaluated for a crystal transition derived from a moderately intense free molecule transition. It is argued, on the basis of a simple model, that Urbach rule behavior can arise from the same class of weak exciton—phonon interactions that determine hot band absorption shapes.     

The temperature dependence of proton relaxation times in triplet exciton ion radical salts

Proton spin lattice relaxation times have been measured for a variety of single crystal orientations of the (φ₃AsCH₃)⁺(TCNQ)₂^? ion radical salt over the temperature range of approximately 90 to 370°K. It is shown that the relaxation rate is directly proportional to the triplet exciton density where the exciton production energy is significantly dependent on temperature. The data suggests that exciton—exciton exchange is an important aspect in the relaxation mechanism.     

Exciton mobility and trapping in a MALDI matrix

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.     

Triplet exciton lifetime in crystalline pyrene

The triplet exciton lifetime in crystalline pyrene is found to be 140 ± 10 msec at room temperature, over 500 times longer than previously reported values. The temperature dependence of the triplet lifetime has been measured over the range 80–300°K. The rate of bimolecular annihilation of triplet excitons in pyrene is found to be independent of temperature over the range 150–300°K. It is concluded that the transfer of triplet energy within the crystal may be described using the monomer exciton model.     

'Uphill' energy transfer in a photosynthetic bacterium

Abstract— Detectable energy flow in the ‘uphill’ direction was demonstrated by exciting the fluorescence of a shorter wavelength band of Rhodopseudomonas spheroides with light absorbed by a longer wavelength band. A variation in the fluorescence spectrum which depends on the state of the traps was also demonstrated. This information, coupled with fluorescence data obtained with monochromatic light, leads to the conclusion that for the longest wavelength bands the ‘uphill’ transfer is about as fast as transfer to the traps; i.e. the ‘average’ rate constant is on the order of 10⁹ sec. The ratio of the uphill to downhill rate agrees with a theoretical estimate of about 0·5. Some possible ramifications of such uphill flow are discussed.     

Masking of trap effects on exciton bands by exciton-phonon coupling

In a rigid lattice the effect on the exciton band of a trap with a lower transition energy than the host is to push a level down from the band edge to an extent depending on the trap depth and the structure of the band. In a non-rigid lattice, in which the levels of the band have mixed exciton-phonon character, interferences caused by the phonon additions to the eigenstates belonging to the same total wavevector (exciton plus phonon) can, especially for shallow traps, greatly weaken the effect of the traps. The effective trap depth may be reduced to a small fraction of its true value. In such cases shallow traps will not be detectable by their influence on exciton band structure. The effect of deep traps is not changed by this mechanism.     

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-h₁₀, ¹³C-monosubstituted A-h₁₀, A-d₁h₉, A-d₂h_g in A-d₁₀ and A-d₁₀ in A-h₁₀) 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.     

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 T² 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.     

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.     

Quantifying the efficiency of förster‐assisted optical gain in semiconducting polymer blends by excitation wavelength selective amplified spontaneous emission

We study the correlation between Förster resonance energy transfer (FRET) and optical gain properties in conjugated polymer blends based on regioregular poly(3‐hexylthiophene) (P3HT) and poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). First, FRET dynamics are investigated with femtosecond transient absorption spectroscopy observing a sub‐picosecond energy transfer from F8BT to P3HT (550 fs) at medium doping levels (40% wt P3HT in F8BT). Amplified spontaneous emission (ASE) is then characterized in blends upon exciting predominantly the host and guest polymers, respectively. The corresponding density of absorbed photons at threshold conditions is compared upon host or guest photoexcitation as a method to quantitatively determine the FRET‐assisted ASE efficiencies. We observe a reduction in ASE efficiency upon host photoexcitation of 20%, in the best case, respect to guest photoexcitation. Our results confirm that even in strongly coupled host:guest mixtures delayed exciton population by energy transfer is subtle to losses ascribed to exciton–exciton annihilation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2311–2317.     

Inter-sublattice energy transfer in MnF2

The magnetically induced circular emission (MCE) of an exciton—magnon pair from the third nearest neighbour impurity trap [Mgσ(III)] in MnF₂ shows an unusual lineshape. The field dependence of the MCE suggests that this is a consequence of a magnon assisted transfer of excitation between sublattices via the intrinsic exciton band.     

Dynamics of energy transport in ternary molecular solids. II. Time evolution of naphthalene fluorescence

The spectrally resolved time-evolution of free and trapped singlet excitons was obtained at liquid-helium temperature for ternary crystals of perdeuteronaphthalene/naphthalene/betamethylnaphthalene (host/guest/supertrap). The naphthalene guest (donor) concentration varied between 0.30 and 0.99 mole fraction, while the supertrap (acceptor) concentrations were 10^?4–10^?5. At the lower guest concentrations (0.50 and below) the naphthalene-exciton decay time approaches the natural lifetime (≈ 122 ns). At higher concentrations, the decay is much shorter and extremely non-exponential. This behavior is inconsistent with simple homogeneous kinetics schemes that use a time-independent rate constant for energy transport. Above the percolation concentration (0.60 naphthalene) we fitted the experimental results with a random-flight-kinetic model, incorporating correlated random walks on the percolating guest cluster. The best fit was obtained for a “coherence length” (mean free path) of ≈ 10² lattice units. These results are in good agreement with previous steady-state studies on the same samples, and seem to indicate a partial coherence of the exciton transport in both pure and substitutionally disordered crystals at these low temperatures.