Nature of the lowest triplet states of 4'-substituted N-phenylphenothiazine derivatives

Article abstract of DOI:10.1039/b005058j

The nature of the lowest triplet state of the donor-acceptor N-phenylphenothiazine derivatives has been studied by means of phosphorescence and EPR spectroscopy in various low temperature glasses. The triplet excitation of N-phenylphenothiazine and N-(p-a

Full text of DOI:10.1039/b005058j

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Nature of the lowest triplet states of 4º-substituted
N-phenylphenothiazine derivatives¤
Pawe¡ Borowicz,a Jerzy Herbich,a Andrzej Kapturkiewicz,*a Romana Anulewicz-Ostrowska,b
Jacek Nowackib and Gunter Gramppc
a Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224,
W arsaw, Poland. E-mail: akaptur=ichf.edu.pl
b Department of Chemistry, W arsaw University, Pasteura 1, 02-093 W arsaw, Poland
c Institute of Physical and T heoretical Chemistry, T echnical University of Graz,
T echnikerstrasse 4-1, A-8019 Graz, Austria
Received 23rd June 2000, Accepted 8th August 2000
First published as an Advance Article on the web 8th September 2000
The nature of the lowest triplet state of the donorÈacceptor N-phenylphenothiazine derivatives has been
studied by means of phosphorescence and EPR spectroscopy in various low temperature glasses. The triplet
excitation of N-phenylphenothiazine and N-(p-anisyl)-phenothiazine is localised within the phenothiazine
subunit. On the contrary, the 77 K phosphorescence of the molecules containing an electron acceptor group
(i.e. ÈCN, ÈCOCH or ÈCOC H at the 4@ position) is similar to that for benzonitrile, acetophenone or
3
6 5
benzophenone, respectively, although the energy levels of their T states are higher than that of phenothiazine.
1
With increasing temperature, however, their phosphorescence becomes similar to that characteristic for
phenothiazine. This Ðnding has been explained in terms of the excited-state intramolecular energy transfer
accompanied by the planarisation of the phenothiazine kernel. The results of crystallographic investigations
support this hypothesis.
and the average energy spacing hl of the vibronic states
1
Introduction
i
coupled to electron transfer) relevant for electron transfer in
The concept of excited-state intramolecular electron transfer
in acceptor (A)Èdonor (D) compounds linked by a single bond
plays a central role in the discussion of their photophysical
properties.1h12 It has been shown previously that in the case
of the AÈD molecules containing large aromatic and/or het-
eroaromatic subunits the primary excited singlet state
1(AÈD)* undergoes a solvent assisted femto/picosecond relax-
ation to a polar charge-transfer (CT) Ñuorescent state 1(A~È
D`). Alternatively, the excitation can lead directly to a 1CT
state. The solvatochromic e†ects on the spectral position and
proÐle of the stationary Ñuorescence spectra proved the CT
character of the emitting singlet states. The nonradiative
depopulation of the excited 1(A~ÈD`) state is usually con-
trolled by two competitive mechanisms. The Ðrst (with the
the Marcus inverted region. Correspondingly, the electronic
coupling elements V can be estimated from analysis of the
solvent polarity e†ects15 on the experimental electronic tran-
sition dipole moment M in terms of the Mulliken16,17 and
Murrel18 models. Using these parameters, k values can be
nr
calculated in terms of a non-adiabatic theory of electron
transfer and compared with experimental ones. This allows us
to verify the theoretical predictions.
A precondition for such a study is to Ðnd a particular AÈD
system with the appropriate energy levels. An acceptable strat-
egy can involve a lowering of the energy of the CT 1,3(A~È
D`) states with respect to those of the locally excited triplet
states 3(AÈD)* by selection of the acceptor and donor sub-
units with appropriate redox potentials. Of course, this strat-
egy assumes that the energy levels of the particular donor or
acceptor chromophore remain unchanged after forming the
AÈD molecule. In the approximation of a weakly interacting
radical ion pair the states 1(A~ÈD`) and 3(A~ÈD`) should be
nearly degenerate. The energy splitting between both 1,3CT
states, however, is usually not equal to zero, e.g., because of
the exchange interactions.
rate constant k ) which corresponds to a direct radiationless
nr
charge recombination in the singlet manifold leads to the
ground state S of the particular AÈD molecule. In the second
0
the lowest excited triplet 3(AÈD)* state, localised typically in
the A or D subunit, is populated by the intersystem crossing
process (ISC with the rate constant k ), most probably via a
isc
CT triplet state 3(A~ÈD`).
In some AÈD systems, with the energies of the 1,3(A~ÈD`)
states lower than those of the 3(AÈD)* states, this deactivation
channel seems not to be operative (Fig. 1). This case is espe-
cially interesting from the theoretical point of view, because it
In fact, we have successfully applied the above mentioned
strategy in such cases as the carbazol-9-yl derivatives of aro-
matic nitriles19,20 and ketones,21 as well as phenoxazine and
phenothiazine derivatives containing benzonitrile as an elec-
allows examination of the relationship between radiative (k )
tron acceptor.22 The experimental and computed k values
f
and nonradiative (k ) rate constants of electron transfer in the
nr
nr
were in satisfactory agreement for *G values lower than the
CT
inverted Marcus region.13,14 CT Ñuorescence band shape
energies of the lowest locally excited triplet states (as taken for
analysis leads to the quantities (the free energy of the CT
the isolated chromophores). This indicates that a particular
AÈD molecule can be treated as a superposition of the donor
and acceptor subunits, only slightly perturbed by their inter-
actions. UVÈVIS absorption spectra (corresponding to the
sum of the donor and acceptor bands with additional CT
process *G , inner j and outer j reorganisation energies
CT
i
o
¤ Electronic Supplementary Information available. See http://
www.rsc.org/suppdata/cp/b0/b005058j/
DOI: 10.1039/b005058j
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Fig. 1 Energy level diagrams for AÈD systems with the energies of the lowest locally excited triplet(s) 3(AÈD)* below (A) and above (B) the
excited intramolecular charge transfer states 1,3(A~ÈD`).
excitation) as well as the results from the phosphorescence
The solvents used for our studies: methylcyclohexane
(MCH), 3-methylpentane (3MP), and n-propanol (PrOH) were
of spectroscopic or Ñuorescence grade (Aldrich or Merck). All
solvents showed no traces of luminescence. The solutions for
phosphorescence and EPR studies were deoxygenated by
saturation with argon.
studies (with spectral features characteristic of the T states of
1
A or D) have supported the above conclusions. Unexpected
results were obtained for the AÈD derivatives of pheno-
thiazine which are N-substituted by acetophenone, benzophe-
none or benzonitrile as an electron acceptor (Fig. 2). The
spectral positions and shapes of their phosphorescence spectra
in various glasses at 77 K are similar to those of the acceptor
chromophores in spite of the fact that their energy levels are
ca. 0.30È0.40 eV higher than that of phenothiazine. Contrary
to that, the phosphorescence of the analogous derivatives of
phenoxazine is similar to the emission from the donor subunit.
Explanation of the above Ðnding is the aim of the present
work.
2.2 Experimental techniques
Absorption spectra were recorded using a Shimadzu UV 2401
PC spectrophotometer. Fluorescence and excitation spectra
(corrected for the spectral sensitivity of the instrument) were
measured by means of an Edinburgh Instruments FS900
steady-state Ñuorimeter. Phosphorescence spectra and life-
times were measured with
a
““Jasny multifunctional
2
Experimental
spectroÑuorimeterÏÏ.25 The phosphorescence decays were
recorded by means of a Tektronix-TDS420A digitising oscillo-
scope. The s2 test and the distribution of residuals were the
main criteria in the evaluation of the quality of the mono-
exponential Ðt of the experimental decay curves.
EPR spectra were recorded with a modiÐed JEOL-3BX
spectrometer operating at the X-band with 100 kHz magnetic
Ðeld modulation. Magnetic Ðeld intensity measurements were
performed with a NMR gaussmeter Drusch NMR-2. A 500 W
high-pressure mercury arc lamp was used for irradiation of
the sample within the cavity of the spectrometer.
The X-ray measurements of single crystals of PCS were
carried out on a KM-4 KUMA di†ractometer with graphite
monochromated CuKa radiation. The data were collected for
Lorentz and polarisation e†ects and absorption correction
was used.26 The structure was solved by a direct method27
and reÐned using SHELXL.28 The non-hydrogen atoms were
reÐned anisotropically, whereas the hydrogen atoms were
2.1 Materials
4-Phenothiazin-10-yl-anisole (APS), N-phenylphenothiazine
(PPS), (4-phenothiazin-10-yl-phenyl)-phenylmethanone (PBS),
1-(4-phenothiazin-10-yl-phenyl)-ethanone (PAS), and 4-
phenothiazin-10-yl-benzonitrile (PCS) were prepared by
Ullmann arylation23 of phenothiazine with an appropriate
aryl bromide. The reagents in 1 : 1 molar ratio were reÑuxed
for 24 h in pyridine in the presence of potassium carbonate
and copper bronze. The crude products, obtained after distil-
lation of the pyridine solvent, were puriÐed on a column of
alumina or silicagel with hexaneÈmethylene chloride (1 : 1) or
methylene chloride as eluent and recrystallised from ethanol.
Satisfactory results of NMR and elementary analyses were
obtained for all the studied compounds (with the melting
points in agreement with the literature data24).
Fig. 2 Formulae of the phenothiazine derivatives studied and their abbreviations used in the text.
Phys. Chem. Chem. Phys., 2000, 2, 4275È4280
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placed in the calculated positions, and their thermal param-
eters were reÐned isotropically. The atomic scattering factors
were taken from International Tables.29 Details of the X-ray
measurements and crystal data for the studied 4-phenothiazin-
10-yl-benzonitrile (PCS) are given in Table 1. Crystallographic
data are available in .cif format as Electronic Supplementary
Information (CCDC ref. no. 1326/11).¤
(3.6 ] 103 M~1 cm~1) or APS. The opposite trend is observed
for the second band. This Ðnding is most probably related to
the changes in the conformation of phenothiazine (vide infra)
induced by the strong electron acceptor substituent at the N10
nitrogen atom. The e†ect of the lowest transitions in the
acceptor moieties32,33 (with relatively low intensities) is mani-
fested only by an increase in the width and intensity of the
Ðrst and second absorption bands of the AÈD molecules with
respect to those of PPS. Moreover, the AÈD phenothiazine
derivatives (PAS, PBS and PCS) showed a long-wave shoul-
der of relatively low intensity which has been attributed to a
CT transition.22
3
Results and discussion
3.1 UV–VIS absorption and Ñuorescence spectra
All the studied compounds emit a single Ñuorescence band
at room temperature.22 The spectral position of the emission
maximum of PPS and APS does not depend on solvent
polarity. Contrary to that, the energies corresponding to the
spectral position of the Ñuorescence maxima of PAS, PBS and
PCS are correlated (in the given solvent) with the di†erence in
the standard redox potentials Eo (D) [ Eo (A) (corresponding
to the oxidation of the donor and the reduction of the accep-
tor subunits, respectively). This Ðnding, as well as a consider-
able red shift of the emission with a parallel increase of the
Stokes shift and of the emission bandwidth with increasing
solvent polarity, points to the distinct CT character of the
Ñuorescent states.
Room temperature absorption spectra of the studied com-
pounds in MCH are presented in Fig. 3. The near-UV spectra
show two absorption regions centred around 29 750È31 550
cm~1 and 39 000È40 000 cm~1. Both bands are related to the
electronic transitions localised mainly in the phenothiazine
subunit. The molar absorption coefficients of the Ðrst band are
similar for PCS, PBS and PAS (being 1.4 ] 104, 1.3 ] 104 and
1.7 ] 104 M~1 cm~1, respectively) but markedly higher than
that of phenothiazine30 (4.0 ] 103 M~1 cm~1) and PPS31
ox
red
An analysis of the CT Ñuorescence (performed in our pre-
vious work22 for PAS, PBS and PCS) leads to the quantities
(i.e. *G , j , j and hl ) relevant to radiative electron transfer
CT
o
i
i
in the Marcus inverted region. The derived data allowed us to
discuss quantitatively the relationship between *G and the
CT
solvent polarity. The increase in the solvent polarity (and, cor-
respondingly, the decrease in the CT 1,3(A~ÈD`) states
energy) should lead to changes in the electronic structure of
the lowest triplet states in a particular AÈD molecule. It arises
from the interactions between the 3(A~ÈD`) state and the
locally excited 3(AÈD)* triplet states resulting from the elec-
tronic transitions localised either in the acceptor or donor
subunits. In the polar solvent the Ñuorescent 1(A~ÈD`) states
are lying close to (or below) the lowest excited triplet states of
phenothiazine34 (2.62 eV). Consequently, in a highly polar
environment, the 3(A~ÈD`) states seem to be localised below
the 3(AÈD)* state for the studied AÈD systems. On the other
hand, the opposite sequence was observed in the non-polar
media, the electronic interactions destabilise the 3(AÈD`)
states because these states are located above the locally
excited triplet states.
Fig. 3 Room-temperature absorption spectra of APS, PPS, PBS,
PCS and PAS in MCH. Low-energy parts of the absorption spectra
are expanded by a factor of 5 (for APS and PPS) or 25 (for PBS, PCS
and PAS). The spectra are respectively shifted along the y-axis (by
2 ] 104 M~1 cm~1 or 4 ] 104 M~1 cm~1).
3.2 Phosphorescence spectra
The results of the phosphorescence studies, which indicate the
dominant 3(AÈD)* character of the phosphorescent state,
Table 1 Crystal data and structure reÐnement for 4-phenothiazin-10-yl-benzonitrile (PCS)
Parameter
Value
Chemical formula
Formula weight
Crystal system
C
300.37
Triclinic
H
N S
19 12
2
Unit-cell dimensions
a \ 10.437(2) A; a \ 91.22(3)¡
b \ 12.707(3) A; b \ 108.34(3)¡
c \ 12.841(3) A c \ 111.03(3)¡
Unit-cell volume
Temperature of data collection
Space group symbol
Number of formula units in unit-cell
Linear absorption coefÐcient
Measured reÑections
1491.4(6) AŽ 3
293(2) K
P-1
4
0.214 mm~1
5723
Independent reÑections
R indices (all data)
Final R indices [I [ 2p(I)]
5409, R \ 0.0265
int
0.1759, wR(F2) \ 0.1828
0.0471, wR(F2) \ 0.1225
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agree well with the above hypothesis. The spectral position
and shape of the PPS and APS phosphorescence spectra are
very similar to that obtained for phenothiazine. Somewhat
surprisingly, however, the phosphorescence spectra of PAS,
PBS and PCS recorded at 77 K can be hardly attributed to
the triplet states localised in the phenothiazine subunit. The
energies of the lowest (most probably 0,0) bands are distinctly
higher than that of PPS or APS (cf. Fig. 4). The observed
shapes and spectral positions of phosphorescence suggest that
the excitation is mainly localised in the acceptor subunits of
the studied AÈD compounds. The observed shifts of the phos-
phorescence maxima to lower energies (ca. 2000È3000 cm~1)
as compared to the unsubstituted acceptors chromophores are
similar to those reported35 for the corresponding 4@-amino
derivatives. These Ðndings indicate that the sequence of the
locally excited 3(AÈD)* states in PAS, PBS and PCS in rigid
media is di†erent to that in PPS and APS. Similarly, to the
e†ects observed in UVÈVIS absorption spectra, this may
probably be related to the changes in the conformation of the
phenothiazine moiety (described below), induced by the strong
electron acceptor subunit connected to the N10 nitrogen
atom.
phorescence at temperatures higher than 77 K. This was per-
formed in 3MP glass. Fig. 5 presents the most representative
results for PCS, but nearly the same behaviour was observed
for PBS and PAS. With increase in temperature the emission
intensity related to the acceptor part of the particular AÈD
molecule decreases, and a new phosphorescence band appears,
with spectral position and shape characteristic for pheno-
thiazine. Most probably, it is accompanied by the change of
molecular conformation which follows photoexcitation. Con-
trary to the situation at 77 K, the softening of 3MP glass at
higher temperatures allows molecular rearrangement. Taking
into account the fact that it corresponds to changes in the
energetic sequence of the 3(AÈD)* states (i.e. the lowering of
the energy level of the triplet state attributed to the donor
subunit) a Ñattening of phenothiazine may be postulated. The
process corresponds to intramolecular energy transfer accom-
panied by changes in the molecular structure.
3.3 EPR investigation
EPR spectra of the studied compounds were measured in
frozen PrOH (Fig. 6). Relatively long lifetimes of the phos-
phorescent triplet states of PPS and PCS allow us to observe
the signals under photostationary conditions. In our investiga-
tions we have recorded the transitions in the *m \ 2 region
The postulated nature of the T states in rigid media is sup-
1
ported by the results of the phosphorescence lifetime q mea-
pho
surements in PrOH glass at 77 K. The value of q is found to
pho
corresponding to the lowest resonance Ðelds B . The zero-
be D90 ms for both PPS and APS, in agreement with the
min
Ðeld splitting (ZFS) parameters D*, characterising the particu-
literature data (q ^ 80 ms) for phenothiazine.36 The phos-
pho
lar, randomly oriented, triplet state, were calculated from the
expression:40
phorescence lifetimes of PAS and PBS (e.g., 0.44 and 0.12 s in
PrOH at 77 K, respectively) are distinctly longer than those of
the parent carbonyl compounds (acetophenone or
benzophenone) corresponding to the lowest 3(n, p*) states,35
but similar to those found for 4@-amino-acetophenone (0.72
s)37 or 4@-amino-benzophenone (0.20 s).38 These results may
be explained by the change in the phosphorescent state char-
acter from 3(n, p*) to 3(p, p*), induced by the substitution of
an electron-donating group at the 4@-position. The long phos-
phorescence lifetime for PCS (0.96 s) points to a 3(p, p*) char-
D* \ J(D2 ] 3E2) \ J3(X2 ] Y 2 ] Z2)
2
\ J3(hl)2 ] 3(gk B )2
(1)
4
B min
where D and E are ZFS parameters related to the principal
values X, Y and Z of the spinÈspin coupling tensor; hl is a
resonance microwave energy, and k denotes the Bohr magne-
B
ton. The isotropic value of g \ 2.0023 has been assumed in
calculations.
acter of the T state localised mainly on the benzonitrile
The value of the parameter D* \ 0.125 cm~1 for PPS at 77
K is (within experimental error) the same as that reported for
phenothiazine.41,42 On the contrary, the high-Ðeld position of
the dominant EPR band for PCS at 77 K results in a mark-
edly smaller value of D* \ 0.102 cm~1. This value is distinctly
di†erent from that reported for benzonitrile43 (D* \ 0.139
cm~1) but agrees very well with those obtained for the series
of p-amino derivatives of benzonitrile39 (D* \ 0.097È0.107
1
subunit.39
In order to gain more insight into the nature of the lowest
triplet states in AÈD molecules we attempted to observe phos-
cm~1). This indicates that the spin density in the lowest T
1
state is mainly distributed over the p-system of the benzonitri-
le subunit. However, an additional weak, low-Ðeld, EPR
signal corresponds to the D* parameter characteristic for
phenothiazine. Most probably, two di†erent types of mol-
Fig. 4 Phosphorescence spectra (in PrOH glass at 77 K) of the
studied phenothiazine derivatives.
Fig. 5 Temperature e†ect on the phosphorescence spectra of 4-
phenothiazin-10-yl-benzonitrile (PCS) in 3MP glass.
4278
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data for phenothiazine,44 the molecule is folded about the
SÈN axis with a dihedral angle of 158.5¡. PPS45 is a pheno-
thiazine derivative with two unique molecules in the asym-
metric unit. Introduction of an additional phenyl ring does
not cause, however, any essential changes in the phenothiazine
kernel, the corresponding dihedral angles are 162.6¡ and
150.7¡, respectively. The plane of the phenyl ring nearly bisects
the benzo planes in each of the two molecules, with the corre-
sponding angles of 78.4¡ and 84.6¡, respectively. Due to the
similar geometry of PPS and phenothiazine, the similar
behaviour of both molecules seems reasonable.
Introduction of a ÈCN group at the 4@-position leads to
intrinsic changes. Similarly, as was found for PPS,45 the crys-
tallographic data indicate the presence of two, somewhat dif-
ferent, molecular conformations. The phenothiazine subunit is
more folded about the SÈN axis with the corresponding dihe-
dral angles of 135.5¡ and 131.5¡. The 4@-cyanophenyl ring is
nearly perpendicular to the plane bisecting the folding angle
of the phenothiazine ring (cf. Fig. 7). Similar changes caused
by the presence of the acceptor group were previously report-
ed for 4@-nitro-10-phenylphenothiazine46 and 10-(2@-pyrazyl)-
phenothiazine.47 It should be noted that 4@-bromophenyl-10-
phenothiazine,48 i.e. the N-phenylphenothiazine derivative
with the weaker acceptor group at the 4@-position is less folded
around the SÈN axis (corresponding dihedral angles are 150.8¡
and 144.9¡).
The central ring of phenothiazine reveals an approximate
boat conformation, depending on the acceptor affinity of the
N10 substituent. Further, the butterÑy structure of the central
ring prevents the overlap of the non-bonding orbital of sulfur
with the pÈsystem. This leads to a more or less pronounced
aromatic character of the phenothiazine kernel, accompanied
by changes of the N10 atom hybridisation from nearly sp3 to
nearly sp2. Such conformation allows for maximum overlap
and, correspondingly, for the electronic interactions between
the lone pair of the phenothiazine N10 atom with p-electrons
of the acceptor benzene ring. These interactions between the
nitrogen N10 lone pair and 4@-substituted phenyl ring seem to
be mostly responsible for the observed structural changes.
Fig. 6 EPR spectra of the triplet state of N-phenylphenothiazine
(PPS) and 4-phenothiazin-10-yl-benzonitrile (PCS) recorded in frozen
PrOH. The spectrum of PCS recorded at 77 K shows two di†erent
signals, corresponding either to the phenothiazine p-system (low-Ðeld
signal) or to the p-amino substituted benzonitrile subunit (high-Ðeld
signal).
ecules with the triplet excitation localised either within the
acceptor or the donor p-system (as postulated above in the
discussion of the phosphorescence data) takes place already at
77 K (probably the small number of the latter molecules is
insufficient to allow luminescence conÐrmation). At higher
temperatures (e.g., 127 K, just below the melting point of
PrOH) only one EPR signal (related to phenothiazine) was
found for PCS, in nice agreement with the results of the phos-
phorescence investigations. It is expected that, at the interme-
diate temperatures, the relative intensities of both EPR signals
(high-Ðeld and low-Ðeld, respectively) will change similarly to
those of the phosphorescence (cf. Fig. 4). Unfortunately, our
EPR instrumentation does not allow for such kind of mea-
surements.
4
Concluding remarks
The observed phosphorescence behaviour of the AÈD mol-
ecules under study may be understood assuming the changes
in the geometry of the phenothiazine subunit which follow
electronic excitation. Flattening of the donor part of the mol-
ecule increases its aromatic character, leading, in consequence,
to lowering of the energy level of the excited triplet state local-
ised within the donor subunit. This process, which is not pos-
sible in the rigid phase, may be observed in a more soft
environment or in Ñuid solvents. Similar changes in molecular
conformation have been postulated to explain the e†ects
related to photoinduced intramolecular electron transfer in
AÈD phenothiazine derivatives.22 The geometrical rearrange-
ment most probably corresponds to rehybridisation of the
N10 atom to more ““pureÏÏ sp2 hybridisation, as was found for
the oxidation of the phenothiazine derivatives to the corre-
sponding radical cations.49,50
3.4 Crystallographic structures
The e†ects observed in the UVÈVIS absorption and the low-
temperature emission as well as in the EPR spectra of the
AÈD compounds can be explained by geometrical changes in
the phenothiazine subunit. According to the crystallographic
The Ñattening process of folded molecules of the pheno-
thiazine derivatives may be described as a change of the
valence angles of the central atoms S and N. The energy
needed to change the two angles is only ca. 0.10È0.12 eV.51
This is distinctly smaller than the amount of energy released
in the postulated intramolecular energy transfer accompanied
by the changes in the molecular structure (0.30È0.40 eV). Con-
sequently, the reaction is strongly favoured thermodynami-
cally and the observed kinetic inhibition at 77 K may be
clearly attributed to the rigidity of the reaction medium. In
Ñuid solvents the lowest locally excited triplet states of PAS,
PBS and PCS seem to be localised within the phenothiazine
Fig. 7 ORTEP schemes of N-phenylphenothiazine (PPS) and 4-
phenothiazin-10-yl-benzonitrile (PCS) molecules. ORTEP plot for
PPS is drawn using literature crystallographic data.45
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20 A. Kapturkiewicz, J. Herbich, J. Karpiuk and J. Nowacki, J.
Phys. Chem. A, 1997, 101, 2332.
subunit, in agreement with the kinetic data for intramolecular
electron transfer in these AÈD molecules.
21 A. Kapturkiewicz and J. Nowacki, J. Phys. Chem. A, 1999, 103,
8145.
22 P. Borowicz, J. Herbich, A.Kapturkiewicz, M. Opallo and J.
Nowacki, Chem. Phys., 1999, 249, 49.
5
Acknowledgements
This work was sponsored by the Grant 3T09A06214 from the
Committee of ScientiÐc Research. A.K. wishes to thank the
Austrian Academic Exchange Service (OAAD) for Ðnancial
support to visit the Technical University of Graz. Special
thanks are also due to the Foundation for Polish Science
(““FastkinÏÏ program) for donation of the Edinburgh Instru-
ments FS900 and FL900 spectrometers. The technical assist-
ance of Mrs. A. Zielinska and B. Osinska is highly
appreciated.
23 M. A. Khan and J. B. Polya, J. Chem. Soc. C, 1970, 5.
24 D. Clarke, B. C. Gilbert and P. Hanson, J. Chem. Soc., Perkin
T rans. 2, 1976, 114.
25 J. Jasny, J. L umin., 1978, 17, 143.
26 J. Walker and J. Stuart, Acta Crystallogr. Sect. A, 1983, 39, 158.
27 G. M. Sheldrick, Acta Crystallogr. Sect. A, 1990, 46, 467.
28 G. M. Sheldrick, SHEL XL 93 Program for ReÐnement of Crystal
Structures, University of Gottingen, Gottingen, 1993.
29 International T ables for X-ray Crystallography, Kynoch Press,
Birmingham, 1974, vol. IV.
30 T. J. Kemp, P. Moore and G. R. Quick, J. Chem. Soc., Perkin
T rans. 2, 1980, 291.
31 K. Yasukouchi, I. Taniguchi, H. Yamaguchi, J. Ayukava, K.
Ohtsuka and Y. Tsuruta, J. Org. Chem., 1981, 8, 1679.
32 DMS UV Atlas of Organic Compounds, Verlag Chemie, Wein-
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