Photoinduced intramolecular charge transfer in diphenylamino-substituted triphenylbenzene, biphenyl, and fluorene

Article abstract of DOI:10.1021/jp970666o

The photophysical properties of a diphenyl amino-substituted triphenylbenzene, (pEFTP), biphenyl (pEFBP), and fluorene (pEFF) derivative are compared. The similarity of the photophysical properties and their solvent dependence for the triphenylbenzene and the biphenyl model compound indicate the formation of a polar excited state, localized in one branch of pEFTP. Further comparison with the fluorene model compound suggests an excited state relaxation process of the biphenyl moiety in solvents of medium and high polarity toward a more planar geometry. The depopulation of the excited state is explained in the framework of the model developed to rationalize the photophysics of substituted biphenyl.

Full text of DOI:10.1021/jp970666o

J. Phys. Chem. A 1997, 101, 8157-8165
8157
ARTICLES
Photoinduced Intramolecular Charge Transfer in Diphenylamino-Substituted
Triphenylbenzene, Biphenyl, and Fluorene
Wouter Verbouwe, Lucien Viaene, Mark Van der Auweraer,* and Frans C. De Schryver*
Laboratory for Molecular Dynamics and Spectroscopy, K.U. LeuVen, Celestijnenlaan 200F,
3001 HeVerlee, Belgium
H. Masuhara
Faculty of Engineering, Osaka UniVersity Suita, Osaka 565, Japan
R. Pansu and J. Faure
Department of Chemistry, Ecole Normale Supe´rieure, 61 AV. du Pre´sident Wilson,
F-94235 Cachan Ce´dex, France
ReceiVed: February 24, 1997; In Final Form: July 10, 1997X
The photophysical properties of a diphenyl amino-substituted triphenylbenzene, (pEFTP), biphenyl (pEFBP),
and fluorene (pEFF) derivative are compared. The similarity of the photophysical properties and their solvent
dependence for the triphenylbenzene and the biphenyl model compound indicate the formation of a polar
excited state, localized in one branch of pEFTP. Further comparison with the fluorene model compound
suggests an excited state relaxation process of the biphenyl moiety in solvents of medium and high polarity
toward a more planar geometry. The depopulation of the excited state is explained in the framework of the
model developed to rationalize the photophysics of substituted biphenyl.
Introduction
excited into twisted Franck-Condon conformations of the S1
state. However, this state possesses a minimum for a more
Although symmetric donor-acceptor substituted chro-
mophores are expected to emit from a non polar excited state,
it has been observed that photoinduced charge transfer occurs
in the excited state of compounds with 2-fold (bianthryl1,2 and
biperylenyl3) and 3-fold (amino-substituted triphenylphosphines4
and ruthenium(II) tris(bipyridine) complexes5-7) symmetry.
Amino-substituted triphenylbenzene derivatives, which were
developed as good hole transporting materials8,9 emit from a
highly polar excited state despite their 3-fold symmetry. The
nature of the excited state of several amino-substituted tri-
phenylbenzene derivatives has been discussed in the framework
of ICT (intramolecular charge transfer) formation.10 When a
benzene ring links the electron donor and electron acceptor
moiety, conjugated planar structures11,12 as well as TICT (twisted
intramolecular charge transfer) states, in which the donor moiety
rotates out of the plane of the acceptor13,14 have been proposed
for the polar excited state. Due to the large fluorescence rate
constants observed for most amino-substituted triphenylbenzene
derivatives, the formation of a TICT state has been excluded
and a planar, delocalized excited state has been suggested.10
Furthermore, preliminary results obtained for a biphenyl model
compound suggested that the excited state become localized in
one branch of the amino-substituted triphenylbenzene.15
The excited state properties of biphenyl and substituted
analogues have been a point of interest over the past decades
and have been explained by Rapp and co-workers in terms of
an excited state relaxation process to a planar geometry.16 Due
to the ground state steric hindrance to planarity, biphenyls are
planar conformation. This model explained as well the presence
and absence of structure in the emission and absorption spectra,
respectively, as the temperature dependence of the vibrational
structure of the emission spectrum of biphenyl. It can be
extended to describe the analogous spectral properties in
4-alkyloxy-4′-cyanobiphenyls17 and 4-(dimethylamino)-4′-cy-
anobiphenyl.18 The relaxation occurred within the time range
of 50-100 ps depending on the solvent.17 Lahmani and co-
workers used the same model as described for biphenyl to
explain the large transition dipole moment of donor-acceptor
biaryls derivatives substituted in the para position by cyano and
N,N′-dimethylamino.19,20 It has to be noticed that the photo-
induced charge transfer process in the latter compounds occurs
between the donor moiety (dimethylanilino) and the acceptor
moiety (cyanophenyl). Komatsu and co-workers have studied
for biphenyl the role of planarity on the rate constants for
intersystem crossing.21 It was concluded that the Ti r S1
intersystem crossing rate constant and the S0 r T1 radiative
rates are much faster in twisted biphenyls than in planar
biphenyls.
In this contribution we want to focus on the radiative as well
the nonradiative deactivation of the excited state of pEFTP,
pEFBP, and pEFF (Figure 1). In this spectral and decay time
study, the localization of the excited state in the 3-fold
symmetric system is examined. In contrast to previous studies
on donor substituted biphenyls,17-19 the entire biphenyl moiety
acts as acceptor.
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5639(97)00666-X CCC: $14.00 © 1997 American Chemical Society

8158 J. Phys. Chem. A, Vol. 101, No. 44, 1997
Verbouwe et al.
analysis of the fluorescence decays was performed as described
previously26-28 using reference convolution. POPOP (1,4-bis-
(2-(5-phenyloxazoyl))benzene) in methylcyclohexane having a
decay time of 1.1 ns at room temperature was used as reference
compound. For time increments bellow 10 ps/channel, a Ludox
scatter solution was used instead of a reference. Fluorescence
spectra and decays were obtained from samples degassed by
several freeze-pump-thaw cycles. The experimental setup for
the laser-induced optoacoustic spectroscopy (LIOAS) is de-
scribed in detail elsewhere.29,30 The absorbance of the sample
solutions used in the LIOAS experiments was always below
0.1, and the samples were deoxygenated by bubbling argon
through the solution for 15 min. Transient absorption spectra
of pEFTP, from a few picoseconds up to several nanoseconds
after excitation, were obtained with a pump-probe setup where
a frequency tripled Nd:YAG laser and a white continuum were
used for pumping and probing, respectively.31-33 In this way,
transient absorption spectra were obtained for different delays
of the pump beam.
For the model compound, pEFBP, the sample was excited
by the amplified and tripled pulses (35 ps) of a mode-locked
picosecond Nd:YAG laser34,35 (BMI 502 PS). The analyzing
light was generated by focusing the fundamental on a tungsten
electrode in a glass cell filled with xenon at a pressure of 2 bar.
Both beams were focused in a nearly collinear arrangement in
the middle of a cell (1 mm optical path), with an angle of 20°
between both beams. After passing through a monochromator
the analyzing light was detected by a streak camera (ARP,
Strasbourg, France) used in single shot mode. For each data
point, 20-300 traces of the analyzing pulse in the presence and
absence of the excitation pulse were collected. The data were
corrected for the fluorescence generated by the excitation beam
and dark counts of the streak camera. The time resolution of
the setup amounted 50-100 ps. This procedure was repeated
every 10 or every 20 nm between 380 and 720 nm. Global
analysis of transient absorption traces at different wavelengths
allowed to reconstruct decay and species associated transient
absorption spectra.
Figure 1. Structure of pEFTP, pEFBP, and pEFF.
Experimental Section
Synthesis. The preparation and purification of 5′-(4-(bis(4-
ethylphenyl)-amino)phenyl)-N,N,N′,N′-tetrakis(4-ethylphenyl)-
(1,1′:3′,1′′-terphenyl)-4,4′′-diamine (pEFTP) has been reported
earlier.22 The biphenyl (pEFBP, N,N-di(4-ethylphenyl)-(1,1′-
biphenyl)-4-amine) and fluorene (pEFF, N,N-di(4-ethylphenyl)-
(9H-fluoren)-2-amine) model compounds were synthesized in
a similar way, starting from the corresponding amine. 4-Ni-
trobiphenyl was reduced by hydrazine monohydrate in ethanol
in the presence of a Pd/C catalyst in an almost quantitative
yield.23 Without further purification, 4-aminobiphenyl was used
in a second step: the substitution of the amino group to the
desired biphenylamino group with 4-ethylphenyl iodide in the
presence of Cu bronze, K2CO3, and tris(dioxa-3,6-heptyl)amine
(TDA) in boiling o-dichlorobenzene as described previously.22
The crude product was purified by column chromatography
using silica and 1:4 dichloromethane/hexane as the eluent.
NMR (400 MHz, CDCl3): 1H: 1.24 (6H, t, J ) 7.6 Hz), 2.61
(4H, q, J ) 7.6 Hz), 7.03-7.10 (10H, m), 7.27 (1H, t × t, J )
8.3, 1.4 Hz), 7.39 (2H, t × t, J ) 8.3 , 1.4 Hz), 7.43 (2H, d
(broad)). 13C: 15.5 (CH3, ethyl), 28.2 (CH2, ethyl), 122.8,
124.6, 126.5, 126.6, 127.6, 128.6, 128.7 (CH, phenyl), 134.1,
138.9, 140.8, 145.4, 147.6 (C-ipso, phenyl). Mass spectrum
(EI) m/z (%): 377 (M+.,100), 362 (M•+ - CH3, 67).
The fluorene model compound was prepared in a completely
similar way from 4-aminofluorene. NMR (400 MHz, CDCl3):
1H: 1.24 (6H, t, J ) 7.6 Hz), 2.61 (4H, q, J ) 7.6 Hz), 3.79
(2H, s), 7.02-7.05 (4H, m), 7.07-7.09 (5H, m), 7.20-7.24
(2H, m), 7.33 (1H, t × d, J ) 7.4, 0.9 Hz), 7.47 (1H, d (broad),
J ) 7.4 Hz), 7.61 (1H, d, J ) 8.2 Hz), 7.67 (1H, d, J ) 7.4
Hz). 13C: 15.5 (CH3, ethyl), 28.2 (CH2, ethyl), 36.8 (CH2),
119.1, 120.1, 120.4, 122.5, 124.2, 124.8, 125.7, 126.7, 128.5
(CH, phenyl), 136.0, 138.5, 141.6, 143.0, 144.5, 145.8, 147.4
(C-ipso, phenyl). Mass spectrum (EI) m/z (%): 389 (M•+, 100),
374 (M•+ - CH3,59).
Solvents. The solvents (isooctane, Merck; dibutyl ether,
Merck; diisopropyl ether, Romil; diethyl ether, Merck; butyl
acetate, Janssen Chimica; ethyl acetate, Merck; THF, Rathburn;
methyl ethyl ketone, Rathburn; acetone, Romil; propionitrile,
Rathburn; acetonitrile, Merck) were of spectroscopic grade and
were used as received and checked for fluorescence before use.
Methods. Absorption spectra were recorded with a Perkin-
Elmer Lambda 6 UV/vis spectrometer. Corrected fluorescence
and excitation spectra were obtained with a SLM 8000C
spectrofluorimeter in L-format. The fluorescence quantum
yields were determined using quinine sulfate in 1 N H2SO4 as
a reference (ΦF ) 0.55).24 The fluorescence decays were
obtained by the single photon timing technique.25 For the SPT
measurements an excitation wavelength of 320 nm was used
and decays were measured over the whole emission range. All
fluorescence decay curves were observed at magic angle (55°),
contained 10 000 counts at the maximum, and were collected
in 511 channels of the multichannel analyzer. The global
A frequency-tripled Nd:YAG laser pulse and a pulsed Xenon
lamp as excitation and probe source, respectively, in combina-
tion whit a OMA-3 system were used to obtain transient
absorption spectra 10 ns to several microseconds after excita-
tion.36
Results
Steady State Spectra. The absorption maxima and shape
of the absorption spectra in acetonitrile of pEFTP, pEFBP, and
pEFF consist of two partially overlapping bands. The lowest
absorption bands are located at 342, 324, and 334 nm for pEFTP,
pEFBP, and pEFF, respectively. The second absorption band
around 310 nm is similar for the three compounds. In the far-
UV region a third band and the onset of a fourth band are
observed around 250 and 220 nm (Figure 2). The absorption
maxima and shapes of the spectra are not dependent on the
solvent polarity. The conjugation of the π-system increases
from pEFBP over pEFF to pEFTP which is reflected in the shift
of the long-wavelength absorption band. None of the absorption
spectra of the three compounds shows vibrational fine structure
in any solvent.
The emission spectra in isooctane consists of a maximum at
385, 377, and 378 nm for pEFTP, pEFBP, and pEFF, respec-
tively, and a shoulder at 400, 392, and 390 nm, respectively. In
polar solvents, this vibrational structured spectrum disappears
and a broad structureless band of which the maximum shifts to
longer wavelengths when the solvent polarity is increased, is

Photophysical Properties of pEFTP, pEFBP, and pEFF
J. Phys. Chem. A, Vol. 101, No. 44, 1997 8159
behavior has been observed for dimethylanilino-substituted
anthracene (ADMA)40 and has been attributed by Baumann et
al.41 to the additional shift due to the dipole-induced dipole
interaction exerted by the strongly polar solute on the surround-
ing solvent molecules. However, to the extent that the ground
state and the excited state have a similar polarizability, taking
the latter effect into account is not sufficient to account for the
deviations from the Lippert-Mataga plot.
The data in the polarity range from isooctane to THF could
be fitted to a linear expression. Assuming a value of 7.9 and
5.5 Å for the radius of the solvent cavity in pEFTP and pEFBP
or pEFF, respectively, the dipole moment of the emitting state
can be calculated. The cavity radius is based on the assumption
of a spherical molecule and a density of 0.8 g/cm3. The dipole
moments amount to 22, 13, and 10 D for pEFTP, pEFBP, and
pEFF, respectively. To estimate the dipole moment in polar
solvents, the emission maxima were for solvents ranging from
methyl ethyl ketone to acetonitrile fitted to the Lippert-Mataga
equation. In this way, a very high dipole moment of 40 D was
obtained for pEFTP. For the other compounds, similar dipole
moments were obtained for this solvent range. This suggests
the contribution of other effects to the observed bathochromic
shift.
Figure 2. Absorption spectra in acetonitrile and emission spectra in
isooctane (dashed), THF (full), and acetonitrile (bold) of pEFTP,
pEFBP, and pEFF.
The width at medium height (∆ν1/2) of the emission band
can be related to the coupling of the solvation and molecular
vibrations (inclusive torsions) to the electronic transition.42
observed (Figure 2). This red shift is accompanied by a
broadening of the width of the fluorescence band. The emission
spectra of pEFTP and the pEFBP compound show a similar
solvent dependence.
While the emission spectra of pEFF have similar features,
the emission maximum is situated at shorter wavelengths and
the solvatochromism is less pronounced than for pEFTP and
pEFBP (Table 1). Hence the energy difference between the
emission maximum of pEFBP and pEFF increases from 0.050
eV in dibutyl ether to 0.116 eV in acetonitrile.
(∆ν1/2hc)2
) 2λ0kT + 2λ′ikT + λihνi
(2)
8 ln 2
λ0 corresponds to the outer-sphere solvent reorganization energy,
λ′i is the intramolecular reorganization energy associated with
vibrations for which hνi < kT, and λi is the intramolecular
reorganization energy associated with vibrations for which hνi
> kT. The decreased flexibility of the fluorene derivative, pEFF,
leads to a smaller width of the spectrum (see Table 1) compared
with the biphenyl model, suggesting a difference in the
equilibrium geometry of the S1 and S0 state. The widths of the
fluorescence bands of pEFTP and pEFBP are of the same order
of magnitude and stress again the similarity between the
triphenylbenzene and the biphenyl compound. For a transition
from an excited state with a permanent dipole moment to a
ground state with a negligible dipole moment, λ0 is given by42
The Lippert-Mataga relationship (eq 1a and 1b)37,38 can be
used to relate the solvent-dependent shift of the emission
maximum to the change of the dipole moment upon excitation.
This relationship can be simplified if the dipole moment of the
ground state can be neglected. This assumption can be made
as the absorption spectra of pEFTP, pEFBP, and pEFF do not
depend upon the solvent. Furthermore, the dipole moment of
the ground state of pEFTP is only 1.9 D.39
2
2
2µE
µE
νjF ) νjF0
-
F(ꢀr,n)
(1a)
(1b)
λ0 )
f(ꢀr,n)
(3a)
(3b)
4πꢀ0hca3
4πꢀ0a3
n2 - 1
2(2n2 + 1)
ꢀr - 1
2ꢀr + 1
ꢀr - 1
2ꢀr + 1
n2 - 1
F(ꢀr,n) )
-
f(ꢀr,n) )
-
2n2 + 1
0
0
νjF and νjF correspond to the emission maximum (wavenumbers)
in a solvent with dielectric constant ꢀr and a refractive index n,
and to the emission maximum (wavenumbers) in vacuum,
respectively. The constants ꢀ0, h, c, and a correspond to the
permittivity of vacuum (8.85 × 10-12 C V-1 m-1), Planck’s
constant (6.6 × 10-34 J s), the velocity of light in vacuum (3.0
× 108 m s-1), and the radius of the solvent cavity (in m). µE
(C m) is the dipole moment of the emitting excited state. This
relationship is based on the assumptions that solvation only
occurs by dipolar interactions and that the nature of the excited
state does not depend on the solvent polarity. Figure 3 indicates
that the experimental results do not follow this linear relationship
over the whole solvent polarity range. Apparently for all the
three compounds the absolute value of the slope increases in
highly polar solvents like acetone and acetonitrile. A similar
Combining eqs 1 and 3, the outer-sphere reorganization energy
can be calculated for the solvent range from isooctane to THF
where the Lippert-Mataga equation could be used to relate the
emission maxima to the solvent polarity. The total intramo-
lecular reorganization energy in different solvents is given by
2
λihνi (∆ν1/2hc)2
µE
λ′i +
)
-
f(ꢀr,n)
(4)
4πꢀ0a3
2kT
16kT ln 2
The similarity between pEFTP and pEFBP chromophores is also
found for the reorganization energies (Table 1). The intramo-
lecular reorganization energy, λi′ + λihνi/2kT, amounts to 0.43
eV for these two compounds, while for the more planar fluorene
model compound, pEFF, a value of only 0.28 eV is obtained.

8160 J. Phys. Chem. A, Vol. 101, No. 44, 1997
Verbouwe et al.
TABLE 1: Photophysical Properties of pEFTP, pEFBP, and pEFF at Room Temperaturea
c
λ
max,e (nm)b
∆ν1/2 (cm-1
)
λ0 (eV)d
λ′i + λihνi/2kT (eV)e
solv
F(ꢀr,n) pEFTP pEFBP pEFF pEFTP pEFBP pEFF
pEFTP
pEFBP
pEFF
pEFTP pEFBP pEFF
ISO
0.097
0.193
0.256
0.308
0.392
383
395
403
417
447
377
390
397
409
435
378
384
387
397
418
2555
2940
3135
3375
4020
2735
3025
3245
3430
4075
2265
2465
2658
2960
3626
3 × 10-3 3 × 10-3 2 × 10-3
0.35
0.41
0.43
0.49
0.40
0.44
0.47
0.51
0.25
0.26
0.28
0.35
DBE
DEE
THF
ACN
0.06
0.11
0.13
0.06
0.10
0.13
0.03
0.06
0.07
a ISO ) isooctane, DBE ) n-dibutyl ether, DEE ) diethylether, THF ) tetrahydrofuran, ACN ) acetonitrile. b Emission maximum. c Full width
at medium height. d Solvent reorganization energy. e Intramolecular reorganization energy.
K is shifted to shorter wavelengths compared to that at room
temperature. For pEFF this shift becomes marginal.
At longer wavelengths, phosphorescence is observed with
maxima for pEFTP at 498, 512, 540, and 573 nm, for pEFBP
at 495, 524, 534, and 561 nm, and for pEFF at 477, 497, 516,
538, and 561 nm.
Fluorescence Decay. The fluorescence decays of the three
compounds pEFTP, pEFBP, and pEFF were obtained in different
solvents and at several wavelengths over the whole emission
spectrum. As well using single curve analysis as global analysis
linking decays at different emission wavelengths and time
windows, all decays could be analyzed as a monoexponential
decay. To explore the occurrence of fast processes with a decay
time less than 50 ps, a time increment of 5 ps/channel is used.
Figure 3. Emission maximum as a function of the solvent polarity
for pEFTP (squares), pEFBP (triangles), and pEFF (crosses). Solvents
are isooctane, n-dibutyl ether, diisopropyl ether, diethyl ether, butyl
acetate, ethyl acetate, THF, methyl ethyl ketone, acetone, propionnitrile,
and acetonitrile.
No indications for biexponential decays or distributions of decay
rates were observed within the time resolution (20 ps) of the
setup. As the fluorescence decay could be analyzed as a single-
exponential decay the fluorescence rate constant, kF, and the
rate constant for radiationless decay, kNR, could be obtained
using the following expressions:
kF ) ΦF/τ
(5a)
(5b)
kNR ) (1/τ) - kF
The similarity of the decay parameters of pEFTP and pEFBP
at different solvent polarities stresses the similarity of their
excited state properties (Table 2). The increase of the decay
times upon increasing the polarity is mainly due to the decrease
of the nonradiative decay rate in polar solvents. This increase
of the decay time is to some extent due to the decrease of kF.
The latter effect is mainly due to the decrease of ν3 upon
increasing the solvent polarity. These data contrast with the
important decrease of the rate constant for radiative decay and
the transition dipole moment, which were observed for cyano-
substituted p-dimethylamino biphenyls upon increasing the
solvent polarity.19 The decrease of kNR is for pEFF limited to
acetonitrile.
Laser-Induced Optoacoustic Spectroscopy (LIOAS). The
nonradiative decay of the excited state can be factorized in the
internal conversion to the ground state with a rate constant kIC,
and the intersystem crossing to a triplet state with a rate constant
kISC. These decay parameters can be obtained by a combination
of fluorescence decay analysis and laser-induced optoacoustic
spectroscopy (LIOAS).44-46 In the latter experiment, the
fraction R of the absorbed laser energy, Eabs, that is converted
into heat within the time constant of the experimental setup,
Eth, is determined. The interpretation of the laser-induced
optoacoustic experiments is based on the scheme proposed in
Figure 5.
Figure 4. Emission spectra of pEFTP, pEFBP and pEFF in an
isopentane glass at 77 K and in isooctane at room temperature (bold).
The emission spectra of pEFTP, pEFBP, and pEFF in
isopentane glass at 77 K show two separated bands (Figure 4).
Fluorescence maxima at 378, 370, and 376 nm and a shoulder
at 402, 385, and 395 nm for pEFTP, pEFBP, and pEFF,
respectively, are observed. Although no clear vibrational fine
structure is observed, the shoulder becomes more pronounced
and the bandwidth decreases at 77 K. The latter effect leads to
a sharp emission band for pEFF at 376 nm. The emission
maximum is shifted to higher energy at 77 K compared with
the room-temperature spectrum in isooctane. For pEFBP and
to a smaller extent for pEFTP,43 the emission maximum at 77
R ) Eth/Eabs
(6a)
(6c)
ΦF + ΦISC + ΦIC ) 1

Photophysical Properties of pEFTP, pEFBP, and pEFF
J. Phys. Chem. A, Vol. 101, No. 44, 1997 8161
TABLE 2: Decay Parameters of the Excited Singlet State of pEFTP, pEFBP, and pEFF (for Abbreviations See Table 1)a
b
c
ΦF
pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF
τ (ns)a
kF (107 s-1
)
kNR (107 s-1
)
kF/n2ν3 (10-7 s-1 cm3)d
ISO
0.43
0.30
0.38
0.34
0.35
0.47
0.68
0.27
1.10
1.34
1.34
1.26
1.50
1.99
4.74
1.73
39
22
29
27
23
24
14
16
52
52
47
52
43
26
7
42
110
70
78
82
80
86
65
44
DBE
DEE
THF
ACN
0.28
0.32
0.44
1.47
1.51
2.52
19
21
18
49
45
22
61
67
71
0.53
0.67
2.14
4.48
25
15
22
7
91
79
a Fluorescence decay time. b Rate constant of fluorescence. c Rate constant of nonradiative decay. d kF/n2 is proportional to the Einstein coefficient
of spontaneous emission. kF/n2ν3 is proportional to the oscillator strength.
observed 100 ns after excitation (Figure 6b). This transient
absorption band has been attributed to a Tn r T1 transition.
Very similar transient absorption spectra at short and long
time after excitation were observed in toluene and THF. The
Sn r S1 absorption at 470 nm and the Tn r T1 at 650 nm are
not dependent on the solvent polarity. Even in solvents of
medium polarity like THF there is no indication for absorption
by radical cation produced by photo ionization. In polar solvents
like acetonitrile only ion absorption with maxima at 396 and
779 nm and a shoulder at 665 nm is observed.
pEFBP. Transient absorption traces at different wavelengths
were obtained in toluene and butyl acetate. The absorption
transients were fitted to the following function:
Figure 5. Scheme of the photophysical processes .
Rhνex ) [E(S1*-S1)] + ΦF[E(S0*-S0)] +
A(λ,t) ) R1(λ) exp(-t/τ) + R2(λ)
(7)
Φ
ISC[E(S1-T1)] + ΦIC[E(S1-S0)] (6b)
The transient absorption traces at different wavelengths were
linked, keeping the decay parameter τ constant. Good statistical
parameters were obtained for the global fitting of pEFBP in
toluene and butyl acetate, and a decay time (toluene 1.5 ns),
similar to the fluorescence decay time, was observed. The
parameters R1(λ) and R2(λ) allow to reconstruct decay and
species associated transient absorption spectra. Therefore, R1-
(λ) and R2(λ) are proportional to following expressions:
E(S1*-S1) corresponds to the energy difference between the
absorption maximum and the 0-0 transition of fluorescence.
E(S0*-S0) is the energy difference between the 0-0 transition
of fluorescence and the emission maximum. E(S1-T1) equals
the difference between the 0-0 transitions of fluorescence and
phosphorescence. E(S1-S0) corresponds to the energy of the
0-0 fluorescence. hνex is the energy of a photon at the
excitation wavelength.
R1(λ) ≈ σS fS (λ) - σS fS (λ) -
Due to volume changes around the polar excited triplet state,
the interpretation of the LIOAS experiment could be more
complicated and a more elaborated treatment of the data would
be necessary. To test the validity of the approach used here,
the solvent dependence of the extent of triplet formation was
evaluated by comparing, for the same molecule, the Tn r T1
absorbance in solvents of different polarity. The data used here
were corrected for the amount of excitation light absorbed by
the sample. This implies, however, that the extinction coef-
ficient of the Tn r T1 absorption is not solvent dependent which
is probably a valid approximation as the shape of the Tn r T1
spectra does not change with the solvent polarity (cf. infra).
Table 3 shows that both methods yield a parallel solvent
dependence of the quantum yield of triplet formation.
kIC and kISC also differ in pEFF, compared with pEFTP and
pEFBP in isooctane. This suggests that for these processes the
hindered rotation plays an important role. While kF and kISC
are similar for pEFTP and pEFBP, it is not so clear for kIC
because of the large error on these small values. For both
molecules kISC decreases when the polarity of the solvent
increases.
Transient Absorption. pEFTP. The picosecond transient
absorption spectrum of pEFTP in isooctane immediately after
excitation shows a maximum at 470 nm. As the decay time of
this absorption signal (1.1 ns) equals the fluorescence decay
time, the species absorbing at 470 nm is probably the emitting
species. While the intensity of the absorption band at 470 nm
decreases, a new red-shifted absorption band becomes more
pronounced at longer times after excitation (Figure 6a). The
latter band has a maximum around 650 nm and has also been
1
n
1
0
(1 - ΦISC)σS fS (λ) - ΦISCσT fT (λ)
0
1
1 n
R2(λ) ≈ ΦISC (σ
(λ) - σ
(λ))
S0fS1
T1fTn
R1(λ) + R2(λ) ≈ σS fS (λ) - σS fS (λ) - σS fS (λ)
1
n
1
0
0
1
where σS fS (λ) is the cross section for absorption to higher
1
n
singlet states, σS fS (λ) cross section for induced emission,
1
0
σ
S fS (λ) cross section for ground state absorption (depletion),
0
1
σ
T1fTn
(λ) cross section for triplet-triplet absorption, and ΦISC
quantum yield for intersystem crossing. As for wavelength
range where the spectra were recorded, ground state depletion
can be neglected those expression can be simplified to
R1(λ) ≈ σS fS (λ) - σS fS (λ) - ΦISC
σT fT (λ)
1
n
1
0
1 n
R2(λ) ≈ ΦISC
σ
T fT (λ)
1 n
R1(λ) + R2(λ) ≈ σS fS (λ) - σS fS (λ)
1
n
1
0
The decay-associated transient absorption spectrum of the singlet
is given by R1(λ) + R2(λ)(t)0). The induced emission gives
rise to a negative absorbance and determine the transient
absorption spectrum at short wavelengths. The maximum of
the singlet-singlet absorption is located at 460 nm both in
toluene (Figure 7a) and butyl acetate but can be shorter due to
competition with induced emission.

8162 J. Phys. Chem. A, Vol. 101, No. 44, 1997
Verbouwe et al.
TABLE 3: Rate Constants for the Nonradiative Deactivation of the Excited State, kIC (Internal Conversion) and kISC
(Intersystem Crossing), Quantum Yield of Triplet Formation, ΦISC, and Tn r T1 Absorbance Corrected for Absorbed Light
(S1 r S0)
kF (107 s-1
)
kIC (107 s-1
)
kISC (107 s-1
)
ΦISC
A(Tn r T1)/A(S1 r S0)
pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF pEFTP pEFBP pEFF
ISO
THF
39
25
29
24
16
21
10
4
2
3
36
18
42
18
44
22
6
37
0.46
0.38
0.59
0.45
0.10
0.49
0.45
0.21
1.06
0.89
0.63
0.88
Figure 6. Transient absorption spectra of pEFTP isooctane: (a, top)
picosecond transient absorption spectra from 10 ps to 3 ns after
excitation, and (b, bottom) nanosecond transient absorption spectra 100
ns after excitation.
Figure 7. Decay associated transient absorption spectra of pEFBP in
toluene (a, top) at short time (R1(λ) + R2(λ)) and (b, bottom) at long
time (R2(λ)) after excitation.
The transient absorption spectra long time after excitation is
given by R2(λ)(t)∞). The maximum of the decay associated
transient absorption spectrum is situated at 550 nm in toluene
and butyl acetate (Figure 7b), which is similar to the Tn r T1
absorption 100 ns after excitation where a maximum is observed
at 557, 564, and 564 nm in isooctane, THF, and acetonitrile,
respectively. The transient absorption spectra show even in
butyl acetate or THF no absorption characteristic for radical
cation of the donor moiety or the radical anion of the acceptor
moiety. Those absorption maxima of those species would be
situated at 712 and 630 nm for diphenylamino cation and at
380, 396, and 655 nm and at 354, 410, and 555 nm for biphenyl
and triphenylbenzene anion, respectively.47 In acetonitrile ion
absorption is observed 100 ns after excitation with maxima at
380 and 750 nm and shoulder at 670 nm. This corresponds
well with the nanosecond transient absorption spectrum of
pEFTP in acetonitrile.
In contrast to the fluorescence spectra, where a solvatochro-
mic shift of the emission maximum is observed, the Sn r S1
absorption is not solvent dependent.

Photophysical Properties of pEFTP, pEFBP, and pEFF
J. Phys. Chem. A, Vol. 101, No. 44, 1997 8163
Although the singlet transient absorption spectra of pEFTP
and the model compound are very similar, this is not the case
for the triplet-triplet absorption spectra, obtained using laser
flash photolysis. This suggests that the triplet excited state in
pEFTP is delocalized over the whole molecule.
Discussion
1. pEFTP-pEFBP. The photophysical properties of pEFTP
and pEFBP are very similar. They both absorb and emit in the
same wavelength region and have a similar solvent dependence.
Furthermore, the singlet excited state decays with similar kinetic
parameters and gives rise to similar Sn r S1 transient absorption
spectra. Therefore, the polar excited state of pEFTP should be
considered as localized in one branch of the molecule. This
has also been concluded for the excited state of ruthenium(II)
tris(bipyridine) complexes.5 The similarity of the evolution of
the emission energy of pEFTP and pEFBP in a large range of
solvents polarities (Figure 3) suggests that the triphenylamino
substituents in 3 and 5 position have only a limited influence
on the excited state properties of pEFTP. This can be probably
related to the fact that these moieties, which are expected to
act as weak acceptors, are rotated out of the plane of the
triphenylbenzene moiety (propeller structure).
The different dipole moments of the emitting state of pEFTP
and pEFBP could be an artifact due the overestimation of the
radius of the solvent cavity in the triphenylbenzene derivative.
If the excitation is localized in one branch, a radius of 5.5 Å as
used for pEFBP should be more appropriate than 7.9 Å for
pEFTP. In this case, a smaller dipole moment of 13.0 D is
found which is similar to the excited state dipole moments of
pEFBP. Those dipole moments are significantly smaller than
the values that would be expected for a TICT state.10 The
difference of the absorption spectra of pEFTP and pEFBP
suggests that to some extent the long-wavelength absorption
band is related to the formation of a delocalized excited state.
Immediately after excitation, relaxation to a localized polar
excited state, from where emission takes place, occurs. In this
framework, the isoenergetic intramolecular energy transfer is
possible in pEFTP as has been reported for related triphenyl-
benzene compounds48 and ruthenium(II) tris(bipyridine) com-
plexes.5 Since the singlet excited state properties of pEFTP
and pEFBP are similar, the charge transfer occurs between the
diphenylamino moiety and the biphenyl moiety. For this reason
a TICT state, where the two phenyl groups of the biphenyl
moiety in pEFBP (or in pEFTP) are orthogonal, can be excluded.
Furthermore, this would lead to an excited state with a very
high energy because of the low electron accepting capacity of
the phenyl moiety. In contrast, the electron decoupling occurs
between the two aryl moieties for donor-acceptor biaryls with
cyano acceptor and dimethylamino donor.19
Figure 8. Twist-angle-dependent potential energy scheme for biphenyl
and related compounds (according to ref 16).
2. pEFBP-pEFF. The absorption spectra of pEFBP and
pEFF are similar which indicates that the initially reached
excited states have the similar electronic properties. The long-
wavelength absorption band of pEFF is bathochromic shifted
with respect to that of pEFBP, due to the extended conjugation
in the fluorene model.
The bathochromic shift of the absorption spectra is not
reflected in a bathochromic shift of the fluorescence spectra. A
similar result was obtained by Klock and co-workers for para-
substituted cyanodimethylaminobiphenyl and an analogous
fluorene derivative.18 In spite of the better electron acceptor
capacity of the fluorene moiety in pEFF, the emission occurs
at slightly higher energy compared with pEFBP. Hence it is
difficult to explain this difference by a different stabilization
of an excited state with charge transfer character. Furthermore,
the small difference of the reduction potentials, E1/2 ) -2.70
and -2.65 V vs SCE in a dioxane/water (3/1) for biphenyl and
fluorene, respectively,50 suggests an identical charge distribution
in the excited state. If the difference in E1/2 would matter at
all it would lead to a red-shifted emission compared with
pEFBP.
Since the twisting angle between the phenyl moieties is the
only important difference between pEFBP and pEFF, an
explanation of this unexpected blue shift must be related to the
shape of the ground state and excited state energy curves as a
function of rotational angle. The intramolecular relaxation
process toward a planar excited state has been reported for
biphenyl and related compounds. The ground state equilibrium
angle has been estimated 32°,51 39.5°,52 41.6°,53 or 38.63° 54 in
gas phase and 19°-26° 55 in solution, depending on the method
used. For biphenyl, related compounds, and also for pEFBP, a
lack of structure in the absorption spectrum and a structured
emission band in apolar solvents is observed. This suggests a
similar photophysical behavior as observed for biphenyl and
related compounds. Upon excitation to an electronically delo-
calized and geometrically unfavorable rotated excited state,
relaxation to a more planar geometry occurs. This process is
too fast to be observed by the current time-resolved fluorescence
technique and hence a single-exponential decay of the excited
state was observed. As shown in Figure 8, fluorescence occurs
to an unfavorable planar geometry in the Franck-Condon
ground state, which is followed by vibrational relaxation. Both
The triplet excited state of pEFTP and the biphenyl model
compound differ suggesting that in the triplet state a more
extensive delocalization occurs.
At 77 K, a deviation of the similar spectral properties between
pEFTP and pEFBP is observed. While the emission maximum
of pEFBP at 77 K in isopentane glass is hypsochromically
shifted compared to the emission at room temperature in
isooctane, a smaller shift has been observed for pEFTP. This
would suggest that for pEFTP along the coordinates involving
large-amplitude vibrations, a smaller shift of the potential energy
minimum occurs upon excitation.49 The smaller shift for pEFTP
can also be attributed to a combination of inhomogeneous
broadening and intramolecular energy transfer to the branch with
an environment that stabilized the polar excited state most
effectively.

8164 J. Phys. Chem. A, Vol. 101, No. 44, 1997
Verbouwe et al.
The results discussed here give less information about the
eventual geometric changes of the diphenylamino group with
respect to the triphenylbenzene, biphenyl, or fluorene part. This
is due to the fact that this group is the same for the three
compounds. The smaller intramolecular reorganization energy,
λi′ + λihνi/2kT, obtained for the fluorene compound (0.28 eV
in diethyl ether compared to 0.47 eV for the biphenyl compound
in the same solvent) is probably mainly due to the reduction of
the intramolecular relaxation of the angle between both phenyl
moieties in the fluorene derivative. In contrast to the nonzero
reorganization energy of 0.25 eV observed for pEFF in
isooctane, the emission at 77 K in isopentane is not shifted to
shorter wavelengths compared to pEFF in isooctane. Hence
for pEFBP a hypsochromic shift is observed in isopentane glass
which can be explained by the hindered relaxation around the
two phenyl moieties in the excited state at 77 K. This suggests
that no important large-amplitude vibrations of the diphenyl-
amino part are involved in the stabilization relaxation process
of the excited state in pEFF, pEFBP, and hence also pEFTP.
Figure 9. Energy difference between the emission maxima of pEFBP
and pEFF as a function of the solvent polarity.
excited state relaxation and ground state destabilization con-
tribute to the observed Stokes shift.
This geometric relaxation does not occur, or occurs to a
considerably smaller extent, in the fluorene derivative and the
emission probably takes place from the minimum of S1 to that
of the S0 state. The reduction of the energy difference is due
to this relaxation and destabilization in the Franck-Condon
ground state and the latter effect does not occur in the fluorene
derivative. The emission spectrum of pEFF in isopentane glass
at 77 K shows no hypsochromic shift compared with the
spectrum at room temperature in isooctane. This also suggests
an identical geometric conformation of the S0 and S1 state for
pEFF. In contradiction to pEFF, a hypsochromic shift from
377 to 370 nm of the emission spectra of pEFBP in isopentane
at 77 K is observed. Hence the relaxation to a more planar
geometry determines to a major extent the emission properties
of pEFBP in apolar solvents.
Since the energy of the relaxed excited state of pEFBP and
pEFF will not differ significantly, the difference of the emission
maxima should be given by the difference between the minimum
of the ground state and the Franck-Condon ground state. This
would correspond to the rotation barrier in the ground state.
For this barrier, 0.16,56 0.089,54 or 0.05 eV51 has been
determined for biphenyl. The shift between pEFBP and pEFF
is of the same order of magnitude as those values, keeping in
mind the influence of substitution and solvation on the rotation
barrier (Figure 9). In isooctane no red shift is observed because
the energy difference of the relaxed singlet excited states
compensates that of the Franck-Condon ground state.
Figure 9 and also Figure 3 indicate the increase of the energy
difference in solvents with a higher polarity. In the framework
of the proposed scheme in Figure 8, this can be explained by
an increased planarity of the biphenyl part of the emitting state
of pEFBP in polar solvents. This is supported by the decrease
of rate constant of the intersystem crossing in the excited state
of pEFBP from 4.4 × 108 s-1 in isooctane to 2.3 × 108 s-1 in
THF. In the case of biphenyl, the same angular dependence of
the rate constant of intersystem crossing has been found.21 This
effect can also be due to the decrease of the energy gap between
the polar S1 state and the apolar triplet state.
Acknowledgment. W.V. acknowledges the IWT for a
scholarship. M.V.d.A. is an Onderzoeksdirekteur of the F.W.O.-
Vlaanderen. The authors gratefully acknowledge the F.W.O.,
the Nationale Loterij, and the continuing support from DWTC
(Belgium) through IUAP IV-11. M.V.d.A. acknowledges the
E.N.S.E.T. Cachan for a stay as invited professor.
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