Photoinduced coupled charge and proton transfers in gradually twisted phenol-pyridinium biaryl series

Article abstract of DOI:10.1021/jp904601r

A detailed photophysical analysis of a phenol-pyridinium biphenyl series with gradual twisted geometry is presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of intensity with a progressive blue shift by increasing the twist angle of the central bond (θAD). These effects are well described and quantified within the framework of the Mulliken-Murrel approach, which allows us to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond, excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet-Taft parameters (π*, α, and β). The correlation first confirms the π* dependence of the CSh band shift but also demonstrates a clear β dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing with θ_AD. Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected. The relative increase of its fluorescence band intensity with θ_AD confirms the gradual enhancement of the excited states acidity.

Full text of DOI:10.1021/jp904601r

J. Phys. Chem. A 2010, 114, 2401–2411
2401
Photoinduced Coupled Charge and Proton Transfers in Gradually Twisted
Phenol-Pyridinium Biaryl Series
Jean-Pierre Malval,*,† Vincent Diemer,‡ Fabrice Morlet-Savary,§ Patrice Jacques,§
He´le`ne Chaumeil,‡ Albert Defoin,‡ Christane Carre´,⊥ and Olivier Poizat|
Institut de Sciences des Mate´riaux de Mulhouse, LRC CNRS 7228, UniVersite´ de Haute Alsace, 15 rue Jean Starcky,
68057 Mulhouse, France, De´partement de Photochimie Ge´ne´rale, FRE CNRS 3252, and Laboratoire de Chimie
Organique, Bioorganique et Macromole´culaire, FRE CNRS 3253, UniVersite´ de Haute Alsace, ENSCMu. 3 rue
Alfred Werner, 68093 Mulhouse, France, UniVersité Européenne de Bretagne (UeB), FOTON-CCLO,
CNRS UMR 6082, ENSSAT, BP 80518, 22305 Lannion Cedex, and Laboratoire de Spectrochimie Infrarouge et
Raman, UMR CNRS 8516, UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France
ReceiVed: May 17, 2009; ReVised Manuscript ReceiVed: December 21, 2009
A detailed photophysical analysis of a phenol-pyridinium biphenyl series with gradual twisted geometry is
presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of
intensity with a progressive blue shift by increasing the twist angle of the central bond (ΘAD). These effects
are well described and quantified within the framework of the Mulliken-Murrel approach, which allows us
to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence
bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular
twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond,
excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive
phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet-Taft parameters
(π*, R, and ꢀ). The correlation first confirms the π* dependence of the CSh band shift but also demonstrates
a clear ꢀ dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing
with ΘAD. Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted
conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation
in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected.
The relative increase of its fluorescence band intensity with ΘAD confirms the gradual enhancement of the
excited states acidity.
1. Introduction
proposed model, which has been the source of controversial
discussions for more than 20 years,18-20 relates a photoinduced
Addressing the factors that mainly impact the photoinduced
intramolecular charge-transfer (ICT) process constitutes an
appealing challenge for the development of molecular devices
devoted to a large set of applications in solar energy conversion,
artificial photosynthetic systems,1 chemosensors,2 high-perfor-
mance optoelectronic materials, and proton photorelease systems.3,4
For this purpose, several key parameters can be methodically
modulated on the basis of relevant series of donor-acceptor
models: (i) the nature of the donor and acceptor subunits which
thermodynamically allow the photoinduced ICT5,6 process, (ii)
the nature and the length of the linker which ensures either the
charge delocalization or confinement,7-10 and (iii) the amplitude
of the relative motion between each donor and acceptor subunit
at excited states.11,12 In connection with this latter factor, the
4-(dimethylamino)benzonitrile13-17 has been extensively studied
as a reference example. This simple donor-acceptor molecule
exhibits a dual fluorescence emission in a polar medium. The
ICT process from the amino group to the benzonitrile moiety
with a concomitant rotation between both subunits. Hence the
new fluorescence band stems from the radiative deactivation
of a twisted internal charge-transfer species (TICT). Such a
model has been generalized for larger conjugated π-systems11
or for donor-acceptor biaryl models.15,21-24 The possibility of
conformational twist in the excited state is mainly dependent
on the energetic barrier between the locally excited and the
charge-transfer state. Ultimately, this barrier, which is strongly
influenced by the donor or acceptor nature, can be suppressed,23
as observed for dimethylanilino- or thiophenopyridinum
derivatives.23,25-27 The large electron affinity of pyridinium
group (Ered ) -1.21 eV7) is assumed to be responsible for this
dramatic effect. Therefore, integrated in D-A biaryl systems,
the pyridinium moiety constitutes an interesting candidate that
should both promote a π-conjugated planar conformation in the
ground state and lead to a large amplitude twisting relaxation
in the excited state. By association with a proton donor group,
a coupling with an excited-state proton-transfer (ESPT) process
can be observed. The strategy that consists of enhancing the
excited-state acidity due to the effect of a primary photoinduced
ICT is well established for rigid hydroxyarenes such as
2-naphthol,28-33 pyranine,34-37 and 6-hydroxyquinoline38,39 de-
rivatives. The role of the withdrawing substituent40,41 coupled
with the polarization of the electronic transition42 was empha-
* Corresponding author. Tel: +33 389 33 50 24; Fax: +33 389 33 50
11. E-mail: jean-pierre.malval@uha.fr.
† Institut de Sciences des Mate´riaux de Mulhouse, Universite´ de Haute
Alsace.
‡ Laboratoire de Chimie Organique, Bioorganique et Macromole´culaire,
Universite´ de Haute Alsace.
§ De´partement de Photochimie Ge´ne´rale, Universite´ de Haute Alsace.
⊥ Université Européenne de Bretagne.
| Universite´ des Sciences et Technologies de Lille.
10.1021/jp904601r  2010 American Chemical Society
Published on Web 02/02/2010

2402 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Malval et al.
SCHEME 1: Molecular Structures of Biphenyl
Derivatives (PhPy1 to PhPy4)
Figure 1. Absorption spectra of biaryl derivatives (solvent: acetoni-
trile).
sized to trigger the excited-state acidity. Moreover, it was
demonstrated that the solvation of the anionic conjugated base,
which is subsequently produced, largely contributes to the
exothermy of ESPT.30,43 However, to our knowledge, few
examples44 were proposed to extend the investigation to systems
that correlate ESPT to conformational change.
In the present paper we describe a detailed photophysical
analysis of a phenol-pyridinium biaryl series where the twist
angle relative to the aryl-aryl bond is gradually increased by
sterically hindering substituents (Scheme 1). Such a conforma-
tion modulation has strong consequences at ground and excited
states.
bisulfate in 0.05 molar sulfuric acid (Φf ) 0.52).48 The
fluorescence quantum yields were obtained from five indepen-
dent measurements, and the uncertainties were determined to
(10% for 0.2 > Φf > 0.02 and (20% for 0.02 > Φf > 5 ×
10-3
.
Phosphorescence and steady-state anisotropy measurements
were performed in ethanol at 77 K. The samples are placed in
a 5 mm diameter quartz tube inside a Dewar filled with liquid
nitrogen. Two Glan-Thompson polarizers are placed in the
excitation and emission beams. The anisotropy r is determined
2. Experimental Section
as follows: r ) (IVV - gIVH)/(IVV + 2gIVH) with g ) IHV/IHH
,
where I is the fluorescence intensity. The subscripts denote the
orientation (horizontal H or vertical V) of the excitation and
emission polarizers, respectively. g is an instrumental correction
factor. The proper calibration of the setup was checked using a
recent standard method with rhodamine 101 in glycerol.49
Fluorescence lifetimes were obtained using a Nano LED
emitting at 344 nm as an excitation source with a nano led
controller module, Fluorohub from IBH, operating at 1 MHz.
The detection was based on a R928P type photomultiplier from
Hamamatsu with a high-sensitivity photon-counting mode. The
decays were fitted with the iterative reconvolution method on
the basis of the Marquardt-Levenberg algorithm.50 Such a
reconvolution technique51 allows an overall-time resolution
down to 0.2 ns. The quality of the exponential fits was checked
using the reduced ꢁ2 (e1.2).
2.1. Compounds. Compounds PhPy1 to PhPy4, twisted by
introduction of crowded alkyl groups on the pyridine ring, are
obtained by protonation of their corresponding zwitterion forms
whose synthesis and characterization are detailed elsewhere.45,46
The addition of stoichiometric amount of trifluoroacetic acid
leads to the quantitative formation of the phenol form. The
complete protonation of each chromophore was indeed checked
upon the total disappearance of the last absorption band of the
zwitterion.47 Hence, the counterion of the cationic chromophores
is CF3COO-. The presence of the crowded tert-butyl substituents
in ortho positions of the phenol group prevents self-aggregation,
which was confirmed by checking the Beer-Lambert law in
the millimolar concentration range.
2.2. Solvents. The solvents used for absorption and emission
analysis are as follows: butyl ether (BOE), ethyl ether (EOE),
butyl acetate (BuAc), ethyl acetate (EtAc), butyl chloride
(nBuCl), tetrahydrofuran (THF), dichloromethane (CH2Cl2),
propionitrile (PPCN), acetonitrile (ACN), ethanol (EtOH),
methanol (MeOH), 2-propanol (2-propanol), trifluoroethanol
(TFE). All the solvents employed were Aldrich or Fluka
spectroscopic grade. The absorption and fluorescence of all
solvents were checked for impurities and have been subtracted
from the sample spectra.
Semiempirical quantum chemical calculations were performed
using AM1 method (from HYPERCHEM package52).
3. Results and Discussion
3.1. Absorption and Emission Spectra in Acetonitrile.
Room temperature absorption bands and fluorescence spectra
of compounds are illustrated in Figures 1 and 2. Absorption
and emission data are collected in Tables 1-3. The red-energy
side of the absorption spectrum exhibits a distinctive structure-
less band that is blue-shifted from PhPy1 to PhPy4. This band
shows a concomitant decrease of its intensity that is divided by
a factor 10 (Table 2). By analogy with D-A biphenyls
previously studied,21,53-55 this well separated band can be
2.3. Instrumentation and Procedures. The absorption
measurements were carried out with a Perkin-Elmer Lambda 2
spectrometer. A FluoroMax-4 spectrofluorometer was used for
the fluorescence and phosphorescence measurements. All emis-
sion spectra were corrected for instrumental response and were
converted from the recorded wavelength scale If(λf) to a linear
energy scale according to the relationship If(νf) ) If(λf) × λf2.
The fluorescence quantum yields include the correction due to
solvent refractive index and were determined relative to quinine
1
ascribed to the CT r S0 transition from the phenol fragment
to the pyridinium one. The steady-state excitation anisotropy
spectra depicted in Figure 3 confirm the existence of this
transition: For all chromophores, the anisotropy spectrum clearly

Twisted Phenol-Pyridinium Biaryl Series
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2403
Figure 2. Normalized fluorescence spectra of biaryl derivatives in ethyl ether, ethyl acetate, and acetonitrile (from left to right, respectively).
displays a plateau below 33 000 cm-1 with constant values of
0.34 ( 0.01, 0.35 ( 0.01, 0.35 ( 0.02, and 0.37 ( 0.02 for
PhPy1 to PhPy4, respectively. Moreover, in the high-energy
side of the spectrum, the anisotropy decreases and reaches
another plateau in the 36 000-37 500 cm-1 region, indicating
the presence of an other transition. This transition can be
r S0 (M1La) and the contribution stemming from the direct
1
interaction between the CT state and the ground state), the
transition moment can be expressed as follows:60
V0(µe - µg)
Mabs ≈ c1
+ c2M1La
CT
1
assigned to the La r S0 one corresponding to the A band
hcν
c1 + c2 ) 1
(2)
abs
located around 36 500 cm-1 in acetonitrile. In the same manner,
the B band observed in the high-energy part of the absorption
spectrum (Figure 1) can be assigned to the 1Bb r S0 transition.
Interestingly, the progressive alkylation of the pyridinium moiety
leads to a red-shift of this B band and suggests that the transition
involves molecular orbitals centered on the pyridinium subunit.
A detailed comparison of the absorption spectra for each
2
2
The first term of this approximation corresponds to the Mulliken
two-state model56 where (µe - µg) denotes the difference of
dipole moments between ground and excited states. V0 corre-
sponds to the electronic coupling element between the 1CT state
and the ground state. According to Dogonadze et al.,61 V0 can
be determined by the interactions between the carbons atoms
forming the A-D bond as
1
compound shows that the decrease of the CT r S0 band is
1
related to a slight growth of the La r S0 one. This effect
1
illustrates a possible electronic coupling of the La state with
the low lying 1CT state, which exhibits the same 1A symmetry.
The electronic transition dipole moments can be used to
quantify the electronic coupling between the 1CT and S0 states
in the framework of Mulliken’s two-state model56 or between
V0 ) cDcAꢀDA cos ΘAD
(3)
1
1
the CT and close lying LE state with the same symmetry
according the intensity “borrowing” concept proposed by
Murell.57 Moreover, the electronic transition moment relative
to the CT state constitutes a suitable variable to derive
information about the change of the twist angle ΘA-D between
the ground and excited state. The transition dipole moment Mabs
relative to the 1CT r S0 absorption can be calculated according
to the expression58,59
where ΘAD is the twist angle between the planes of the acceptor
and donor fragments, ꢀAD is the resonance integral of the two
neighboring carbon atoms (ꢀAD ) -2.39 eV for benzene carbon
atoms62,63), cD and cA are the LCAO coefficients of the pπ atomic
orbital relative to the HOMO and LUMO respectively. These
latter coefficients can be obtained by the AM1 method. With
AM1 cD and cA coefficients (0.46 and 0.56 as calculated for
isolated 2,5-di-tert-butylphenol and N-methylpyridinium chro-
mophores), one can estimate V0 coupling constants ranging from
ca. 0.29 eV for PhPy4 to ca. 0.55 for PhPy1 compounds (see
Table 2). The estimated V0 allow prediction of transition dipole
moments Mabs by taking into account 1CT r S0 transition
energies and differences between µe and µg dipole moments. In
the case of the investigated species, electron transfer takes place
from the phenolic oxygen to the nitrogen atom of the N-
methylpyridinium kernel with a charge-shift distance of ca. 6-8
Å and µe - µg difference as high as 30-40 D, correspondingly.
Using the first right-hand term of relation 2 (c1 ∼1), one can
finally calculate Mabs expected values as collected in Table 2
(estimated with µe - µg ) 35 D). From the plot of Mabs vs
3hc103 ln 10
8π3NAn
2
Mabs
)
ε(ν ) d(ln ν )
(1)
∫
A
A
where ε(νΑ) corresponds to the molar extinction coefficient of
the CT band at wavenumber νΑ; n, h, c, and NA are the refractive
index of the solvent, the Planck constant, the speed of light,
and the Avogadro number, respectively. The transition moments
are collected in Table 2. We can clearly notice that the transition
moment decreases from PhPy1 to PhPy4. If we assume that
the value of the transition dipole moments are provided from
two main components (the transition dipole moment of the 1La
cal
Mabs a very good linear correlation is obtained for all

2404 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Malval et al.
TABLE 1: Spectroscopic Data of PhPy 1-4 at Room
Temperature in Various Solvents: Absorption and
Fluorescence Transition Energy Maxima, Stocks Shifts, and
TABLE 2: Room Temperature Absorption Data for Biaryl
Compounds in Acetonitrile: Experimental and Calculated
Values of the Absorption Transition Dipole Moments
cal,
Band Half-Widths (All in cm-1
)
ΘΑ-D
,
V0,
Mabs
Mabs
,
CT
a
CT,b
ꢀ(νabs
)
νabs
eV
deg
eV
D
Dc
max
no.
solvent
νamax
νf
PhPy1
24040
∆νST
HWa
HWf
PhPy1
PhPy2
PhPy3
PhPy4
26400
14700
5600
3.60
3.71
3.82
3.88
28.3
43.7
53.6
65.5
0.55
0.46
0.39
0.29
5.3
4.3
3.6
2.6
5.8
4.7
3.0
2.0
1
2
3
4
5
6
7
8
BOB
EOE
BuAc
EtAc
THF
CH2Cl2
PPCN
ACN
MeOH
EtOH
2-propanol
TFE
28650
28740
29030
28990
28330
26740
28900
28990
28410
28170
28090
27930
27930
4610
4820
5670
5840
5240
3750
6630
6770
4980
4340
4250
5740
5100
3670
3810
4230
4030
4080
3240
3860
4030
4290
4130
4220
3740
3600
4070
3810
3860
3920
3860
3760
4200
4310
4070
3670
3300
4570
3870
23920
23360
23150
23090
22990
22270
22220
23430
23830
23840
22190
22830
2600
a Error is about: 5-10%. b Scatter of results: (0.01 eV.
c Experimental error: 0.1- 0.2 D.
TABLE 3: Solvent Effect on the Fluorescence Quantum
Yields, Decay Times, and Resulting Radiationless and
Radiative Rate Constants, and Electronic Transition Dipole
Moments of Biaryl Derivatives
9
10
11
12
13
knr × 107
kf × 107
Mflu
compd
PhPy1
solvent
Φf
τ, ns
s-1
s-1
D
nBuCl
EOE
0.016
0.47
0.13
0.11
<0.001
0.14
<0.2
5.1
1.7
1.6
<0.2
1.5
PhPy2
CH2Cl2
PPCN
ACN
EtOH
TFE
10.4
51.2
55.6
9.2
7.6
6.9
2.1
2.1
2.1
1
2
3
4
5
6
7
8
BOB
EOE
BuAc
EtAc
THF
CH2Cl2
PPCN
ACN
MeOH
EtOH
2-propanol
TFE
29590
29850
30030
30030
29410
27550
29760
29940
29500
29070
28900
28570
28740
23070
22620
21600
21360
21350
21690
19760
19760
22970
23640
24000
20390
21100
6520
7230
8430
8670
8060
5860
10000
10180
6530
5430
4900
8190
7640
4170
4430
4560
4640
4530
4030
4450
4530
4760
4720
4730
4340
4170
5340
4990
5060
4960
4940
4110
4760
4730
5140
4940
4750
4640
4910
57.3
9.3
2.6
PhPy2
PhPy3
PhPy4
EOE
0.007
0.28
<0.2
10.3
2.1
2.0
<0.2
1.6
<0.2
13.7
3.0
2.6
<0.2
1.9
<0.2
17.3
4.0
3.7
<0.2
2.5
CH2Cl2
PPCN
ACN
EtOH
TFE
7.0
45.2
48.3
2.7
2.4
1.7
1.2
1.4
1.2
0.051
0.034
<0.001
0.042
0.004
0.19
0.034
0.020
<0.001
0.028
0.003
0.20
9
10
11
12
13
59.9
2.6
1.5
EOE
CH2Cl2
PPCN
ACN
EtOH
TFE
EOE
CH2Cl2
PPCN
ACN
EtOH
TFE
5.9
32.2
37.7
1.4
1.1
0.8
0.9
1.0
0.9
nBuCl
PhPy3
1
2
3
4
5
6
7
8
BOB
EOE
BuAc
EtAc
THF
CH2Cl2
PPCN
ACN
MeOH
EtOH
2-propanol
TFE
30400
30770
30680
30770
29940
28010
30400
30770
30210
30030
29500
29410
29670
21980
21980
20920
20410
20490
21410
19310
19190
22570
23250
23830
20080
20880
8420
8790
9750
10360
9450
6600
11090
11580
7640
6780
5670
9330
8800
4780
4960
5100
5260
5120
4330
4700
5180
5290
5300
5290
4800
4530
5530
5180
4970
4820
5010
4030
4410
4360
6270
5320
4370
4390
4620
51.2
1.5
1.2
4.6
24.2
26.5
1.2
0.8
0.5
0.8
0.8
0.7
0.032
0.020
<0.001
0.031
38.8
1.2
1.1
9
10
11
12
13
attached to the pyridinium ring. The stronger the interaction
between 1CT and 1La states, the lower is energy of the 1CT state
and the lower is value of (hcνflu + hcνabs)/2, respectively. If the
nBuCl
PhPy4
1
2
3
4
5
6
7
8
BOB
EOE
BuAc
EtAc
THF
CH2Cl2
PPCN
ACN
MeOH
EtOH
2-propanol
TFE
31060
31300
31450
31450
30680
28490
30860
31250
30770
30400
30120
29500
30210
22580
22250
21210
20760
20680
21460
19460
19280
21290
23090
23730
20260
20870
8480
9050
10230
10690
9990
7030
11410
11970
9480
7310
6390
9240
9340
5750
5500
5410
5820
5770
4800
5250
5550
5840
5740
5610
4940
4940
5030
4810
4760
4700
4680
4090
4410
4320
5270
6720
5100
4340
4540
9
10
11
12
13
nBuCl
compounds (Figure 4). This agreement suggests that one can
neglect the contribution from mixing of the 1La and 1CT states
in all investigated molecules. A similar conclusion can be drawn
by comparing energies of the excited 1CT states calculated from
absorption and emission data. Nearly the same (hcνflu + hcνabs)/2
values (e.g., 3.12 ( 0.05 eV in acetonitrile solutions) are
characteristic for all four of investigated compounds, as could
be expected by assuming only the sterical effect of alkyl group(s)
Figure 3. Excitation anisotropies of D-A biphenyls in glassy matrix
of ethanol at 77 K (νem: 24 690 cm-1): PhPy1 (squares), PhPy2 (circles),
PhPy3 (triangle), PhPy4 (diamond).

Twisted Phenol-Pyridinium Biaryl Series
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2405
64π4
kf )
n3νf3Mf2
(6)
3h
The significant differences between Mf and Mabs values (Tables 2
and 3) confirm the strong electronic and geometrical relaxation in
the excited states. By analogy with (dimethylanilino)pyridinium
derivatives that have been mainly described by Fromherz et
al.,53,54,64 a precursor-successor model can be proposed to account
this photophysical behavior. The model involves dual fluorescence
from a locally excited (LE) species (resonance-type state stabilized
by mesomeric interaction) and a successor charge shift (CSh)
species with twisted geometry. Rettig et al.25,26 clearly demonstrated
from quantum calculations that the ionic D-A biphenyls undergo
an energetic barrierless twist relaxation toward the 90° energetic
minimum with charge shift whereas the neutral donor-acceptor
biphenyls exhibit a sizable rotation barrier in the S1 surface.
Moreover, a subsequent equilibration of the two excited states can
be observed. This rapid thermal equilibration between LE and CSh
states was observed for the 9-mesityl-10-methylacridinium cation66
for instance. In our case, thermal equilibration is also demonstrated
through a temperature dependence analysis. Figure 6 displays the
evolution of emission spectra of PhPy1 and PhPy4 in acetonitrile
upon an increase in temperature over the range 5-80 °C. The
emission band of PhPy1 decreases monotonically without spectral
shift whereas the fluorescence decrease for PhPy4 is accompanied
by a band blue shift. The proximity in energy of LE and CSh states
should promote a thermal population of a high lying LE state from
the relaxed CSh one; the relative ratio for the radiative rate constants
can be well described in terms of the Boltzmann expression with
an energy gap decreasing from ca. 7.5 ( 0.2 kJ mol-1 for PhPy1
to ca. 4.4 ( 0.2 kJ mol-1 for PhPy4. From PhPy1 to PhPy4, the
decrease of the energy gap between the LE and CSh states is in
line with their increasing geometrical equivalency due to the
additional help of pretwisting.
3.2. Analysis of the Solvent Induced Spectral Shift. Solvent
effects on the absorption bands do not follow the classical
Lippert-Mataga67,68 behavior. Despite a significant change of
the dipole moment upon excitation, the CT absorption band
seems slightly sensitive to solvent polarity. This suggests that
specific solute-solvent interactions are not negligible in the
ground state. At the S1 state, the dipole-dipole interactions
become significant since better correlations can be derived.
Therefore, to rationalize all the solute-solvent interactions
at both the ground and excited states, we use the empirical
Kamlet-Taft solvatochromic parameters:69,70 π*, R, and ꢀ. The
values of solvent parameters are collected in Table 4. Parameter
π* is a measure of the nonspecific solvent polarity/polarizability,
whereas R and ꢀ indicate the solvent hydrogen bond (HB)
donating and accepting properties. The spectral shift can
therefore be linearly correlated with these parameters by
Figure 4. Correlation between experimental and calculated values of
the absorption transition dipole moments.
1
1CT state would strongly interact with the La state, one could
expect well pronounced changes in (hcνflu + hcνabs)/2 as well.
Protolytic properties of the chromophores are also strongly
influenced by pretwisting. The pKa relative to the phenolate-
phenol equilibrium was determined by the spectrophotometric
method upon addition of CF3COOH to solutions of acetonitrile
with the corresponding zwitterion. The pKa gradually increases
from a value of ca. 9.10 for PhPy1 to 12.70 for PhPy4. This
tendency is clearly assigned to a charge-transfer enhancement
from the phenol group to the pyridinium one. In the excited
state, such an effect will be investigated hereafter.
The fluorescence spectrum in acetonitrile consists of a
structureless band that is red-shifted and broadens out from
PhPy1 to PhPy4 (Figure 2). The Stokes shift is markedly
increasing with a value of ca. 6770 cm-1 for PhPy1 to a larger
one of ca. 11 970 cm-1 for PhPy4. This hints at a significant
electronic and/or geometrical change between the ground and
excited state. For such ionic compounds, fluorescence can be
attributed to a relaxed charge transfer or more precisely charge
shift state23,25-27,54,64 (CSh) that is connected to a twisted
geometry. Figure 5 shows the fluorescence decays of compounds
in acetonitrile. It should be stressed that, under our time
resolution, decays are monoexponential at all monitoring
wavelengths and do not contain any contribution of long-lived
component. From a first approximation, if we assume an
irreversible charge shift state formation, the radiative and
radiationless rate constant can be determined as follows:
Φf
kf )
(4)
(5)
τ
(1 - Φf)
ν ) ν0 + pπ* + aR + bꢀ
(7)
knr
)
τ
The experimental values of kf show a significative decrease from
ca. 6.9 × 107 s-1 for PhPy1 to ca. 0.5 × 107 s-1 for PhPy4 (Table
3). Hence, sterically hindered derivatives acquire a forbidden
emissive character in accordance with the population of highly
twisted conformation. Interestingly, a methodical increase of knr is
observed by releasing sterical restriction around the central single
bond, this should be ascribed to a much larger amplitude relaxation
toward the twisted geometry. The electronic transition dipole
moments Mf are calculated according to the expression65
Coefficients p, a, and b are related to solute properties: p refers
to the solute dipole moment, a evaluates its ability to donate a
HB to the solvent, b measures the tendency of the solute to
accept a HB from the solvent. Excluding haloaliphatic solvents
which deviate from correlation, multilinear regressions to the
absorption and emission data lead to coefficients listed in Table
5. Figure 7 displays the correlation between the calculated
maxima of fluorescence as a function of the experimental values
in various solvents.

2406 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Malval et al.
Figure 5. (a) Time fluorescence decay of biaryl derivatives in acetonitrile; instrumental response function (IRF). (B) Residual graphs relative to
monoexponential fits.
Figure 6. Temperature effect on the fluorescence spectra of PhPy1 (A) and PhPy4 (B). Inset: Arrhenius plot for the emission quantum yield
(solvent: acetonitrile).
TABLE 4: Values of Kamlet and Taft Parameters of
Various Solvents
hydrogen, the presence of a negative partial charge on the
oxygen atom should promote mesomeric interactions and lead
no.
solvent
π*
R
ꢀ
to a more planar geometry. The values of the twist angles
calculated for the corresponding zwitterion forms47 confirms
such a planarization tendency. The absorption CT band is
consequently shifted to the low-energy side of the spectrum.
Second, in the excited state, an important contribution for the
fluorescence band shift stems from solute-solvent polarity
interactions. The stabilizing effect (p < 0) increases with steric
hindrance around the phenol-pyridinium bond. We have here
another confirmation that enhanced twist angle provides a
charge-transfer enhancement at excited states. Third, the H-
bond-accepting property of the solvent contributes to a strong
destabilization of the excited state (b > 0). This effect is very
pronounced for highly twisted compound. For instance, the
contribution of the solvent basicity accounts for 37% of the total
spectral shift for PhPy1; this value rises to 52% for PhPy4. As
previously observed for PhPy1,71 the energy level of the emitting
excited states rises by increasing the solvent basicity. Therefore,
we confirm and extend such an atypical effect to more twisted
1
2
3
4
5
6
7
8
BOB
EOE
BuAc
EtAc
THF
CH2Cl2
PPCN
ACN
MeOH
EtOH
2-propanol
TFE
0.18
0.27
0.51
0.55
0.58
0.82
0.64
0.75
0.60
0.54
0.48
0.73
0.40
0
0
0.06
0.06
0
0.46
0.47
0.45
0.45
0.55
0.10
0.37
0.40
0.66
0.75
0.90
0
0.30
0
0.19
0.98
0.86
0.76
1.51
0
9
10
11
12
13
nBuCl
0
Three important points can be noted. In the ground state, first
the solvent induced spectral shift is clearly governed by the ꢀ
parameter for all compounds. We can observe that increasing
solvent basicity induces a bathochromism in the CT band (b <
0). Consecutive to the lability enhancement of the hydroxylic

Twisted Phenol-Pyridinium Biaryl Series
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2407
TABLE 5: Kamlet-Taft Coefficients of Chromophores
S0
S1
p (cm-1
)
a (cm-1
)
b (cm-1
)
r
p (cm-1
)
a (cm-1
)
b (cm-1
)
r
PhPy1a
PhPy2
PhPy3
PhPy4
120
10
-480
-480
130
160
550
240
-2820
-2390
-3200
-2720
0.976
0.908
0.889
0.864
-3140
-5810
-5240
-5540
860
1470
1730
140
2340
3860
4410
6050
0.954
0.988
0.993
0.980
a From ref 72.
Figure 7. Plots of the calculated maxima of fluorescence as function of the corresponding experimental maximum for various solvents.
compounds. Scheme 2 depicts the sequential steps leading to
the deactivation of the excited biaryl derivatives. Because of a
very efficient nonradiative deactivation pathway associated with
the phenolate-like species generated from ESPT and with the
assumption that this latter process constitutes the major relax-
ation channel from both LE and CSh states, this ꢀ dependence
effect should be ascribed to a dramatic increase of the excited-
state proton-transfer rate for highly twisted conformation (i.e.,
kpt , kpt in Scheme 2). Such a differential aspect can be
explained by a better solvent stabilization of the excited twisted
phenolate form (ESPTCSh) with respect to that of the more planar
one (ESPTLE). It should be stressed that the fluorescence band
hypsochromy upon increasing solvent basicity is an indirect
effect of the differential deactivation of the excited twisted
species.
Moreover, the evolution of the fluorescence quantum yields
(Φf) and fluorescence lifetimes in various solvent confirms that
the radiationless process is strongly correlated to the basicity
of the solvent (Table 3): for instance, in ethanol, Φf exhibits
very weak values below 10-3 whereas in trifluoroethanol, a non-
H-bond accepting solvent, Φf is at least multiplied by a factor
30 for PhPy4 to a factor 140 for PhPy1.
3.3. Fluorescence Properties in Glassy Matrix (77 K). To
suppress the radiationless processes that are linked with the
intramolecular twisting relaxation, fluorescence measurements
are performed in a rigid medium. Figure 8A shows the
fluorescence spectra of biaryl derivatives in a glassy matrix of
ethanol at 77 K and Table 6 gathers the corresponding
spectroscopic data. Each spectrum can be divided into three
parts. First, the high-energy side of the spectrum corresponds
to the emission of the LE state. The band is clearly blue-shifted
with respect to that observed at room temperature. The band
intensity is strongly temperature dependent since its value is
multiplied by a factor 100 for PhPy1 to a factor 500 for PhPy4.
Figure 9 shows the corresponding LE fluorescence decays for
each compound. The fluorescence lifetimes that initially exhibit
short values below 200 ps at room temperature are strongly
increased in the glassy matrix with sizable values ranging from
2.20 ns for PhPy1 to 4.70 ns for PhPy4. Therefore, we have
clear evidence that the radiationless processes associated with
the conformational change in the excited state are mainly
suppressed in rigid medium.
LE
CSh
Second, the middle part of the spectrum exhibits a distinctive
band. According to the time gated measurements, such a

2408 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Malval et al.
SCHEME 2: Mechanism of Stepwise Photoreaction with Radiative and Radiationless Deactivation of the Primary (LE)
and Secondary (CSh) Rotamer
structured band is assigned to the phosphorescence emission
(inset Figure 8A). For all derivatives, the phosphorescence
lifetime (τp) is longer than 1 s (as collected in Table 6) and
suggests a ππ* character for the emitting triplet state. The
phosphorescence exhibits a vibronic structure whose associated
wavenumber is in 1200-1300 cm-1 range. Such vibrational
wavenumbers correspond to the symmetric interaryl stretching
modes.72 Whereas the gradual increase of the twist angle ΘAD
leads to a slight red shift of the fluorescence band, the
phosphorescence one is clearly blue-shifted with a concomitant
increase of its relative spectral contribution. Hence, the spin-orbit
coupling is promoted by a perpendicular conformation. This
was also observed for twisted biphenyls that exhibit higher kISC
than planar ones.27,73 Indeed, the theoretically spin-forbidden
T1 f S0 transition is assumed to be strengthened through
vibronic mixing and singlet-state mixing with polarization
normal to the molecular plane.74
Finally, the red-energy side of the spectrum clearly shows a
broad band. It should be stressed that this band is totally
invariant to the dilution effect, which excludes the emission of
aggregates. The excitation spectrum is in excellent agreement
with that collected from the LE band and both excitation
anisotropy spectra are also equivalent. Therefore, we have clear
indications that the red emitting species stems from the LE state.
We propose that this new band is ascribed to the radiative
deactivation of a phenolate-like species produced by an excited-
state proton tranfer process from the LE state. The fluorescence
of the conjugated phenolate base can be detected in a rigid
medium, as displayed in Figure 10A. Such broad bands are also
located in the same spectral region but are markedly blue-shifted
with respect to that of their corresponding acid forms. This
spectral difference should be assigned to a more planar geometry
for the LE state of the phenolate derivatives. The mesomeric
interactions are indeed more efficient for the zwitterion species.
Moreover, the phenolate-like species generated by ESPT exhibit
long fluorescence lifetimes ranging from 3.93 ns for PhPy1 to
6.93 ns for PhPhy4, as can be derived from the fluorescence
decay curves depicted in Figure 9. These values are quite
comparable with that obtained for the zwitterion species (see
Table 6). Interestingly, we can note in Figure 8A that the
increase of the twist angle ΘAD leads to a red shift of the ESPTLE
band with a clear increase of the ESPT-to-LE quantum yield
ratio (see also Figure S1, Supporting Information). As shown
in Table 6, this ratio is multiplied by a factor of 5 from PhPy1
to PhPh4. In the glassy matrix, the photoinduced intramolecular
twisting relaxation is mainly suppressed. Therefore, only the
sequential steps inherent in the deactivation of the LE state are
occurring here (left side of Scheme 2). With the assumption
Figure 8. Emission spectra of biaryl derivatives in glassy matrix of
(A) ethanol (77K) and (B) CH2Cl2/ACN, 4v/1v mixed solvents (dashed
lines: deconvoluted phosphorescence spectra). Insets: normalized time
gated phosphorescence spectra (delay: 5 ms).

Twisted Phenol-Pyridinium Biaryl Series
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2409
TABLE 6: Photophysical Properties in a Glassy Matrix of Ethanol and Mixed Solvents of CH2Cl2/ACN at 77 K
Zwi
S1
S1
T1
a
νLE (cm-1
)
νESPT (cm-1
)
τLE (ns)
τESPT (ns)
ΦESPT/ΦLE
τd (ns)
νP (cm-1
)
τP (s)
ν00 (cm-1
)
∆
(kJ mol-1
)
S-T
EtOH (77 K)
0.20
PhPy1
PhPy2
PhPy3
PhPy4
25380
24810
24750
24940
19575
17730
16920
16640
2.20
3.16
4.56
4.70
3.54
4.85
5.35c
6.25c
3.20
5.60
6.30
7.10
20530
21185
22450b
22780b
3.01
2.51
1.87
1.84
1250
1300
1200
1270
58.1
43.4
27.6
25.9
0.28
0.43
0.92
CH2Cl2/ACN (77 K)
PhPy1
PhPy2
PhPy3
PhPy4
25730
25000
24250
24850
3.13
3.62
4.95
5.72
4.00
4.80
6.80
7.25
20800
21400
22480
22750
0.87
1.65
1.55
1.45
1310
1290
1220
1190
59.0
43.1
21.2
25.2
a Ratio calculated after subtraction of phosphorescence emission. b Fit of the spectral profile to four Gaussians functions. c Presence of a short
time component (τ < 0.2 ns) that represents less than 5% of the decay. d Fluorescence lifetimes of the zwitterion species (Zwi).
Figure 9. Time fluorescence decays of the LE and ESPT states of each biaryl derivative in a glassy matrix of ethanol (77 K). Insets: residual
graphs relative to monoexponential fits.
that the ratio of radiative rate constants relative to LE and
ESPTLE states does not change significantly from PhPy1 to
PhPhy4, the correlation observed between the increase of the
twist angle ΘAD and the relative growth of the ESPTLE band
can be assigned to the increase of kptLE. In good accordance
Kamlet-Taft analysis, we have shown that a strongly twisted
conformation leads to the enhancement of the acidity of the
excited state.
It is noteworthy that ESPT is strongly dependent on the
H-bond-accepting property of the solvent. To illustrate such a
pre-eminent factor, we have performed the same experiment in
a glassy matrix of CH2Cl2/ACN (4v/1v) mixed solvents. The
basicity of the medium should be strongly reduced due to the
presence of a large amount of dichloromethane. At room
temperature, the fluorescence bands of the biaryl compounds
can be used to probe the ꢀ parameter of this binary solvent
mixture. According to the Kamlet-Taft linear correlations that
were previously derived for each chromophore, the ꢀ parameter
was then estimated to a value of ca. 0.16. This value, which is
intermediate to that of acetonitrile and dichloromethane, con-
firms the low basicity of the medium as compared to ethanol,
which exhibits a ꢀ of ca. 0.75. Figure 8B presents the emission
spectra of the compounds in a glassy matrix of CH2Cl2/ACN.
As previously observed for ethanol, the fluorescence bands
undergo a shift to the high-energy region with a noticeable
intensity enhancement with respect to the fluorescence emission
at room temperature. For instance, the fluorescence intensity
for PhPhy1 and PhPy4 is multiplied by a factor 10 and 19,
respectively. However, these enhancement factors are consider-
ably lower than that measured in ethanol, which indicates that
the nonradiative deactivation processes associated with confor-
mational change at the excited state (e.g., ESPTCSh) are strongly
reduced in CH2Cl2/ACN. Moreover, on going from PhPy1 to
PhPh4, the fluorescence undergoes a strong band broadening.
This spectral effect is ascribed to the increasing contribution of
the red emitting phosphorescence whose spectra are displayed
in Figure 8B. The phosphorescence bands were extracted from
the global emission spectra using a time gated method (see

2410 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Malval et al.
reaction. This latter effect is responsible for a strong decrease
of the proton-transfer rate constant in a glassy matrix. According
to the activation model formalized by Agmon and Levine,75,76
the intrinsic free-energy barrier for proton transfer (Ga) is
modified by the total free-energy change (∆G) so that the rate
of proton transfer (kptLE) may be correlated to an Arrhenius plot:
Ga
LE
pt
k
) k°pLt E exp -
(8)
(
)
RT
with
Ga ) ∆G - Ga#
(9)
°LE
k
and Ga#, which depend on the nature of the solvent, denote
pt
the intrinsic proton-transfer rate and an intrinsic barrier,
respectively. Hence, the role of specific solute-solvent interac-
tion is all the more decisive in a glassy matrix since it constitutes
the ultimate driving force for the promotion of the breaking or
formation of hydrogen bonds that should contribute to a decrease
of the activation barrier. In a glassy matrix of CH2Cl2/ACN,
due to the weak basicity of the medium that does not allow
any relevant specific solute-solvent interaction, it is clear that
the rate of proton transfer is too weak to observe any emission
from the phenolate-like species. However, in glassy ethanol,
solvent molecules in close contact with the excited chromophore
(R*OH) should mediate the proton-transfer reaction by the
formation a solvent-ion pair R*OH · · ·S intermediate that
promotes the proton dissociation. This mediating effect, which
mainly stems from the H-bond-accepting property of ethanol,
should lower the activation barrier for proton transfer and lead
to the emission of the phenolate-like species.
Figure 10. Emission spectra of zwitterion forms of PhPy1 (squares),
PhPy2 (circles), PhPy3 (triangle), and PhPy4 (diamond) in a glassy
matrix of (A) ethanol and (B) CH2Cl2/ACN, 4v/1v (77 K).
4. Conclusion
Supporting Information). Finally, the emission from the phenolate-
like species was not detected whatever the biaryl derivative.
This contrasts with the sizable emission of the conjugated
phenolate base observed in the low-energy region and displayed
in Figure 10B. Such a result indicates that excited-state proton
transfer does not occur in a glassy matrix of CH2Cl2/ACN. The
suppression of ESPT even for highly twisted compounds that
exhibit an enhanced excited-state acidity suggests that proton
transfer to solvent is clearly dependent on the nature of the
solvent and on its H-bond-accepting propensity more particu-
larly. The mechanism of excited-state proton transfer is indeed
the result of successive fast steps32,35,40 such as both solvation
of the excited species (R*OH) and byproduct, hydrogen bond
formation between R*OH and a specific solvent molecule,
proton dissociation, and diffusion. Such processes proceed
through distinctive time regimes. For instance, the excited-state
proton-transfer dynamics from 8-hydroxypyrene-1,3,6-trisul-
fonate (HPTS)36 to water is composed of a first step of ∼0.3 ps
corresponding to the solvation of HPTS in the excited state, a
second step of 2.2 ps for the solute-solvent hydrogen bond
formation (R*OH· · ·S), and a longer time process of 87 ps
within the proton is dissociated and diffuses. In a glassy matrix,
solvation processes are mainly suppressed, which should
particularly impact the stabilization of the ESPT level,40 whose
energy is intimately related to the solvation of the proton and
the phenolate-like species.30,43 As a consequence, the energy
gap between LE and ESPT levels should drastically decrease
as well as the exothermic character of the proton-transfer
The photophysical properties of a donor-acceptor biaryl
family that incorporates a phenol and a pyridinium moiety into
gradually twisted conformations have been investigated. In the
ground state, the increase of the twist angle ΘAD leads to a
progressive blue shift of the absorption CT band with a
concomitant decrease of its intensity. Such an effect is well
addressed within the framework of the Mulliken-Murrel. model
which indicates that this approach is applicable in charge-shift
process. In the excited state, a rapid charge transfer occurs within
the LE state to generate the corresponding CSh state with a
highly twisted geometry. The fluorescence emission and emis-
sivity of biaryl compounds are strongly solvent dependent. The
correlation of the solvent induced spectral shift with Kamlet-Taft
parameters first confirms the role of the nonspecific dipole-dipole
interaction but also indicates a strong ꢀ dependence. The ꢀ
contribution is increasing with twist angle ΘAD and leads to an
unusual destabilization of the excited emitting state. This effect
was indirectly ascribed to a differential nonradiative deactivation
of LE and CSh states. An excited-state proton-transfer process
is proposed as the major radiationless pathway of both excited
states, which leads to a nonemissive phenolate-like form in
liquid medium. In a rigid glassy matrix, despite the absence of
the intramolecular twisting relaxation, the fluorescence of the
phenolate species is detected and confirms the ESPT from the
LE state. The relative increase of the phenolate band with twist
angle ΘAD suggests that the enhancement of the excited states
acidity can be triggered by intramolecular twisting change.

Twisted Phenol-Pyridinium Biaryl Series
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2411
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