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)
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 »
- 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
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 .
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 . 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 E-type complexes characterized by broad (Dn
» 100 cm-1), structureless excitation spectra and
red shifted ( Dn » 3000 cm-1
) diffuse (Dn » 2000 cm-1 )
exciplex emission with a ~ 300 ns decay time emitted from the A-D+
state. The A*D ~~> A-D+ electron transfer takes place
at the subpicosecond time scale even from the lowest levels of the A*D
the R-type systems show structured, narrow band excitation spectra and
the resonant narrow band fluorescence (t »
20 ns) upon the excitation of low vibrational levels. PET followed by the
exciplex emission is observed only upon the excitation of higher levels.
The onset of PET varies from case to case in the 20 to ~ 300 cm-1
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
(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 . 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.
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