Fluorene Crystals Research Articles

Stimulated emission from anthracene aggregates in fluorene crystals fabricated by cell method

AbstractIn fluorene(80%)–anthracene(20%) mixed crystals, mainly observed luminescence is broad D2 bands, which originates from an excitation state localized on two anthracene molecules and is accompanied by lattice relaxation. Under high intensity excitation, several sharp luminescence lines emerged in the high energy side of the D2 0–1 bands and their intensity increased superlinearly with excitation intensity. The decay time of the luminescence lines is 30 ps, which is two orders of magnitude smaller than decay time under low intensity excitation. These results suggest that sharp luminescence lines are stimulated luminescence from D2 centers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Difference in optical properties of anthracene molecule aggregates in fluorene and phenanthrene crystals

AbstractOptical properties of anthracene molecules in phenanthrene crystals with anthracene concentration of 0.03% to 8.4% were investigated and compared with those in fluorene crystals. In contrast to fluorene system where two kinds of dimers, one of which is accompanied by lattice relaxation, are observed, it was found that only one kind of dimers without lattice relaxation are observed in phenanthrene system. Also in aggregates of anthracene in higher concentration samples, the luminescence is a ‘free exciton’‐type in phenanthrene crystals in contrast to a ‘self‐trapped exciton’‐type in fluorene crystals. With reference to the variation of the magnitude of exciton–phonon coupling constant on nearest neighbour distance of molecules known for bulk crystals of aromatic molecules, the difference is attributed to longer inter molecular distance between anthracene molecules in phenanthrene matrix in comparison to those in fluorene matrix (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical study of anthracene molecules doped in fluorene single crystals

Properties of two luminescence centers, which are observed in anthracene-doped fluorene crystal as concentration increases, have been investigated to clarify their origin. Excitation spectra and concentration dependence of absorption spectra were measured and two types of luminescence are attributed to anthracene molecule pair centers with weak and strong interaction. Dipole–dipole interactions between two anthracence molecules of various configurations are estimated and possible configuration for two types of luminescence is proposed.

Computation of the tunneling H-transfer reaction kinetics in the fluorene molecular crystal

The semi-empirical computation of the rate constant is fully performed for the photochemical bimolecular hydrogen atom transfer reaction in condensed phase, proceeding in the deep tunneling regime. For the system considered (fluorene + acridine in its first triplet state, inside the fluorene molecular crystal) the kinetics have been thoroughly investigated experimentally over wide temperature range (http://bernd-prass.de). In the present theoretical work the potential energy surface is constructed by means of the reported earlier QM/MM approach, covering the interaction of the reaction pair with the crystalline environment. Direct evaluation of two-dimensional tunneling amplitudes on this surface is made by means of the accurate quantum-mechanical numerical method. The kinetic part of the rate calculation invokes Golden Rule methodology in its recently elaborated modification while consistently accounting for the dynamical energy exchange between the tunneling pattern and the medium. The careful calibration of transition amplitudes for H- and D-transfer reactions provided the parameters controlling their exponential dependence on the promotion mode (i.e. the distance between the terminal heavy atoms of the reaction centre), which establishes the effective tunneling length of the transfer process. The experimental temperature dependence of the reaction rates for both cases is well reproduced by tuning the parameters of the phonon frequency spectrum projected on the promotion mode. The fitted values of these spectral density parameters are contained within the range located in the preliminary independent computation of the lattice dynamics for the fluorene crystal. Most difficult is the part of the computation concerned with the absolute values of rate constants for both H- and D-reactions. The magnitude of the isotope effect can be reasonably interpreted at a qualitative level by complementing the amplitude quantum-mechanical data with empirical Frank–Condon (FC) factors inherent to the reorganization of several intra-molecular modes, which is known to accompany the background transfer reaction. However, this treatment, based on the oversimplified one-dimensional model of such reorganization, is not fully consistent, since the pertaining FC parameters were found to differ significantly for the cases of H- and D-transfer.

The role of intermolecular vibrations and reorganization of a reaction system in tunneling reactions with H atom transfer. A Debye model for the medium

Mechanisms of temperature dependence of the rate constants for two types of solid-state tunneling chemical reactions, namely, transfer of an H atom between two molecules and intramolecular transfer, are analyzed. To this end, an analytical expression for the rate constant for tunneling atom transfer in solids is derived in the framework of a modified theory of nonradiative transitions. The mechanisms of the temperature dependence of the rate constant considered in this work include oscillations of the potential barrier to chemical reaction in intermolecular fluctuations and reorganization of the medium. The effect of pressure on the distance between reactants and on the frequency of intermolecular vibrations is taken into account. The theory developed is used to interpret experimental data on tunneling transfer of an H atom in two reactions: a) intramolecular hydrogen transfer in a matrix-isolated formic acid molecule entrapped in an argon crystal and b) H atom transfer from a fluorene molecule to an excited acridine molecule in a fluorene crystal.

Phonon spectrum of a fluorene molecular crystal

The dynamics of the fluorene molecular crystal lattice is calculated by the atom-atom potential method. The fundamental lattice vibration frequencies are determined, their assignment is made, and shapes of the vibrations are studied over a pressure range of from 1 bar to 30 kbar. For acoustic phonon vibrations, the distribution of Debye frequencies over directions of the wave vector is estimated. The results obtained are used to discuss the photochemical reaction of detachment of a hydrogen atom in a fluorene crystal, a process occurring by the tunneling mechanism.

Structure and optical properties of single crystal of fluorene–thiophene oligomers

Solid-state optical properties of polymorph 2,7-(2-thienyl)-9,9-dihexylfluorene [TFT(C6) 2] and 2,7-di(2-thienyl)-9,9-cyclopentanefluorene [TFT] related to their crystal structure are reported. 3D phase of the first molecule displays a very peculiar arrangement, tetragonal system, where adjacent molecules cannot face to each other, due to fourfold alternating axis, therefore preventing close intermolecular interactions. The 3D packing of the second molecule shows a herringbone arrangement, where dimers are alternatively shifted. Both exhibit significant photoluminescence (PL), which changes in position and shape as a function of the solid state organization. Such different arrangements are related to different PL quantum yield values preliminarily determined.

Sensitization of fluorescence of crystalline fluorene by anthracene

Sensitization of fluorene fluorescence by highly fluorescent anthracene has been studied at room temperature. The weak violet fluorescence of fluorene is found to be enhanced by anthracene impurity without any change in spectral energy distribution.The rate of sensitization as a linear function of anthracene impurity is observed in the 10 −3–10 −2 mol per mol fluorene range. The mechanism proposed to explain sensitization consists of an inductive resonant energy transfer between singlet-excited anthracene and ground-state fluorene to produce triplet-excited fluorene. The triplets of fluorene further undergo a biphotonic triplet–triplet annihilation, in which the first excited singlet formed by energetic interaction deactivates radiatively and enhances the fluorescence. From the fluorescence spectra, it is also noted that the mixed crystal of fluorene containing 8 × 10 −1 mol anthracene shows a shift of 1408.4 cm −1 towards red.

Deuterium Effect on the Pressure Coefficient of the Tunneling Rate in the Acridine−Fluorene Solid-State Photoreactive System

We report the pressure effect on the intermolecular deuterium transfer tunneling rate in the acridine-doped fluorene crystal at 77, 150, and 200 K. Similar to the hydrogen transfer, the tunneling rate is exponentially enhanced by pressure. The pressure slope for this exponential enhancement, however, is found to be more temperature dependent for deuterium than for hydrogen tunneling. The ratio of the pressure coefficients for H and D stands at 2.6 at 77 K, gradually decreasing with increasing temperature until it becomes essentially unity at room temperature. An intuitive model based on the mass dependence of the tunneling distance is presented to rationalize these observations.

High pressure studies of the acridine/fluorene photoreaction: Vibration assisted tunneling

We report a multifaceted investigation of the hydrogen transfer photoreaction in acridine-doped fluorene crystals at higher temperature. The purpose is to elucidate the role of vibrationally assisted tunneling in this reaction system. Raman experiments were conducted at various pressures and 77 K to document the change of vibrational frequency for the promoting mode(s). Upon compression, a line with a large pressure coefficient emerges from under the strong phonon mode at 96.5 cm−1. Through polarization studies under pressure, we have identified it as a molecular butterfly mode of B1 symmetry. We have measured the reaction rate at 150 K in order to examine the effect of a suggested promoting mode at ∼440 cm−1. The reaction rate again increases exponentially with pressure, but with a significantly higher pressure coefficient than that at 1.4 and 77 K. Mode patterns based on a recently published [J. Phys. Chem. 98, 12 223 (1994)] normal coordinate analysis of fluorene are used to help establish the promoting modes for this reaction. This consideration suggests that the 95 and 238 cm−1 modes are likely promoting modes in addition to the 125 cm−1 libration. A computation of the Franck–Condon factor for the H-transfer process indicates that a small population of a high overtone of a promoting mode may make a disproportionally large contribution to the reaction rate. This calculation fails to account for the greater pressure coefficient of the reaction rate at higher temperature. Instead, such an increase may come partly from a greater compressibility at higher temperature.

Polarizing proton spins by electron-spin locking of photo-excited triplet state molecules

The transfer of the electron spin polarization from photo-excited triplet state molecules to proton spins has been studied in single crystals of fluorene doped with 2000 ppm acridine in three isotopically different forms. From spin-locking experiments it appears that the proton spins on the acridine guest molecules play a minor role in this process and that the evolution of the nuclear spin polarization is determined by the second moment of the dipolar interaction with the proton spins on neighbouring fluorene molecules. In contrast to recent reports we do not observe fast Rabi oscillations in the polarization transfer process. This is explained by the fact that the magnetic field in our experiments is large, resulting in a weak contact between the nuclear Zeeman reservoir and the electron dipole-dipole reservoir.

Pressure dependence of the low-temperature tunneling rate for the hydrogen transfer in acridine-doped fluorene crystals

Pressure dependence of the tunneling rate for the hydrogen transfer photoreaction in acridine: fluorene was investigated at 1.4 K. Above 10 kbar, the rate rises exponentially with increasing pressure. These data constitute a measurement of tunneling rate from the vibrational ground state. Its physical implications are discussed.

Nuclear magnetic ordening in fluorene

The present paper considers the possibility to obtain a magnetic ordering of the proton spins in molecular crystals of fluorene using dynamic nuclear polarization (DNP) followed by adibatic demagnetization in the rotating frame (ADRF). It is found that such an ordering is most easily reached for T<0 AND B 0 ∥c-axis. Then a polarization of only 26% needs to be reached by DNP in order to obtain a ferromagnetic ordering. The importance of fluorene lies in the possibility to use photoexcited triplet states for DNP. Then, during and after the ADRF the sample is free of paramagnetic centres and a cleaner study of the ordering can be made

Generalized golden rule approach to the H-transfer problem. II. Application to a photochemical reaction

The golden rule approach generalized for arbitrary values of the electronic coupling integral, as reported earlier, is applied to interpret the kinetic data and discuss the driving forces of the fast photochemical H- and D-transfer in acridine-doped fluorene crystal. The excited-state proton transfer (ESPT) is treated as multidimensional tunneling. Both the stretching and bending vibrations of the transferred H atom and the lattice phonons which affect the tunneling distance are taken into account. The relatively large value of the electronic coupling J between the initial and final state corresponding to the fluorene-triplet acridine, and triplet acridinyl-fluorenyl radical pair complex, respectively, is found to be the main reason for the unusually rapid reaction. This conclusion is supported by a direct estimate of J using Slater orbitals.

Optical Nuclear Polarization of Deuterium Spins: Mechanisms and Applications in Solid State NMR

AbstractA gain in detection sensitivity of more than three orders of magnitude has been achieved in high resolution solid state 2H NMR of monocrystalline systems by applying optical nuclear polarization (ONP) via excited triplet states. The method is demonstrated for crystals of fluorene doped with optically active guest molecules which have localized triplet states and for the 1: 1 mixed crystal of anthracene/tetracyanobenzene with excitonic states. The factors determining the efficiency of ONP are discussed and compared to the case of proton spin systems.

Direct estimate of the electronic coupling driving the photochemical proton transfer in acridine-doped fluorene crystal

The electronic coupling integral J of a photochemical hydrogen abstraction in an acridine-doped fluorene crystal was estimated in order to clarify the controversial question of its driving force. The features of the reaction sytem strongly motivate the adoption of the “diabatic” procedure for calculating J instead of its calculation as an adiabatic quantity. A simple model was applied for direct construction of the diabatic states employing a single determinant presentation of the wavefunctions. AO s being Slater orbitals. The present estimates support the previously made conclusion from the analysis of the kinetic data based on a golden-rule approach generalized for arbitrary values of the coupling, that J is mainly responsible for the unusually rapid reaction.

Generalized golden rule treatment of a photochemical solid-state reaction involving hydrogen tunneling

Fast photochemical H- and D-transfer in acridine doped fluorene crystals is treated as multidimensional tunneling. Both the stretching and bending vibrations of the transferred atom and the lattice phonons which affect the tunneling distance are taken into account. The golden rule approach is applied as generalized for arbitrary values of the electronic coupling integral, as reported earlier. Its relatively large value is found to the main reason for the unusually rapid reaction.

Spin coherence effects in the electron—nuclear polarization transfer process

The nuclear spin polarization resulting from optical pumping of molecular triplet states, ONP, has been studied in a time-resolving experiment by synchronized irradiation of light and rf pulses. After laser flash excitation of T 1 triplet states of acridine doped into a fluorene crystal, an rf pulse of variable intensity and duration is applied near the resonance of an electronic spin transition. It leads to partial transfer of optically generated electronic polarization to the nuclear spin reservoir (rf-ONP). For sufficiently high rf-intensity, the polarization transfer shows an oscillatory behaviour when varying the pulse length in the submicrosecond range, which reflects the initial coherence among the spins. Critical tests for the analysis are provided by experiments under different rf excitation conditions and for various isotopic compositions. The transfer process is shown to involve two steps on different time scales, the first of which is closely related to nutations of electron spins about the rotating B 1 field.

ChemInform Abstract: Photochemical Reactions in Doped Fluorene Single Crystals Involving Hydrogen Abstraction in Different Reaction Geometries

ChemInformVolume 19, Issue 21 Preparative Organic Chemistry ChemInform Abstract: Photochemical Reactions in Doped Fluorene Single Crystals Involving Hydrogen Abstraction in Different Reaction Geometries P. STEIDL, P. STEIDL Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorC. VON BORCZYSKOWSKI, C. VON BORCZYSKOWSKI Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorF. FUJARA, F. FUJARA Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorB. PRASS, B. PRASS Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorD. STEHLIK, D. STEHLIK Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this author P. STEIDL, P. STEIDL Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorC. VON BORCZYSKOWSKI, C. VON BORCZYSKOWSKI Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorF. FUJARA, F. FUJARA Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorB. PRASS, B. PRASS Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this authorD. STEHLIK, D. STEHLIK Fachbereich Phys., FU Berlin, D-1000 Berlin 33Search for more papers by this author First published: May 24, 1988 https://doi.org/10.1002/chin.198821084Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume19, Issue21May 24, 1988 RelatedInformation

Inhomogeneous Photochemistry and Hole Burning in Dihydrophenazine Doped Molecular Crystals

Abstract A series of photochemical products of 5,10 dihydrophenazine (PH2) doped in fluorene (Fl) crystals can be observed via optical and ESR spectroscopy upon two photon excitation of PH2. ESR spectra reveal the formation of host-guest triplet radical pairs as a product of a double hydrogen abstraction. The non-planar PH2 ground state leads to a strong site distribution. Photochemical hole burning experiments indicate that this distribution determines to some extent the reaction rate.