ELECTRON TRANSFER IN JET COOLED MOLECULAR COMPLEXES

F. Piuzzi1, A. Tramer2, V. Brenner1, and P. Millié1,

1 CEA-Centre d'Etudes Nucléaires de Saclay, DRECAM-SPAM, 91191 Gif sur Yvette Cedex, France (e-mail:Piuzzi@drecam.cea.fr;FAX:(33) 1 69 08 87 07)
2 Laboratoire de Photophysique Moléculaire CNRS, Université de Paris-Sud, 91405 Orsay Cedex, France (e-mail:Tramer@ppm.u.psud.fr;FAX:(33) 1 69 15 67 77)
 

Introduction.

 The photo-induced electron transfer (PET) was studied for jet cooled donor-acceptor complexes AD. We are interested by the dependence of the PET rate on such parameters as :
 - slight modifications of donor properties by « inert » substituents,
 - the configuration of the complex (its isomeric forms),
 -  internal (vibrational) energy and its redistribution among different internal and external modes.
 The electron transfer in AD complexes induced by a selective excitation of the acceptor -A, i.e. by excitation of the locally excited A*D state may be described either (Figure 1):

Figure 1: Representation of ground, locally excited, ionic diabatic states and of adiabatic IM> state (dotted curve).

(i) in the basis of diabatic AD, A*D and A-D+ states as the radiationless A*D ~~> A-D+ transition or
(ii) as the evolution on the single surface of the adiabatic M-state :
 |M(Q)> = a(Q)|A*D> + b(Q)| A-D+ >
from the a>>b to the b>>a region. Its signature is the replacement of the direct (resonant) A*D®AD emission by the strongly red shifted exciplex fluorescence.
     The complexes under study involve anthracene as electron acceptor -A and as donors a series of aniline derivatives : N,N-dimethyl-aniline (DMA), N,N-dimethyl-p-toluidine (DMPT), N,N-dimethyl-m-toluidine (DMMT), N,N-dimethyl-o-toluidine (DMOT) and N,N-diethyl-aniline (DEA).
    Fluorescence spectra and excitation spectra of individual fluorescence components as well as fluorescence decay curves were recorded. The « hole burning » spectra allowed us to assign all spectral features to individual isomeric forms of complexes and the MS-R2PI (mass selected two-photon ionization) spectra confirm the 1:1 stochiometry of all systems.
     On the other hand, the information about potential energy surfaces of AD, A*D and A-D+ states of different complexes (energy minima, saddle points, intersections of A*D and A-D+ surfaces) is deduced from semi-empirical computations based on the « exchange perturbation » theory [1].
     The preliminary results are also reported for AD complexes involving chiral derivatives of both anthracene and aniline.

 Isomeric R- and E-forms of complexes.

By combining the techniques of fluorescence and hole burning (Figure 2) we identified a number (2 to 5) of isomeric forms for five complexes under study [1]. They may be divided into two groups :     This difference may be correlated with the existence and height of the energy barriers between the minima corresponding to A*D and A-D+ electronic configurations on the M-state surface. As a matter of fact, the semi-empirical calculations show that energies of A*D/A-D+ intersection depend strongly on the configuration of isomers and vary in wide limits. In E isomers PET is a direct process while in R isomers PET results from a tunneling across the energy barrier with a rate strongly dependent on the vibrational energy in the initially excited configuration.

 The role of intermolecular vibrational redistribution.

 It is known that in complexes of this size the vibrational energy injected in an external (intermolecular) mode of  n > 70 cm-1 is redistributed with the rate 1011 ³  kIVR ³109 s-1[2,3]. The PET rate depends on the nature of the initially excited mode only in complexes in which the onset of PET is significantly lower than 70 cm-1. For the vibrational energies exceeding the IVR threshold the PET rate is no more mode selective but increases monotonically with Evib. This means that in this energy range kIVR ³ kPET while for low energies PET is the primary process.
 
Figure2:. Characterization of the anthracene-dimethylaniline complex absorption: 
(a) fluorescence excitation spectra  with detection of the resonant fluorescence (375 nm region) with 'hot' bands of free anthracene marked by *,  
(b) hole burning spectra with probe laser fixed on the most intense narrow band and detection of the resonant fluorescence,  
(c) fluorescence excitation spectra with detection of the exciplex fluorescence (450 nm region) and  
(d) hole burning spectra with probe laser fixed on the maximum of the broad exciplex absorption band and detection of the exciplex fluorescence. 

Ionization of excited complexes.

 The E-type complexes are efficiently ionized by two photon absorption. For R-type isomers, the efficiency strongly increases upon the excitation of higher vibrational levels, above the PET onset. One can rationalize this result by assuming two different ionization paths (Figure 3): 
 (i) AD + hn ® A*D,  A*D + hn ® A+D + e 
     ( » ionization of the excited A* molecule) 
(ii) AD + hn ® A*D ® A-D+ , A-D+ + hn ® AD+ + e 
     ( » photo-detachment from the A- ion). 
The first scheme corresponds to the excitation of low levels of the R-isomer, the latter one to the case of high levels of the R-isomer and of the E-type form. We assume that the cross section is significantly larger for photo-detachment than for the photo-ionization process. 
 
 

Figure 3: Schematic representation of the two ionization channels in AD complexes

PET in AD complexes of chiral molecules.

    Because of the strong dependence of PET on stereochemical factors, we can expect important differences between complexes of chiral compounds [4]. In order to remain close to previously studied systems and in cooperation with A. Zehnacker and K. Lebarbu, we choose 2-trifluoro 1-(9-anthryl)ethanol (TFAE) as acceptor and N-methyl, N-ethyl-aniline (EMA) as donor. The spectra of the TFAE-EMA complex were compared to those of TFEA-DMA and TFEA-DEA complexes involving a non-chiral donor and to those with non-chiral acceptor and donor, A-DMA, A-EMA and A-DEA in order to show the influence of chirality on the PET.

1.  a)V. Brenner, P. Millié, F. Piuzzi and A. Tramer, J. Chem. Soc., Faraday Trans., 1997, 93, 3277 b) A. Tramer, V. Brenner, P. Millié, F. Piuzzi, J. Phys. Chem. A (in press April or May 1998)
2.  M. Castella, Ph. Millié, F. Piuzzi, J. Caillet, J. Langlet, P. Claverie and A. Tramer, J. Phys. Chem. 1989, 93, 3941
3.  H. Abe, N. Mikami, M. Ito and Y. Udagawa, Chem. Phys. Letters 1982, 86, 217
4.  K. Le Barbu, V. Brenner, Ph. Millié, F. Lahmani, and A. Zehnacker-Rentien J. Phys. Chem. A 1998,102,128