Synthesis, photophysics and nonlinear optical properties of stilbenoid pyrimidine-based dyes bearing methylenepyran donor groups

Article abstract of DOI:10.1002/cphc.201300419

The nonlinear properties and the photophysical behavior of two π-conjugated chromophores that incorporate an electron-deficient pyrimidine core (A) and γ-methylenepyrans as terminal donor (D) groups have been thoroughly investigated. Both dipolar and quadrupolar branching strategies are explored and rationalized on the basis of the Frenkel exciton model. Even though a cooperative effect is clearly observed if the dimensionality is increased, the nonlinear optical (NLO) response of this series is moderate if one considers the nature of the D/A couple and the size of the chromophores (as measured by the number of π electrons). This effect was attributed to a disruption in the electronic conjugation within the dyes' scaffold for which the geometry deviates from planarity owing to a noticeable twisting of the pyranylidene end-groups. This latter structural parameter also has a strong influence on the excited-state dynamics, which leads to a very efficient fluorescence quenching. Copyright

Full text of DOI:10.1002/cphc.201300419

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Let’s get (photo)physical! The nonlin-
ear properties and the photophysical
behavior of a series of p-conjugated
chromophores that incorporate an elec-
tron-deficient pyrimidine core (A) and g-
methylenepyrans as terminal donor (D)
groups are thoroughly investigated.
S. Achelle,* J.-P. Malval,* S. Aloꢀse,
A. Barsella, A. Spangenberg, L. Mager,
H. Akdas-Kilig, J.-L. Fillaut, B. Caro,
F. Robin-le Guen
&& – &&
Synthesis, Photophysics and Nonlinear
Optical Properties of Stilbenoid
Pyrimidine-Based Dyes Bearing
Methylenepyran Donor Groups
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DOI: 10.1002/cphc.201300419
Synthesis, Photophysics and Nonlinear Optical Properties
of Stilbenoid Pyrimidine-Based Dyes Bearing
Methylenepyran Donor Groups
Sylvain Achelle,*[a] Jean-Pierre Malval,*[b] Stꢁphane Aloꢂse,[c] Alberto Barsella,[d]
Arnaud Spangenberg,[b] Loic Mager,[d] Huriye Akdas-Kilig,[e] Jean-Luc Fillaut,[e]
Bertrand Caro,[a] and FranÅoise Robin-le Guen[a]
The nonlinear properties and the photophysical behavior of
two p-conjugated chromophores that incorporate an electron-
deficient pyrimidine core (A) and g-methylenepyrans as termi-
nal donor (D) groups have been thoroughly investigated. Both
dipolar and quadrupolar branching strategies are explored and
rationalized on the basis of the Frenkel exciton model. Even
though a cooperative effect is clearly observed if the dimen-
sionality is increased, the nonlinear optical (NLO) response of
this series is moderate if one considers the nature of the D/A
couple and the size of the chromophores (as measured by the
number of p electrons). This effect was attributed to a disrup-
tion in the electronic conjugation within the dyes’ scaffold for
which the geometry deviates from planarity owing to a noticea-
ble twisting of the pyranylidene end-groups. This latter struc-
tural parameter also has a strong influence on the excited-
state dynamics, which leads to a very efficient fluorescence
quenching.
1. Introduction
In recent years, there has been a rapid progress in the devel-
opment of nonlinear optical (NLO) materials, which owes to
their great impact and promising potentials in a wide range of
applications such as multiphoton microscopy,[1] photodynamic
therapy,[2] optical limiting,[3] multiphoton fabrication[4] and high
density optical data storage.[5] A fundamental issue that consti-
tutes the cornerstone of all these applications concerns the
elaboration of valuable design strategies to tune the NLO re-
sponse of the materials in a methodical manner. Some key
principles and structure–property relationships have been
clearly identified especially for organic materials.[6] One of the
simplest integrated systems consists of an electron-donor (D)
and an electron-acceptor (A) group that are connected togeth-
er by using a p-conjugated bridge as an “electron relay”.
Within this prototypal dipolar configuration, several parameters
account for the choice of the central p-conjugated fragment:
1) the nature of linkers (double vs triple bonds),[7] 2) the size of
the bridge as measured by the number of p electrons,[8] 3) the
donor or acceptor character of the bridge, 4) the inherent ri-
gidity or flexibility of the bridge and 5) the aromatic character
of the bridge. Several p-spacers have been successfully em-
ployed, examples are: phenylene-vinylene,[9] phenylene-ethyny-
[a] Dr. S. Achelle, Prof. B. Caro, Prof. F. Robin-le Guen
Institut des Sciences Chimiques de Rennes
UMR CNRS 6226, IUT de Lannion
rue Edouard Branly, BP 30219
22302 Lannion Cedex (France)
E-mail: sylvain.achelle@univ-rennes1.fr
[b] Dr. J.-P. Malval, Dr. A. Spangenberg
Institut de Science des Matꢁriaux de Mulhouse
UMR CNRS 7361, Universitꢁ de Haute-Alsace
15 rue Jean Starcky ,68057 Mulhouse (France)
E-mail: jean-pierre.malval@uha.fr
lene,[7] fluorene,[10] dithienothiophene[11] and porphyrin.[12]
A
second fruitful strategy involves an increase in the dimension-
ality of the molecule with a recursive implementation of sever-
al D-p-A chromophores into a multibanched configuration[13]
present in quadrupoles,[9b,14] octupoles[13,14c,15] or dendrimers.[16]
Owing to a cooperative interaction among each arm, the mag-
nitude of the resulting NLO response is found to be much
greater than the sum of the discrete NLO responses.
[c] Dr. S. Aloꢀse
Laboratoire de Spectrochimie Infrarouge et Raman
UMR CNRS 8516
Universitꢁ des Sciences et Technologies de Lille
59655 Villeneuve d’Ascq Cedex (France)
[d] Dr. A. Barsella, Dr. L. Mager
Dꢁpartement d’Optique ultra-rapide et Nanophotonique
IPCMS-CNRS 23 Rue du Loess, BP 43
67034 Strasbourg Cedex 2 (France)
In line with this branching strategy, this paper presents the
photophysical feature of two chromophores that result from
the molecular association of a new D/A couple linked by flexi-
ble phenylene-vinylene bridges (Scheme 1). The acceptor sub-
unit is a substituted pyrimidine ring (1,3-diazine) that has been
extensively used as a building block for the synthesis of func-
tionalized p-conjugated NLO materials.[17] The structural shape
[e] Dr. H. Akdas-Kilig, Dr. J.-L. Fillaut
Institut des Sciences Chimiques de Rennes UMR CNRS 6226
Campus de Beaulieu, 263 av. du Gꢁnꢁral Leclerc
35042 Rennes (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201300419.
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Scheme 2. Reaction scheme for the formation of 3. i) 4-(diethoxymethyl)-
benzaldehyde, Aliquat 336, aqeous NaOH (5m), 2 h, D then HCl, acetone,
5 min at room temperature ii) nBuLi, THF, 2 h, T=ꢀ788C!RT.
Scheme 1. Molecular structures of chromophores. Representation of the ef-
fects of the interbranch coupling on the electronic levels on going from 3 to
5.
of the investigated pyrimidines gives rise to either one-dimen-
sional or two-dimensional D-p-A or D-p-A-p-D type molecules,
respectively. The donor terminal group is a g-methylenepyran
system, which is considered as a “proaromatic” donor[18] be-
cause it gains aromaticity through a charge transfer process.
This effect influences the degree of mixing between the qui-
noidal and zwitterionic limiting resonance forms of the ground
state. As previously emphasized[19] the NLO properties are di-
rectly connected to the optimization of this latter degree of
mixing. In the present contribution, we show that the twist
angle within stilbenoid pyrimidine-based dyes bearing methyl-
enpyran donor groups has a strong influence on the NLO
properties but also drives the major relaxation process at excit-
ed state in this series.
Scheme 3. Reaction scheme for the formation of 5. i) 4-(diethoxymethyl)-
benzaldehyde, Aliquat 336, aqeous NaOH (5m), 2 h, D then HCl, acetone,
5 min at room temperature ii) nBuLi, THF, 2 h, ꢀ788C!RT.
state and can be stored without the need for special precau-
tions.
2.2 Electronic and Linear Absorption Properties of the
Ground State
2. Results and Discussion
2.1 Synthesis
Figure 1 shows the absorption spectra of 3 and 5 in dichloro-
methane. Both dyes exhibit three absorption peaks at approxi-
mately l=255, 290 and >400 nm. Notably, a slight shoulder
located at the blue edge of the longest wavelength absorption
band is observed for compound 5. The UV absorption band at
The linear push-pull compound 3 has been obtained in two
steps from readily available 4-methylpyrimidine (Scheme 2).
The first step consists of a condensation reaction between 4-
methylpyrimidine and 4-(diethoxymethyl)benzaldehyde in boil-
ing aqueous NaOH (5M) by using Aliquat 336 as a phase-trans-
fer catalyst. This is followed by deprotection of the acetal
group in acidic media, which leads to compound 1 according
to the procedure initially described by Vanden Eynde.[20] The
second step consists of a Wittig reaction between aldehyde
1 and phosphonium salt 2[21], which leads to compound 3.[22]
Quadrupolar derivative 5 has been obtained by the same syn-
thetic procedure that starts from 4,6-dimethylpyrimidine
(Scheme 3). These materials are perfectly stable in the solid
1
l=255 nm should be attributed to the presence of the La and
1Lb electronic transitions centered on the 4-methylene-4H-
pyran moiety.[23] Interestingly, this band is 2-fold intensive for
the two-arm chromophore as compared to the dipolar one.
Such a linear correlation first indicates that, for chromophore
5, no intramolecular interaction at the ground state occurs be-
tween the pyranylydene groups, and the electronic conjuga-
tion between the 4-methylenepyran and the pyrimidine-based
fragments remains comparable to that observed for 3 despite
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hardly affects the planarity observed between the pyrimidine
and its styryl substituents, which results in hyperchromic and
bathochromic effects. The longest wavelength absorption
band that should encompass several p–p* electronic transi-
tions partially localized on the substituted pyrimidine moiety
shifts to the low energy side of the spectrum and undergoes
a 1.65-fold increase in intensity with emax of approximately
40900mꢀ1 cmꢀ1 and 67400mꢀ1 cmꢀ1 for compounds 3 and 5,
respectively.
Figure 3 displays the cyclic voltamogramms of the deriva-
tives in dichloromethane. Each compound exhibits an irreversi-
ble electrochemical oxidation wave with peaks at 0.73 V and
0.78 V versus the saturated calomel electrode (SCE) for 3 and
5, respectively. These anodic potentials should be attributed to
the oxidation of the methylenepyran group that probably
Figure 1. Absorption spectra of derivatives in dichloromethane.
the increase in dimensionality for the V-shaped structure. The
fully optimized structures calculated at density functional
theory (DFT) level are presented in Figure 2. Both dyes show
quite similar geometries. The 4-methylenepyran and the 4-styr-
Figure 3. Cyclic voltamogramms of 3 and 5 in dichloromethane and
(nBu)4NPF6 (0.1m) on a platinum electrode at 200 mVsꢀ1 (concentrations
within mM range).
leads to the formation of dimer byproducts as previously ob-
served for similar derivatives.[22,24] Interestingly, the oxidation
potentials of both compounds are quite comparable. This
effect should be connected to the equivalent degree of elec-
tronic conjugation between the pyran and styrylpyrimidine
subunits within both chromophores, which is consistent with
the equivalent twisted geometries derived from DFT calcula-
tions. It was previously established that the oxidation potential
of the methylenepyran group is strongly sensitive to the sub-
stituent on the exocyclic C=C bond. For instance, with
a phenyl substituent, Eox is approximately 0.73 V versus the
SCE, and strongly increases to 1.03 V versus the SCE with a 4-
pyridyl group.[25] Therefore, the oxidation potentials obtained
for 3 and 5 highlight the low communication between the two
heterocycle parts of the molecules.
Figure 2. Optimized geometry of chromophores (PBE0/6-31G(d) level).
ylpyrimidine fragments adopt a quasi-planar conformation but
are twisted between each other, which leads to a torsional
angle (V1 in Scheme 1) of approximately 288. Such a twisted
geometry is independent of the substitution effects on the
pyrimidine core. Moreover, increasing the 2-dimensionality on
going from a 4-(styryl) to a 4,6-bis(styryl)pyrimidine subunit
The cathodic part of both voltamogramms shows a quasi re-
versible reduction wave with a half-wave potential of approxi-
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mately ꢀ1.68 V versus the SCE for 3 and ꢀ1.53 V versus the
SCE for 5. This reduction wave corresponds to the formation of
a radical anion located on the pyrimidine moiety. It is strongly
shifted to the high potential region with respect to that of the
pyrimidine (i.e. ꢀ2.56 V vs the SCE).[26] Such an electrochemical
effect was previously observed for 4,6-dithienyl or 4,6-diaryl-
pyrimidine derivatives[26,27] and should be attributed to an effi-
cient stabilization of the generated radical anion, which owes
to a charge delocalization along the styryl branches. The ex-
tending charge delocalization within the multibranched chro-
mophore 5 stabilizes the radical anion all the more, and leads
to a further increase in the reduction potential (i.e. +150 mV)
with respect to that of 3.
dependent density functional theory (TD-DFT) method are also
displayed on top of each experimental spectrum. The electron-
ic distributions of the frontier orbitals are shown in Figure 5.
The calculated energies of the electronic S0!Sn transitions and
the experimental values of the 0–0 transition energies (E00) are
Figure 4 shows the steady-state excitation anisotropies of 3
and 5 along with their respective absorption and fluorescence
spectra recorded in a glassy matrix of 2-methyl tetrahydrofuran
(2 MTHF). The calculated absorption spectra by using a time-
Figure 5. Representation of the molecular orbitals involved in the lowest
energy electronic transitions of chromophores 3 and 5.
reported in Table 1. The vertical S0!S1 transition energies
agree well with the experimental E00 values; these are mea-
sured from the intercept of the normalized absorption and
fluorescence spectra recorded at T=77 K in a glassy matrix of
2MTHF. In the case of 3, the constant anisotropy within the
long wavelength absorption band (410–530 nm) is consistent
with the presence of a single electronic transition. The calcula-
tions indicate that this S0!S1 transition is a pure HOMO–
LUMO transition with a high oscillator strength (f>1). This
transition implies some delocalization of charge all along the
chromophore with a pronounced localization on the donor
group (methylenepyran) in the HOMO and on the pyrimidine
acceptor in the LUMO. Below lexc ꢁ400 nm, the anisotropy
strongly decreases, which suggests the presence of other elec-
tronic transitions with distinctive electronic symmetries.
The evolution of the excitation anisotropy spectrum of 5 is
less straightforward. On going from l=545 to 480 nm, the ani-
sotropy decreases sequentially from two narrow plateaus, then
undergoes a strong drop below l=475 nm and remains con-
stant in the l=400–330 nm range. These changes in the aniso-
tropy spectrum are consistent with TD-DFT calculations, which
predict the occurrence of two energetically close and strongly
allowed transitions that dominate the longest wavelength ab-
sorption band. The S0!S1 transition mainly corresponds to the
HOMO–LUMO (73%) transition, whereas the S0!S2 transition
implies a major contribution (70%) from the HOMOꢀ1-LUMO
(see Figure 5 for orbital symmetry). Both transitions clearly cor-
respond to conjugated p–p* transitions and exhibit a charge
Figure 4. Top) Positions of vertical energy transitions along with correspond-
ing electronic oscillator strengths calculated from the TDDFT method for
both 3 and 5. Bottom) Normalized absorption and fluorescence spectra (full
lines) of chromophores in glassy matrix of 2-MTHF (T=77 K). Excitation ani-
sotropy spectra (circles) in glassy matrix of 2-MTHF (T=77 K) for both 3 and
5.
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Table 1. TDDFT and experimental electronic transitions of chromophores.
3
5
Transition
Experimental Theoretical
Experimental Theoretical
E00
Eth
f
Pol.[a] Major natural transitions
(fractions)
E00
Eth
f
Pol.[a] Major natural transitions
(fractions)
[eV]
[eV]
[eV]
[eV]
S0!S1
S0!S2
S0!S3
S0!S4
S0!S5
S0!S6
2.47
2.46 1.872 x,y[b]
3.09 0.064 x,y[b]
3.45 0.144 x,y[b]
H-L
2.36
2.37 1.803
2.53 1.598
2.82 0.258
2.84 0.211
3.08 0.067 x,z
3.09 0.051 x,z
y
x
x
y
H-L (0.73), Hꢀ1-L+1 (0.27)
H-L+1 (0.30), Hꢀ1-L (0.70)
H
H
H
H
H
ꢀ1-L (0.54), Hꢀ1-L+1 (0.46)
ꢀ1-L+1(0.46), H-L+2 (0.54)
ꢀ2-L (0.20), Hꢀ1-L (0.80)
ꢀ2-L (0.69), Hꢀ1-L (0.18)c
ꢀ3-L (0.64), Hꢀ3-L+1 (0.19)
H
ꢀ1-L (0.31), H-L+1 (0.69)
H-L (0.27), Hꢀ1-L+1 (0.73)
ꢀ1-L+3 (0.27), H-L+3 (0.27)
3.96 0.455
3.99 0.160
4.01 0.002
X
y
z
c
H
c
c
H-L+4 (0.18), Hꢀ1-L+4 (0.19)
[a] Orientation of transition dipole. xyz axis are displayed in Scheme 1. [b] The main component is underlined. [c] The two principal natural transitions are
displayed.
transfer from the periphery (pyran fragments) to the core (pyri-
midine ring) of the dye.
The occurrence of these two transitions is well predicted
within the framework of the Frenkel exciton model.[13,28] For
a two-branched V-shaped chromophore, it is assumed that in-
terbranch electrostatic interactions lead to the splitting of the
one-exciton transition relative to the molecular synthon.[29] This
twofold degeneracy occurs symmetrically for a homo-dimeric
chromophore (see inset of Scheme 1) and leads to a S1–S2 gap
of 2V in which V denotes the interbranch coupling. Such a sym-
metrical splitting is well-reproduced here: calculations show
that the midpoint energy between the S1 (2.37 eV) and S2
(2.53 eV) levels of chromophore 5 matches the S1 energy of 3
(i.e. 2.46 eV). The interbranch coupling can be estimated at ap-
proximately E=0.09 eV. This value is moderate with respect to
interbranch couplings calculated in the E=0.15–0.20 eV range
for symmetrical V-shaped structures that bear strong electron-
withdrawing groups.[14c,d,15a] This moderate coupling should in-
dicate a weak dipole moment change upon dye excitation
from the ground to Franck–Condon states. In the high-energy
region (i.e. lexc <475 nm), two sets of two close-lying transi-
tions are predicted. The first set (namely S0!S3 and S0!S4) is
positioned within the region that corresponds to the strong
change of the anisotropy spectrum. This latter effect should be
attributed to an important change in the relative orientation of
the transition dipole moments associated with each transition
(see Table 1). Interestingly, the second set of S0!Sn transitions
located at approximately l=400 nm are weakly allowed (f<
Figure 6. One- (full lines) and two- (data points) photon absorption spectra
of dyes in dichloromethane for 3 and 5. (The 2PA spectra are plotted against
half the excitation wavelength). Inset: Typical Z-Scan graph and its best fit-
ting curve (see text).
0.1) but exhibit similar electronic polarizations in line with the
constant anisotropy observed in this spectral region.
2.3 Two-Photon Absorption Properties
The two-photon absorption spectra have been measured in
the l=740–980 nm range by using the open aperture Z-scan
technique.[30] The one- (1PA) and two-photon (2PA) absorption
spectra of the compounds are shown in Figure 6. The 2PA
bands are plotted against half the excitation wavelength to
have a direct comparison with the linear absorption spectra.
According to our spectral resolution, it appears that the lowest
energy 2PA band of 3 reasonably matches that of its 1PA one
and shows a maximum 2PA cross-section (d) of 86ꢂ9 GM at
l=880 nm. Despite the strong electron-withdrawing character
of the pyrimidine core and the good electron-donating ability
of the pyran group for which the oxidation potential is equiva-
lent to that of a N,N’-dimethylaniline group (0.78 V vs SCE),[31]
the dmax value obtained for 3 is relatively weak with respect to
2PA cross-sections of other linear “push-pull” stilbenoid sys-
tems. For instance, 4-dialkylamino-4’-nitrostilbene[32] or 4-
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alkoxy-4’-nitrostibene[4f] exhibit a dmax of approximately 191 GM
and 180 GM, respectively. Cui et al.[33] reported that an elec-
tron-deficient triazine core connected by a vinylene linker to
a dimethylaniline group leads to a dmax of 340 GM. In addition,
if we normalize the 2PA cross-section by the effective number
of p electrons (Ne) as proposed by Kuzyk et al.[8] the weak 2PA
ability of 3 becomes even more sizeable because the ratio d/
Ne drops to 2.7 for 3, which is 5 times lower than the d/Ne
values for the other cited stilbenes.
This approximation is usually used for push-pull organic and
organometallic molecules.
gEFISH ¼ mb=5kT þ g0ðꢀ2w,w,w,0Þ
ð1Þ
Measurements are performed at l=1907 nm, and obtained
from a Raman-shifted Nd:YAG+ laser source, which allowed us
to work far from the resonance peaks of the compounds we
investigated (3 and 5). mb values of 400 and 770 10ꢀ48 esu
were obtained respectively for compounds 3 and 5. The mb
values of the two compounds are positive, which indicates
that the excited states are more polarized than the ground
states (me>mg). In addition, this implies that the ground and
excited states are polarized in the same direction. The positive
values are in accordance with the emission solvatochromism
observed for the compounds (see later). The mb value obtained
for compound 3 is higher than the value obtained for a 4-styr-
ylpyrimidine substituted by a dimethylamino group (330
10ꢀ48 esu).[17b] Notably, the D-p-A-p-D dye 5 is not completely
symmetric, and this has been shown on the DFT-optimized ge-
ometry. This compound exhibit a higher mb value than the di-
polar compound 3.
We assume that the flexible single bond that connects the
stilbenyl moiety to the terminal donor group noticeably affects
the nonlinear absorption properties of our derivatives. The flex-
ible rotation between the donor and acceptor subunits (i.e. V1
in Scheme 1) does not guarantee an optimum planar geometry
that should maximize the p-orbital overlap. Such an influence
of the twisted geometry on the 2PA cross-section was rational-
ized by Ahn et al.[12a] for a series of meso-linked porphyrin
dimers that exhibit a gradual increase in the twist angle be-
tween the two porphyrins. On going from a near planar to per-
pendicular structure, the 2PA cross-section collapses by
a factor of 75. As shown in Figure 6, the 2PA maximum of the
two-branched chromophore 5 is blueshifted with respect to
twice its linear absorption maximum. This should be attributed
to the near centrosymmetry of the D-p-A-p-D dye, which leads
theoretically to a two-photon forbidden S0!S1 transition. The
longest wavelength 2PA band of 5 shows a maximum located
at approximately l=900 nm with dmax of 271ꢂ28 GM, which
leads to d/Ne of approximately 4.7. Therefore, a cooperative en-
hancement of the 2PA efficiency is clearly evidenced on going
from a one-dimensional D-p-A system to a quadrupolar
branching structure.
2.5 Excited-States Properties
Both chromophores are nonfluorescent in apolar and low polar
solvents (e.g. hexane or ethyl ether) at room temperature, and
are weakly emissive if the solvent polarity is increased
(Table 2). 5 is more emissive than 3. In dichloromethane, the
fluorescence quantum yield (Ff) is approximately 10ꢀ3 for 3
and 2.5ꢃ10ꢀ3 for 5. In both cases, the structureless fluores-
cence band is very broad (full width at half maximal (FWHM)
ꢁ4500 cmꢀ1) and is located in the l=450–800 nm range (Fig-
ure S1 in the Supporting Information). 3 and 5 clearly exhibit
strong Stokes shifts (DuST), which increase with solvent polarity.
The fluorescence spectra undergo a pronounced bathochromic
effect, whereas only a small solvent-induced shift is observed
for the absorption bands upon an increase in the solvent po-
larity. This provides clear evidence for a significant electronic
and/or geometrical change between ground and emitting
states.
2.4 Second-Order Nonlinear Measurements
Second-order nonlinear properties have been studied in CHCl3
solution by using the electric-field-induced second-harmonic
generation technique (EFISHG), which provides information
about the scalar product mb (2w) of the vector component of
the first hyperpolarisability tensor b and the dipole moment
vector.[34] This product is derived according to Equation (1)
taking into consideration that g0(ꢀ2w,w,w,0), the third-order
term, is negligible for the push-pull compounds under study.
Table 2. Spectroscopic data of the ground and excited states of compounds in various solvents.
3
la
5
la
Solvent[a]
lf
FWHM[b]
Ff ꢃ102
Stokes
Shift
lf
FWHM[a]
[cmꢀ1
]
Ff ꢃ102
Stokes
Shift
[cmꢀ1
]
[nm]
[nm]
[cmꢀ1
]
[nm]
[nm]
[cmꢀ1
]
ETA
2MTHF
THF
DCM
ACT
DMF
425
428
431
436
427
434
546
557
556
566
581
592
4690
4840
4982
4675
4260
4505
0.04
0.04
0.06
0.10
0.09
0.19
5214
5411
5216
5268
6207
6150
446
450
452
458
447
456
568
576
577
609
630
652
4145
4334
4190
4095
4040
4020
0.12
0.13
0.17
0.22
0.19
0.28
4816
4861
4793
5414
6498
6592
[a] Solvents are: ETA=ethyl acetate, 2MTF=2-methyltetrahydrofuran, THF=tetrahydrofuran, DCM=dichloromethane, ACT=acetone and DMF=dimethyl-
formamide. [b] Full width at half maximum for fluorescence bands.
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The dipole moment change (Dmge) between ground and ex-
cited states can be estimated by using the Lippert–Mataga
equation[35] [Eq. (2)]:
This assumption is also in line with the very weak phosphor-
escence measured for 3 and 5 in a glassy matrix of 2-methyl
tetrahydrofuran (2 MTHF) (Figure S2). The phosphorescence
spectrum, located in the l=600–750 nm region is strongly
overlapped with the fluorescence band. This suggests a rela-
tively small energy difference between the S1 and T1 levels. As
a consequence, a thermally activated delayed fluorescence
emission is observed even at T=77 K. Moreover the delayed
fluorescence lifetime is quite similar to that obtained for phos-
phorescence emission with values of approximately 15 ms. No-
tably, the very short lifetime measured for the phosphores-
cence of T1 state is a clear indication of its np* character.
Hence, it appears that additional nonradiative processes at the
S1 state other than trans-to-cis photoisomerization and inter-
system crossing should be taken into account to explain the
weak fluorescence of the chromophores at room temperature.
Finally, we observed that high-viscosity solvents freeze down
the molecular relaxation precesses in 3 and 5. The temperature
dependence of steady-state fluorescence spectra of 3 and 5 in
2MTHF is shown in Figure 7. The fluorescence quantum yields
notably increase upon cooling the solutions. From T=293 K to
80 K Ff is multiplied by a factor of 430 and 105 for 3 and 5, re-
spectively. Two distinctive steps are clearly observed and both
ꢀ
ꢁ
2Dm2eg
e ꢀ 1
n2 ꢀ 1
ð2Þ
DuST
¼
ꢀ
þ Const:
hca30 2e þ 1 2n2 þ 1
Within this dielectric continuum model, e is the relative permit-
tivity and n the refractive index of the solvent, h is the Planck
constant and c is the speed of light. The Onsager radius, a0, de-
fined as the solvent shell around the molecule was estimated
at a value of approximately 5.5 ꢄ for both molecules. This
value corresponds to 40% of the longest axis of the com-
pound 3 as suggested by Lippert for nonspherical molecule-
s.[35a] It should be emphasized that, for chromophore 5, we
assume that the relaxed S1 state is not fully delocalized within
the entire two-branched structure but is mainly localized on
a single branch of the molecule. Such an assumption has been
theoretically predicted for the relaxed S1 state of multi-
branched chromophores with quadrupolar V-shaped geome-
tries[14d] or octupolar trigonal symmetries.[36] Even though the
solvatochromic measurements are performed over a limited
range of the solvent polarity scale, the dipole moment change
(Dmge) can be estimated to 15ꢂ1 D and 19ꢂ2 D for chromo-
phores 3 and 5, respectively. Such values are of the same mag-
nitude as those obtained for “push-pull” trans-stilbenes such
as 4-cyano-4’-dimethylaminostilbene[37] (~15 D) or 4-methoxy-
4’-nitrostilbenes[4f,38] (~23 D) and therefore illustrate the signifi-
cant charge transfer character of the emitting states.
The very low fluorescence quantum yield measured for 3
and 5 is a clear indication of the occurrence of very efficient
nonradiative deactivation processes at the singlet excited
state. We assume that the main relaxation process responsible
for the fluorescence quenching of 3 and 5 corresponds to
a conformational change of the excited state.
Trans-to-cis photoisomerization should be first excluded as
a major nonradiative channel. Indeed, the measured quantum
yield for this photoreaction (Ft!c) has a very low value (below
10ꢀ3) both in hexane and dimethylformamide (N2-degassed sol-
utions). Such a weakly efficient photoisomerization presumably
owes to a large activation barrier between the S1 state and the
1
double-bond-twisted excited species P.[39] This so-called “phan-
1
tom” P state acts as a photochemical funnel (possibly through
conical intersection) toward the ground state surface from
which an equally probable isomerization to cis and trans
occurs.[40] We can also exclude that a parallel photoisomeriza-
tion reaction may occur by intersystem crossing (ISC).[41] In this
case, the planar triplet state would undergo a barrierless C=C
torsion to the 3P state. However, one would expect that
Ft!c should be at least equal to half the value of FISC even
though we assume that the triplet state mechanism consti-
tutes the major photoisomerization pathway. Owing to the
very low quantum yield measured for the photoisomerization
(<10ꢀ3), intersystem crossing should be considered as a negli-
gible process with respect to the other deactivation pathways
at the singlet excited state.
Figure 7. Fluorescence spectra of 3 and 5 in 2MTHF over the temperature
range of 80-293 K.
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dyes behave similarly in a qualitative sense. Decreasing the
temperature from 293 K to 140 K results in a fluorescence en-
hancement of approximately eight times, and a gradual band
with the very low trans-to-cis quantum yields observed for 3
and 5. A first mechanism was proposed by Lapouyade et al.
that is based on ring-bridged model stilbene derivatives.[37bc,46]
1
redshift with
a
thermochromic slope of approximately
In this mechanism, the formation of the TICT state results in
11 cmꢀ1 Kꢀ1 for 3 and approximately 14 cmꢀ1 Kꢀ1 for 5. Upon
further cooling, the bathochromic effects level off and below
T=130 K the fluorescence bands reversely shift to the high-
energy region with a sizeable increase in intensity.
a rotation about a single bond adjacent to the central ethylene
one. This hypothesis was proposed for trans-4-(N,N-dimethyla-
mino)stilbenes or trans-4-(alkoxy)stilbenes with strong terminal
acceptor groups (e.g. ꢀCN, ꢀNO2) in the 4’ position. Such a pre-
cursor–successor mechanism affects the central part of the
chromophore. In our case, this relaxation mode would induce
a very large amplitude motion if the crowded V-shaped struc-
ture of 5 is considered. Therefore, one would expect to ob-
serve a significant activation barrier (Ea) between LE and TICT
states (see Scheme 4). Interestingly, these latter activation bar-
riers can be determined for 3 and 5. Indeed, the nonradiative
rate constant (knr) can be derived from the following relation-
ship [Eq. (3)]:
It is well established that the dielectric constant and viscosi-
ty are two main physical properties of the surrounding
medium that affect the fluorescence properties of chromo-
phores.[42] In reference to 2MTHF, the relative influence of
these two parameters evolves sequentially depending on the
temperature range.[43] In the high-temperature region (i.e.
above the glass-transition temperature, Tg ꢁ130 K), the ther-
mochromic shift should be mainly attributed to an increase in
the effective solvent polarity owing to a continuous growth in
the dielectric constant from 7 at T=293 K to a maximum value
of 18 at T=140 K.[44] During the glass-transition step, the die-
lectric constant drops abruptly to 2.6 and remains quite con-
stant from T=110 K to 77 K. Meanwhile, the viscosity increases
exponentially from 102 Pasꢀ1 to 3.5ꢃ1019 Pasꢀ1. In this low
temperature region, all conformational changes are severely re-
stricted. This change in the solvent viscosity is clearly responsi-
ble for the dramatic fluorescence enhancement for 3 and 5
that is associated with band hypsochromy. As a consequence,
we propose that the main relaxation process responsible for
the fluorescence quenching of 3 and 5 corresponds to a con-
formational change from a nearly planar locally excited (1LE)
state to a twisted geometry; presumably a twisted intramolec-
ular charge transfer (1TICT) state.[45] This D/A decoupled geom-
etry is expected to exhibit a forbidden optical transition that
leads to a non-emissive (or weakly emissive) excited state
(Scheme 4).
1
1
knr kIC þ kISC þ ka 1 ꢀ Ff
ð3Þ
¼
¼
kr
kr
Ff
In this equation, the radiative rate constant of the emitting
state (kr) is considered not to correlate with temperature and
the internal conversion (IC) is normally an unactivated pro-
cess.[47] If we assume that intersystem crossing can be neglect-
1
ed as compared to the 1LE! TICT process (ka) and internal
conversion, then one can derive that knr ꢁkIC +ka. The Arrhe-
nius plots of ln(knr/kr) versus 1/T are presented in Figure 8 and
In reference to donor–acceptor-substituted stilbenes, two
scenarios can be investigated. They all rely on the molecular
structure of the stilbene, the nature of the D/A couples and
the determination of the single bond involved in the torsional
motion. Interestingly both twisting mechanisms strongly com-
pete with the formation of the 1P state, which is consistent
Figure 8. Arrhenius plot for the nonradiative-to-radiative rate constants
ratio.
correlations are sufficiently linear to derive activation barriers
in the 1.7–1.8 kcalmolꢀ1 range. These values nicely coincide
with the solvent viscosity barrier (Eh ꢁ1.82 kcalmolꢀ1 for
1
1
2MTHF)[48], which indicates that the LE! TICT reaction does
not proceed over a significant activation barrier and does not
1
require a large-amplitude conformational change from the LE
state. Therefore this first mechanism is not reasonably trans-
posable to our chromophores.
A second relaxation mechanism has been evidenced and ex-
tensively studied by Yang et al.[47a,49] This model was proposed
for some aryl-substituted stilbenes such as N-aryl-substituted
trans-4-aminostilbenes. Notably, the molecular structure of
these derivatives is quite similar to that of our compounds.
Scheme 4. Representation of the adiabatic TICT reaction pathway, in which
S0 and S1 represent the ground and first electronic states, respectively.
kr =radiative rate constant, ka =Arrhenius rate constant, knr =nonradiative
rate constant, Ea =activation energy, LE=locally excited state, FC=Franck—
Condon state, and TICT=twisted intramolecular charge transfer.
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(0.9 equiv and 0.45 in the case of compound 4) was added. The so-
lution was stirred at T=ꢀ788C for 30 min, and allowed to warm to
RT for 2 h. The THF was evaporated and the residue was dissolved
in a 1:1 mixture of CH2Cl2/water (100 mL) and the organic layer
was separated. The aqueous layer was extracted with CH2Cl2 (2ꢃ
50 mL). The combined organic extracts were dried with MgSO4 and
the solvents evaporated.
The conformational change at the excited state results in a ro-
tation about the single bond that connects the stilbenyl struc-
ture with the external aromatic subunit. This mechanism is
more in line with the barrierless nature of the reaction because
it only implies the relative motion of a peripherical part of the
molecule with the rotation of a methylenepyran group from
a twist angle of approximately 288 (see DFT calculation) to 908
with respect to the 4-styrylpyrimidine fragment (Scheme 1).
(E)4-{2-[4-(2,6-Diphenyl-pyran-4-ylidenemethyl)-phenyl]-vinyl}-pyri-
midine (3): Red solid; obtained according to general procedure
and purified by column chromatography (SiO2, CH2Cl2/ETA, 1:1) fol-
lowed by crystallization from CHCl3/n-heptane; yield 65%
(277 mg); mp: 227–2288C (decomposes); 1H NMR (500 MHz,
CDCl3): d=5.94 (s, 1H), 6.44 (s, 1H), 7.04 (s, 1H), 7.05 (d, 1H, J=
16 Hz), 7.31 (d, 1H, J=5 Hz), 7.42–7.46 (m, 8H), 7.59 (d, 2H, J=
8 Hz), 7.76–7.78 (m, 4H), 7.89 (d, 1H, J=16 Hz), 7.66 (d, 1H, J=
5 Hz), 9.16 ppm (s, 1H); 13C NMR and JMOD (125 MHz, CDCl3): d=
162.5 (C), 158.9 (CH), 157.3 (CH), 153.3 (C), 151.3 (C), 140.3 (C),
137.3 (CH), 133.4 (C), 133.2 (C), 132.6 (C), 130.5 (C), 129.5 (CH),
129.2 (CH), 128.8 (CH), 128.70 (CH), 128.70 (CH), 128.1 (CH), 125.0
(CH), 124.6 (CH), 124.3 (CH), 118.6 (CH), 113.7 (CH), 108.7 (CH),
102.2 ppm (CH). HRMS (ESI/ASAP) m/z: calcd for C30H23N2O [M+
H]+: 427.1810; found: 427.1804.
3. Conclusions
A linear and a V-shaped pyrimidine-based chromophore bear-
ing g-methylenepyrans as terminal donor groups have been
synthesized. According to a large set of experimental data sup-
ported by DFT theoretical calculations, the photophysical prop-
erties of these new NLO organic materials have been methodi-
cally untangled. The branching effect is well described within
the Frenkel exciton model and leads to a moderate inter-
branch coupling. The ground state structure of both deriva-
tives shows a significant twisting of the pyranilydene groups
with respect to the planar electron-deficient pyrimidine core.
Such a deviation from planarity of the chromophores does not
allow an optimal electronic conjugation all along the molecular
scaffold. As a consequence, the NLO response is modest rela-
tive to that of other p-conjugated derivatives with similar size
and architecture. In the same manner, the rotation of the
donor end-group has a decisive influence on the excited-state
dynamics, which leads to a detrimental fluorescence quench-
ing effect. Therefore, we demonstrate here that the flexibility
of a simple structural parameter both affects the NLO proper-
ties as well as the dyes emissivity. Preventing this specific rota-
tion would presumably lead to a synergetic effect. A ring-
bridged strategy is currently underway.
4,6-Bis-{2-[4-(2,6-diphenyl-pyran-4-ylidenemethyl)-phenyl]-vinyl}-
pyrimidine (5): Red solid; obtained according to general procedure
and purified by column chromatography (SiO2, CH2Cl2/ETA, 7:3) fol-
lowed by crystallization from CHCl3/n-heptane; yield 31%
(240 mg); mp: 243–2448C (decomposes); 1H NMR (500 MHz,
CDCl3): d=5.95 (s, 2H), 6.45 (s, 2H), 7.06 (s, 2H), 7.08 (d, 2H, J=
16 Hz), 7.28 (s, 1H), 7.40–7.48 (m, 16H), 7.61 (d, 4H, J=8 Hz), 7.77–
7.79 (m, 8H), 7.92 (d, 2H, J=16 Hz), 9.10 ppm (s, 1H); 13C NMR and
JMOD (125 MHz, CDCl3): d=162.9 (C), 158.7 (CH), 153.2 (C), 151.2
(C), 140.1 (C), 136.8 (CH), 133.4 (C), 133.2 (C), 132.8 (C), 130.5 (C),
129.5 (CH), 129.2 (CH), 128.71 (CH), 128.69 (CH), 128.1 (CH), 128.0
(CH), 125.0 (CH), 124.7 (CH), 124.6 (CH), 116.4 (CH), 113.8 (CH),
108.8 (CH), 102.2 ppm (CH). HRMS (ESI): m/z: calcd for C56H41N2O2
[M+H]+: 773.3168; found: 773.3163.
Steady-state absorption and fluorescence spectra: The absorption
measurements were performed with a PerkinElmer Lambda 2 spec-
trometer. Steady-state fluorescence and phosphorescence spectra
were collected from a FluoroMax-4 spectrofluorometer. Emission
spectra were spectrally corrected, and fluorescence quantum yields
include the correction owing to solvent refractive index and were
determined relative to quinine bisulfate in sulfuric acid (0.05M,
Fref =0.52)[50] Owing to the very weak emissive chromophores
(Ff <10ꢀ3), the fluorescence quantum yields were obtained from 5
independent measurements and the uncertainties were deter-
mined to ꢂ20%. Phosphorescence and steady-state anisotropy
measurements were performed in 2-methyl tetrahydrofuran at T=
77 K. The samples were placed in a 5 mm diameter quartz tube
inside a Dewar flask filled with liquid nitrogen.
Experimental Section
Materials and general methods: In air and moisture-sensitive reac-
tions, all glassware was flame-dried and cooled under nitrogen. All
other commercially available reactants were used without further
purification. The solvents used for absorption and emission analysis
were as follows: ethyl acetate (ETA), 2-methyl tetrahydrofuran
(2MTHF), tetrahydrofuran (THF), dichloromethane (CH2Cl2), acetone
(ACT), dimethylformamide (DMF). All the solvents employed were
Aldrich, Fluka or Merck spectroscopic grade. Compounds 1 and 4
were prepared as previously described.[21b] NMR spectra were ac-
quired at RT on a Bruker DRX500 spectrometer at the Service
Commun de Recherche de Rꢁsonance Magnꢁtique Nuclꢁaire et de
Rꢁsonance Paramagnꢁtique Electronique de l’Universitꢁ de Bre-
tagne Occidentale. Chemical shifts are given in parts per million
relative to TMS (1H, d=0.0 ppm) and CDCl3 (13C, d=77.0 ppm).
Acidic impurities in CDCl3 were removed by treatment with anhy-
drous K2CO3. High resolution mass analyses were performed at the
Centre Rꢁgional de Mesures Physiques de l’Ouest (CRMPO, Univer-
sity of Rennes1) by using a Bruker MicroTOF-Q II apparatus.
Time-gated luminescence: The phosphorescence lifetimes were
measured by using a FluoroMax-4 spectrofluorometer that was
also equipped with a Xe-pulsed lamp operating at up to f=25 Hz.
The phosphorescence decays were obtained according to a time-
gated method. The emission was recorded by using a control
module that included a gate-and-delay generator, which allowed
the integration of the signal during a specific period after a flash
(delay) and for a pre-determined time window. The total signal
was accumulated for a large number of excited pulses.
General procedure for the Wittig reaction: A small excess of n-bu-
tyllithium (1.1 equiv; 2.5m in hexanes) was added dropwise at T=
ꢀ788C, under a nitrogen atmosphere, to a degassed solution of
2,6-diphenyl-4H-pyran-4-yl triphenylphosphonium tetrafluorobo-
rate salt 2[21] (1 equiv) in dry THF. After 15 min, the carboxaldehyde
Fluorescence anisotropy: For the steady-state anisotropy measure-
ments, two Glan-Thompson polarizers were placed in the excita-
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tion and emission beams. The anisotropy r is determined as follows
in Equation (4):
in which z0 is the coordinate along the propagation direction of
the focal point of the beam, l the optical path length. The 2PA
cross-section,
d,
(in
units
of
1 GM:
10ꢀ50 cm4 sꢀ1 photonꢀ1 moleculeꢀ1) is then determined by using the
relationship [Eq. (6)]:
IVV ꢀ gIVH
IHV
IHH
ð4Þ
r ¼
g ¼
IVV þ 2:g:IVH
dNAd
hu
b ¼
10ꢀ3
ð6Þ
in which I is the fluorescence intensity. The subscripts denote the
orientation (horizontal H or vertical V) of the excitation and emis-
sion polarizers, respectively. g is an instrumental correction factor.
The proper calibration of the set-up was checked by using a recent
standard method with rhodamine 101 in glycerol.[51]
in which h is the Planck constant, u the frequency of the incident
laser beam, NA the Avogadro constant and d is the concentration
of the chromophore (molLꢀ1). The rhodamine 6G in methanol[56]
(16.2ꢂ2.4 GM at 806 nm) was used for the calibration of our mea-
surement technique. The uncertainty in d is of the order of 20%.
Low-temperature measurements: Low-temperature emission meas-
urements were performed in 2MTHF by using an Oxford Cryogen-
ics OptistatDN cryostat fitted with an Oxford Instruments ITC503S
temperature controller. The temperature dependency of the refrac-
tive index and the density of the solvent were taken into account
for the measurement of Ff.[52]
EFISHG measurements: The molecular quadratic optical nonlineari-
ties of chromophores, in the form of mgb products (mg is the
ground state permanent dipole moment and b the first hyperpo-
larizability of the molecules, according to convention B),[57] were
measured on a standard Electric Field Induced Second Harmonic
Generation (EFISHG) set-up[58] operating at a l=1907 nm funda-
mental wavelength, obtained by Raman-shifting the emission from
a Nd:YAG laser, which has the advantage of distance from the ab-
sorption bands of the molecules.
Cyclic voltammetry: The cyclic voltammetry experiments[53] (con-
ducted by using a computer-controlled Radiometer Voltalab 6 po-
tentiostat with a three-electrode single compartment cell; the
working electrode was a platinum disk; a saturated calomel elec-
trode (SCE) used as a reference was placed in a separate compart-
ment with a salt bridge containing the supporting electrolyte)
were performed at T=300 K, in N2-degassed dichloromethane with
a constant concentration (0.1m) of nBu4BF4. Ferrocene (Fc) was
used as an internal reference (considering EFc/Fc+ =0.53 V in CH2Cl2
vs the aqueous SCE).[54]
DFT calculations: The theoretical absorption spectra were comput-
ed based on Density Functional Theory (DFT) and Time-Dependent
DFT (TDDFT). The overall computation strategy was defined as fol-
lows: after initial AM1 optimization calculations (vacuum), subse-
quent optimization of geometrical structures of the derivatives
were performed using the PBE0/6-31G(d) level of calculation. (No-
tably, the PBE0 is a good functional for the description of photo-
functional molecules).[59] Finally, the TDDFT vertical transitions were
computed by using the same level of calculations. All calculations
were performed by using the Gaussian 09 package.[60]
Actinometry: quantum yields for trans!cis photoisomerization
were measured under irradiation at l=436 nm by using a 100 W
Mercury-Xenon Lamp (Hamamatsu, L2422-02) equipped with
a band pass filter (THORLAB, FB430-10). All irradiated solutions
were previously N2-degassed. The progress of the photoreaction
was monitored by UV/Vis absorption spectra. The absorbance at
the excitation wavelength had to be greater than 2 to assume
a total absorption of the incident photons. The incident light inten-
sity was measured by ferrioxalate actinometry.[55]
Keywords: chromophores
· conjugation · heterocycles ·
nonlinear optics · photophysics
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blI0
pffiffiffi
TðzÞ ¼ 1 ꢀ
ð5Þ
z
2
2
2ð1 þ ðz Þ Þ
0
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