rf ONP Studies of Photochemical Hydrogen Abstraction
The Journal of Physical Chemistry, Vol. 83,
No. 26, 1979 3435
Photochemical Hydrogen Abstraction in Doped Fluorene Crystals. Proton Hyperfine Structure via Optical Nuclear Polarization Detected Radio-Frequency-Saturation Spectroscopy H. M. Vleth, V. Macho, and
D. Stehlik”
Freie Universitat Berlin, Fachbereich Physik (FB 20, WE lB), 1 Berlin 33, West Germany (Received August 13, 1979) Publication costs assisted by Freie Universitht Berlin
A new technique is described which uses optical nuclear polarization (ONP) as an observable and permits NMR detection in both the ground and excited triplet states of molecules embedded in a crystalline matrix. Application is made to the characterization of a photochemical hydrogen abstraction product found in fluorene single crystals doped with acridine or anthracene acting as hydrogen acceptors after optical excitation. In particular, hyperfine coupling constants of the transferred hydrogen could be determined. Satisfactory evidence can be given for the local structure of the molecular groups involved .in the solid state hydrogen transfer reaction.
1. Introduction Photochemical radical pair formation by hydrogen transfer within a crystalline lattice has been established by a series of ESR and ONP experiments in fluorene single crystals doped with acridine,lr2 phenazine, and anthracenea3p4The present state of magnetic resonance characterization of a reaction product as a radical pair triplet state has been evaluated in another contribution to this ~ymposium.~ A point of obvious importance is the position of the transferred hydrogen in the reaction product with respect to its starting position in the CH2 group of the fluorene reaction partner. Hydrogen transfer to an anthracene acceptor should result in the formation of a CH2 group in the sp3 configuration as shown in Figure 1 which summarizes the reaction scheme for fluorene as a hydrogen donor and anthracene as an acceptor, while for an azaaromatic group in acceptor molecules like acridine or phenazine the formation of a planar >N-H fragment is expected. Since totally different and specific proton hyperfine tensors are typical for the two types of fragments? investigation of the hyperfine coupling provides a sensitive tool to probe the local molecular structure of the central groups in the reaction product. With the use of partially deuterated molecules, it is possible to resolve much of the relevant hyperfine structure in standard EPR experiments which resulted in the first correct assignment of the reaction product as a radical pair.l However, complete resolution cannot be obtained in some cases of interest. Recourse to standard double resonance techniques like ENDOR were not possible due to the poor signal-to-noise ratio of the EPR lines. Optical detection techniques are also excluded because no emission can be observed at temperatures for which thermal activation is able to promote the reaction (see Figure 1). We have proposed, recently, an alternative method’ which uses another observable in the optical pumping cycle, the optical nuclear polarization (ONP). It does not depend on radiative transitions and can be used at room temperature in this case. The technique permits the detection of NMR transitions within the pumping cycle in both the ground and excited triplet state; the ground state is here the dissociated state of the diamagnetic reaction partners while the radical pair triplet state of the reaction product represents the triplet state. In the present paper we outline the experimental scheme for the ONP-detected radio-frequency-saturation spectroscopy, while a first theoretical treatment of the various effects is given elsewhere.@ The emphasis here will be 0022-3654/79/2083-343550 1.OO/O
placed on the application of the concept to the determination of essential magnetic resonance parameters of the reaction product of the hydrogen transfer reaction.
2. Experimental Section Method and Materials. Figure 2 summarizes the basic mechanism of optical nuclear polarization (ONP) together with the general concept of the experiment. The upper part of the figure gives the optical excitation cycle involving the lowest excited singlet and triplet states of the molecular unit which is investigated. Following light absorption in the singlet states intersystem crossing (ISC) populates and depopulates the triplet state, a process which can be highly selective for the individual triplet spin states due to symmetry selection rules. We consider a single nuclear spin I = 1/2 added to the spin system. The additional energy splitting in the paramagnetic triplet state is dominated by hyperfine coupling while the singlet states are split into la)= (+1/2) and IP) = 1-1/2) due to the generally smaller nuclear Zeeman energy. Nuclear polarization in the ground state is defined by the normalized population difference p = (n, - n p ) / ( n ,+ np). When the nuclear spin states are coupled to the electronic triplet spin states, e.g., by the static hyperfine coupling, they can participate in the selection processes acting on the electronic spin states. As a result the return rates R, and Rp become different giving rise to a nonzero ONP rate, 0 R, - R,. Referring to the general ONP mechanism@one finds it plausible that this difference is proportional to the square of the electron nuclear spin mixing coefficient, Le., proportional to the squared ratio of a hyperfine coupling constant A and the electronic energy separation AE: 0 ( A / A E ) 2 . This simple argument explains that large ONP can be expected in level crossing regions when A E becomes small, and provides an understanding for general ONP by level anticrossing (LAC).@ig As a starting point for the present experiments we have used this ONP in the level crossing region as the carrier signal for the detection of radio-frequency- (rf) induced transitions. It will become obvious, however, that other electron-nuclear coupling schemes, in particular saturation of “forbidden” ESR transitions with simultaneous electron-nuclear spin flips, can provide an additional and more generally applicable alternative for ONP-detected rf-saturation spectroscopy. In the actual experiment, the time sequence of which is shown in the lower part of Figure 2, the optically generated nuclear polarization is detected via the free in-
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@ 1979 American Chemical Society
3436
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
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Flgure 1. Reaction scheme of the radical pair formation by photochemical hydrogen abstraction from fluorene to anthracene. The reaction begins from a photoexcitedstate of the guest anthracene and requires thermal activation. The transferred hydrogen, denoted by H', and the anthracene mesoproton H, form a CH2 group within the dibenzocyclohexadienyl radical.
s.1
s=o
1.112
s. + z 4 1
\
UV-Ab5
-IAIAEP
:Polarization ~ r fSaturation Detection
,
B
I
I
,
i
-t
pk: __.___
p = p,(l-e-'~''l~~
Flgure 2. Optical pumping cycle with the general concept of the ONP mechanism and the time sequence of the experimental procedure including rf saturation.
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duction decay (FID) amplitude, SFID p , in a fixed magnetic field B, = 7 kG corresponding to a fixed frequency 30-MHz pulse spectrometer. Before a pumping cycle is started all nuclear polarization of the sample is destroyed in order to begin with a defined initial condition: p ( 0 ) = 0. After the polarization field Be is set, which is variable in magnitude and orientation with respect to the crystalline axes, the polarization period starts by irradiating the sample for a time t L with the starting time defined as t = 0. Generally, the nuclear polarization is found to be built up exponentially, p = p,(l - e - t L / T I L ) . For short irradiation times, At
