Fluorene Crystals Research Articles

Photochemical reactions in doped fluorene single crystals involving hydrogen abstraction in different reaction geometries

Irreversible hydrogen abstraction reactions have been observed in dihydrophenazine doped single crystals of fluorene after photoexcitation of the guest molecules. Additional hydrogen abstraction from one fluorene host molecule in any of several inequivalent nearest-neighbor sites leads to the production of stable phenazinyl–fluorenyl radical pairs in the triplet state. Their electron paramagnetic resonance spectra have been analyzed for the different reaction geometries. There are indications that the abstracted hydrogen atoms remain in the crystal as an integral part of the radical pair geometry.

Identification of the lowest energy nuclear fluctuation modes promoting the photochemical H-transfer tunneling reaction in doped fluorene single crystals

Data with improved accuracy are presented for the low temperature reaction rates k(T) of the H-tunneling reaction between one specific host neighbor molecule in fluorene single crystals and a photoexcited acridine guest. The constant low temperature reaction rate k0 cannot be separated from the precursor intramolecular decay rate. This does not affect the analysis of the temperature dependent reaction rate, which renders the identification of two distinct thermally excited nuclear fluctuation modes promoting the H-transfer reaction. The energies are 125(15) and 440(40) cm−1. They agree well with corresponding lines in the Raman spectra of fluorene crystals. For the first time this result offers the possibility to test quantum mechanical reaction mechanisms without the need of averaging over unspecified distributions of nuclear fluctuation modes.

Photochemically induced hydrogen elimination and hole burning in solid state reactions of doped fluorene crystals

Photochemical products of dihydrophenazine (PH 2) in fluorene single crystals are investigated by optical and ESR spectroscopy. ESR spectra reveal the formation of guest-host triplet radical pairs as products of a double hydrogen elimination, whereas optical spectra elucidate some aspects of the primary multi-step decomposition of PH 2. Photochemical hole burning on PH 2 indicates an inhomogeneous photoreaction since PH 2 molecules differing in electronic conformations react with different reaction rates.

The mechanism of microwave-induced optical nuclear polarization in fluorene doped with phenanthrene: A qualitative analysis

Experiments are presented on microwave-induced optical nuclear polarization (MIONP) using the photoexcited triplet states in a crystal of fluorene doped with phenanthrene. The effects of modulation of the magnetic field and the microwave frequency on the dynamic nuclear polarization rate were studied. The results show that MIONP must be described in this case by the solid effect theory taking into account the burning of holes in the inhomogeneously broadened ESR line.

Production of high, long-lasting, dynamic proton polarization by way of photoexcited triplet states.

In a crystal of fluorene ${\mathrm{C}}_{13}$${\mathrm{H}}_{10}$ doped with phenanthrene ${\mathrm{C}}_{14}$${\mathrm{D}}_{10}$ a proton spin polarization of 42% has been obtained by means of microwave-induced optical nuclear polarization involving the photoexcited triplet state of the guest. This result makes the system a promising candidate for frozen targets and for the study of magnetic ordering of nuclear spins, where the paramagnetic centers used in conventional dynamic nuclear polarization cause great complications.

Room-temperature kinetics of the photoexcited triplet state of acridine in fluorene crystals as obtained from electron spin echo studies

It is shown that electron spin echo results allow an analysis of the complete kinetics of excited triplet spin states at room temperature, not only with respect to all relevant kinetic parameters of the triplet decay and spin relaxation but also the spin selectivity of the triplet population and decay processes.

Kinetics of photochemical hydrogen abstraction in doped fluorene crystals studied by time-resolved optical nuclear polarization

The rate of radical-pair formation in single crystals of fluorene doped with acridine resulting from a photoinduced hydrogen-transfer reaction has been determined between 145 and 300 K from the time development of the electron spin alignment in the radical-pair triplet state. The experimental technique described here is a newly developed pulse version of rf-induced optical nuclear polarization with time resolution to better than 10 −7 s. By comparison with the decay rate of the lowest acridine triplet state, it is shown that this state is the precursor of the photoreaction, in contrast to earlier reports.

Symmetry considerations for solid-state photochemical hydrogen-abstraction reactions. Applications to fluorene doped with acridine and related systems

The photochemical hydrogen abstraction by acridine doped in fluorene single crystals is discussed in particular, taking into account recent experimental evidence from time-resolved spectroscopic methods that the 3(ππ*) state of acridine is the precursor. In this reaction a “vertical” symmetry plane is conserved, in contrast to the “horizontal” plane introduced by Salem in his classification concept for photochemical reactions. The consequences of the presence or absence of the various symmetry planes are discussed, mainly for reactions with 3(nπ*) and 3(ππ*) precursors. It is shown that in the fluorene crystal an adiabatic reaction to radical pair products is allowed for 3(ππ*) acridine but forbidden for 3(nπ*) benzophenone. It is also found that the rate of hydrogen abstraction by 3(ππ*) acridine from fluorene in a single-crystal matrix is comparable to the abstraction rates by the 3(nπ*) states of benzophenone and pyrazine from ethanol in glass matrix. It appears that there are exceptions to the rule often formulated as k nπ* ⪢ k ππ*. Further aspects of the acridine-fluorene reaction are discussed such as the spin selectivity of formation and decay of the radical pair in the triplet state. The non-adiabatic decay rate of the radical pair to ground-state molecules has to be faster than the rate of the adiabatic formation of the radical pair.

Mechanism of the Photochemical hydrogen Transfer Reaction in Doped Fluorene Crystals

Abstract New kinetic data from time resolved ONP-data are presented for the solid state photoreaction of a hydrogen transfer from fluorene to photoexcited acridine in doped fluorene single crystals. They remove existing uncertainties about the reaction pathway which proceeds via the lowest excited triplet state of the acceptor. In contrast to general belief, this 3(ππ*) abstraction shows comparable kinetic parameters to 3(nπ*) abstractions observed in solid glass matrix.

Dynamic nuclear polarization of proton spins via photoexcited triplet states: the system phenanthrene in fluorene

The proton spins in a single crystal of fluorene have been polarized by dynamic nuclear polarization via the photoexcited triplet state of phenanthrene guest molecules. It is possible to reach an enhancement of the proton polarization of 90 at 1.2 K where the thermal equilibrium polarization is 0.033%.

The kinetics of the photochemical hydrogen abstraction in crystals of fluorene doped with acridine

Photoexcitation of acridine when doped in fluorene crystals leads to hydrogen abstraction from the central fluorene CH 2 group and formation of a radical pair product. Both, the radical pair formation and a newly detected high temperature decay channel of the acridine triplet state are thermally activated and exhibit a strong isotope effect upon deuteration of the fluorene donor CH 2 group. The analysis of the existing data shows that the two processes are not directly related and hence the triplet state is not the precursor of the radical pair formation.

Host-Guest Energy Transfer Via Dipole-Dipole Interaction in Doped Fluorene Crystals

Abstract Fluorene mixed crystals have been investigated by sensitized fluorescence measurements in order to prove the importance of dipole-dipole interactions for singlet energy transfer. Fluorene: dibenzofuran (F:DBF) and fluorene:benz[f]indan (F:BI) are used. In these systems the optical transition dipole moments of host and guest molecules are perpendicular and parallel to one another, respectively. For these systems the host-guest energy transfer rate has been determined as a function of the guest concentration from the quantum ratio of guest and host emission (F:BI) and from fluorescence decay time measurements (F:DBF). The latter verified the particularly low transfer rate in F : DBF found in previous steady state investigations. The measured transfer rates at 1.3 K could be explained quantitatively taking into account the relative orientations of the host and guest transition dipoles and the oscillator strengths of the optical transitions. An increase of the transfer rate with increasing temperature between 1.3 K and 100 K in the system F: BI could be related to a change from energy transfer within perturbed states to excitonic band states of the fluorene crystal.

Structural aspects of the mechanism of photochemical radical pair formation in crystalline fluorene containing acridine

In previous work [l] experiments have been reported to demonstrate that, in crystalline fluorene (CH the a&dine triplet state is concluded to be an unlikely precursor of this radical pair. We investigated the structural aspects of the acridine molecules as guests in the fluorene crystal, with acridine simulated to be in either the ground state or excited states, by the method of atom-atom potential energy functions. The results show that in either case only a relatively small distance prevails between the nitrogen atom of a guest molecule and a hydrogen atom of a methylene group in adjacent fluorene host molecules, thus facilitating the hydrogen atom transfer process. The structure of the radical pair is also being investigated.

Variation in the spin-lattice relaxation rate of the photoexcited triplet state of pyrene- d10

The orientation and temperature dependence of the spin-lattice relaxation rate in the photoexcited triplet state of pyrene- d 10 in a fluorene crystal was measured using the saturation method. The orientation dependence supports the hypothesis that the relaxation arises from strong anisotropic librations coupled to large modulations of the dipolar coupling tensor. The variation with temperature of the relaxation rate along the x and z axes are different.

Radical pair formation and decay by photochemical hydrogen transfer in doped fluorene crystals as studied by esr transient nutation

The method of ESR transient nutation has been used to investigate the kinetics of formation and decay of a radical pair triplet state due to photochemical hydrogen transfer. The detailed analysis of the observed transients restrains the possible reaction mechanisms.

Photoreaction in aromatic crystals: EPR studies of anthracene doped fluorene

Light absorption in fluorene single crystals doped with anthracene induces a photoreaction. One hydrogen atom from a fluorene host gets transferred to an anthracene guest molecule over a distance of several angstroms. The resulting triplet state radical pair is investigated by EPR. Fine- and hyperfine structure analyses are given. The photoprocess is similar to that found earlier in fluorene crystals doped with acridine or phenazine. Where the nitrogen lone pair orbitals of acridine or phenazine were suitable acceptors for the transferred hydrogen atoms with anthracene, the hydrogen has to enter a tetrahedral sp3 configuration at the guest molecule. Somewhat higher crystal temperatures are required for comparable intensity EPR-lines of the photoproduct.

Photochemical hydrogen abstraction in doped fluorene crystals. Proton hyperfine structure via optical nuclear polarization detected rf-saturation spectroscopy

Photochemical hydrogen abstraction in doped fluorene crystals. Proton hyperfine structure via optical nuclear polarization detected rf-saturation spectroscopy

EPR-study of a photochemical hydrogen transfer reaction in phenazine doped fluorene crystals

Optical excitation of phenazine doped in a fluorene crystal induces a photochemical hydrogen abstraction reaction in which a hydrogen atom is abstracted from the CH 2-group of a fluorene host and transferred to the aza-group of the phenazine guest molecule. The resulting radical pair product in its triplet state is studied by EPR. Hyperfine structure has been resolved and assigned to the two guest nitrogen spins and the two proton spins originating from the fluorene CH 2-group including the transferred hydrogen. The results allow the characterization of the local molecular structure of the relevant groups both in the reactant as well as the product configuration.

Triplet State Properties of Binary Mixed Crystals of Dibenzofuran and Fluorene

Abstract Phosphorescence spectra and ODMR measurements at 1.3 K on a series of 13 mixed crystals of dibenzofuran (DBF) and fluorene (F) as well as of neat crystals of DBF and F (X-traps) are reported. Drastic changes in the spectra with different F:DBF ratios are observed, demonstrating the possibility of investigations of the electronic properties of F monomers, dimers, trimers and larger aggregates in this mixed crystal system of two chemically different molecules.

Effects of impurity traps on exciton-guest, triplet-triplet annihilation in pyrene-d10 doped single crystal of fluorene

Delayed luminescence spectra of pyrene-d10 doped single crystal of fluorene have been observed. At 245 K the delayed fluorescence intensity shows a second-order dependence on the guest phosphorescence intensity, indicating the bi-molecular origin of delayed fluorescence from the pyrene-d10 molecule. Impurity thermal energy levels are obtained from various Arrhenius plots and the fluorene exciton band is located in the energy level diagram. The decay behaviour of delayed fluorescence is given and the delayed fluorescence life times are obtained for various temperatures. We believe that unknown impurities cause the delayed fluorescence in our crystal system. The kinetics for such guest-trap, triplet-triplet annihilation are discussed.