Polarizability and internal charge transfer in thiophene-triphenylamine hybrid π-conjugated systems

Article abstract of DOI:10.1021/jp203759e

Extended star-shaped conjugated systems consisting of dicyanovinyl electron-acceptor units connected to a triphenylamine core by means of thiophene (T), thienylenevinylene (TV), and bithiophene (BT) conjugating spacers have been synthesized. The analysis

Full text of DOI:10.1021/jp203759e

ARTICLE
pubs.acs.org/JPCB
Polarizability and Internal Charge Transfer in
ThiopheneꢀTriphenylamine Hybrid π-Conjugated Systems
Emilie Ripaud,† Yoann Olivier,‡ Philippe Leriche,*,† Jꢀer^ome Cornil,‡ and Jean Roncali*,†
†LUNAM Universitꢀe, MOLTECH—Anjou UMR CNRS 6200, Groupe Systꢁemes Conjuguꢀes Linꢀeaires, University of Angers,
2 Boulevard Lavoisier, 49045 Angers, France
‡Service de Chimie des Matꢀeriaux Nouveaux, Universitꢀe de Mons, Place du Parc 20, B-7000 Mons, Belgium
S Supporting Information
b
ABSTRACT: Extended star-shaped conjugated systems consist-
ing of dicyanovinyl electron-acceptor units connected to a triphe-
nylamine core by means of thiophene (T), thienylenevinylene
(TV), and bithiophene (BT) conjugating spacers have been
synthesized. The analysis of the electronic properties of the
molecules by UVꢀvis absorption spectroscopy, cyclic voltamme-
try, and theoretical calculations shows that the electronic proper-
ties of the systems depend on the length and rigidity of the
conjugating spacer.
’ INTRODUCTION
In our continuing interest in the design of TPA-based donor
materials for molecular BHJ solar cells, we report here an analysis
of the effect of the chemical structure of the conjugating bridge
between the TPA core and the peripheral electron acceptor groups
on the electronic properties of the resulting star-shaped molecules.
To this end, the electronic properties of three compounds involving
dicyanovinyl acceptor units connected to the TPA core by means
of thiophene (T), thienylenevinylene (TV), and bithiophene (BT)
conjugating spacers have been investigated by cyclic voltammetry,
UVꢀvisible absorption spectroscopy, fluorescence emission spec-
troscopy, and quantum-chemical calculations.
Among the numerous classes of organic materials endowed
with specific electronic properties, triphenylamine (TPA) deri-
vatives have attracted sustained interest for many years.
Because of their high hole mobility, isotropic charge transport,
and optical transparency in the visible region, these materials
have been widely used for the design of hole injections and
transporting layers in organic light-emitting diodes.1
The emergence of bulk heterojunction (BHJ) organic solar
cells2 based on soluble small molecules has triggered a renewal of
interest in TPA derivatives as donor materials in such solar cells.4
In this context, we have shown previously that the introduction of
electron acceptor groups in the structure of a TPA-based molecular
donor produces an internal charge transfer (ICT) which leads at
the same time to the following: (i) the broadening of the absorp-
tion spectrum; (ii) a reduction of the optical band gap; and (iii)
an increase in the oxidation potential and, hence, a decrease of the
HOMO level.5 When such compounds are used as donor material
in molecular bulk heterojunction solar cells, this ICT leads to an
improvement of the light-harvesting properties, a bathochromic
extension of the external quantum efficiency spectrum, and an
increased open-circuit voltage; these combined effects result in a
large increase of the power conversion efficiency (PCE).5
During the past few years, many conjugated TPA derivatives
have been used as molecular donors in BHJ solar cells, leading to
a steady improvement of the PCE.6 Values exceeding 4% have
been reported for molecular BHJ solar cells based on TPA
compounds and PC71BM or a new analogue of fullerene.7
Received: April 22, 2011
Revised:
June 9, 2011
Published: June 24, 2011
r
2011 American Chemical Society
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’ EXPERIMENTAL METHODS
Tris[4-(5-dicyanomethylidenemethyl-thiophen-2-ylvinyl)phenyl]-
amine (TV). At room temperature and under inert atmosphere,
250 mg of the aldehyde (3) (0.383 mmol) was dissolved in
300 mL of chloroform. Then, 10 equiv of malonodinitrile (3.83 mmol,
253 mg) and ca. 30 drops of triethylamine were added. The
mixture was stirred at room temperature overnight under nitro-
gen atmosphere after 100 mL of chloroform was added. The
organic phase was washed three times with 150 mL of a solution
of sodium hydroxide (1 M) and then with water. After the
mixture was dried over anhydrous magnesium sulfate and after
the filtration and evaporation of the solvent, the residue was
purified by chromatography on silica gel using dichloromethane
as eluent. A purple powder was isolated (yield: 265 mg, 87%). 1H
NMR (DMSOꢀd6, ppm): 8.61 (s, 1H); 7.87 (d, 1H, 3J = 4 Hz);
7.67 (d, 2H, 3J = 8.5 Hz); 7.55 (d, 1H, 3J = 16 Hz); 7.5 (d, 1H, 3J =
4.5 Hz); 7.36 (d, 1H, 3J = 16 Hz); 7.08 (d, 2H, 3J = 8.5 Hz). 1H
NMR (CDCl3, ppm): 7.76 (s, 1H); 7.60 (d, 1H, 3J = 4 Hz); 7.46
General. Solvents were purified and dried using standard
protocols. 1H NMR and 13C NMR spectra were recorded on a
Bruker AVANCE DRX 300 spectrometer; δ are given in ppm -
(relative to tetramethylsilane), and coupling constants (J) are
given in Hz. Matrix-assisted laser desorption ionization time-of-
flight (MALDI-TOF) mass spectra were recorded with a Bruker
Biflex-III, equipped with a N2 laser (337 nm). For the matrix,
dithranol in CH2Cl2 was used. High resolution mass spectra were
recorded under FAB mode on a Jeol JMS 700 spectrometer.
UVꢀvisible optical data were recorded with a Perkin-Elmer
lambda 950 spectrophotometer. Thermal analyses were performed
using a DSC 2010 CE (TA Instruments). For cyclic voltammetry
(scan rate 100 mV cmꢀ1), the electrochemical apparatus con-
sisted of a potentiostat EG&G PAR 273A and a standard three-
electrode cell. As the working electrode and counterelectrode, a
platinum foil and a platinum wire were used, respectively, while a
SCE electrode was used as a reference.
3
(d, 2H, J = 9 Hz); 7.19ꢀ7.13 (m, 5H). 13C NMR (CDCl3,
ppm): 155.2; 150.1; 147.4; 140.3; 134.0; 133.5; 131.0; 128.5;
126.8; 124.5; 119.1; 114.4; 113.6; 75.8. MALDI-TOF (calcd/
found, [M+.], g molꢀ1): 797.15/797.20. HRMS (calcd/found,
[M + Na]+, g molꢀ1): 820.1382/820.1380. Tf (DSC, °C): 259.
Td (TGA, °C): 357. IR (cmꢀ1): 2219 (ꢀCN).
Synthesis. Tris[4-(thiophen-2-ylvinyl)phenyl]amine (2). At
room temperature and under inert atmosphere, 500 mg of tris-
[4-formylphenyl]amine (1) (1.52 mmol) and 5 equiv of 2-
(diethoxyphosphorylmethyl)thiophene (7.6 mmol, 1.78 g) were
added to 70 mL of dry tetrahydrofuran. Then, 5 equiv of t-BuOK
(20 wt %) in solution in tetrahydrofuran (3.8 ꢁ 10ꢀ2 mol,
4.6 mL) was added. The mixture was stirred overnight. After the
evaporation of the solvent, the residue was purified by chromato-
graphy on silica gel, using a mixture of dichloromethane/petroleum
ether 1:2 as eluent. Compound 2 was precipitated using a mixture
of dichloromethane/petroleum ether and isolated as a yellow
solid (yield: 580 mg, 67%). 1H NMR (CDCl3, ppm): 7.37 (d, 2H,
3J = 8.5 Hz); 7.17 (d, 1H, 3J = 5.5 Hz); 7.15 (d, 1H, 3J = 16 Hz);
7.08 (d, 2H, 3J = 8.5 Hz); 7.045 (d, 1H, 3J = 3.5 Hz); 7.00 (dd,
1H, 3J = 5 Hz, 3J = 3.5 Hz); 6.89 (d, 1H, 3J = 16 Hz). 13C NMR
(CDCl3, ppm): 146.5; 143.1; 131.8; 127.7; 127.6; 127.2; 125.7;
124.2; 124.0; 120.5. Tf (DSC, °C): 181. Td (TGA, °C): 201.
MALDI-TOF (calcd/found, [M+.], g molꢀ1): 569.13/569.41.
HRMS (calcd/found, [M+.], g molꢀ1): 569.1300/569.1300.
Tris[4-(5-formyl-thiophen-2-ylvinyl)phenyl]amine (3). Un-
der inert atmosphere, 280 mg of tris(4-(thiophen-2-ylvinyl)phenyl)-
amine (2) (0.49 mmol) was dissolved in 80 mL of 1,2-dichlor-
oethane. Then, 4 equiv of dimethylformamide (1.96 mmol,
0.15 mL) and 4 equiv of phosphorus oxytrichloride (1.96 mmol,
0.18 mL) were added. The mixture was refluxed overnight under
N2. The mixture was allowed to cool to room temperature, and
50 mL of dichloromethane was added. The organic layer was
washed with 150 mL of an aqueous solution of sodium acetate,
and then it was washed three times with water. After the mixture
was dried over anhydrous magnesium sulfate, and after the filtra-
tion and evaporation of the solvent, the residue was purified by
chromatography on silica gel, using dichloromethane/ethyl
acetate 95:5 as eluent. An orange powder was recovered (yield:
96 mg, 30%). 1H NMR (C6D6, ppm): 9.52 (s, 1H); 7.07 (m, 4H);
6.92 (d, 1H, 3J = 16 Hz); 6.89 (d, 1H, 3J = 4 Hz); 6.78 (d, 1H, 3J =
16 Hz); 6.55 (d, 1H, 3J = 4 Hz). 1H NMR (CDCl3, ppm): 9.85
(s, 1H); 7.66 (d, 1H, 3J = 4 Hz); 7.42 (d, 2H, 3J = 8.5 Hz); 7.12 (m,
5H). 13C NMR (C6D6, ppm): 181.8; 151.8; 147.5; 142.4; 136.6;
131.8; 128.5; 128.1; 126.3; 124.6; 120.3. 13C NMR (CDCl3, ppm):
182.5; 152.7; 147.1; 141.2; 137.3; 132.1; 131.1; 128.1; 126.3; 124.4;
119.7. MALDI-TOF (calcd/found, [M+.], g molꢀ1): 653.10/
653.17. HRMS (calcd/found, [M + Na]+, g molꢀ1): 676.10453/
676.10450. Td (Kofler, °C): 123. IR (cmꢀ1): 1646 (—CdO).
(50-Dimethoxymethyl-[2,20]bithiophen-5-yl)-trimethyl-stannane
(4). Under inert atmosphere, 6 g of 2,20-bithienyl-5-carboxalde-
hyde (30.9 mmol) and 0.1 equiv of PTSA (3.09 mmol, 533 mg)
were introduced in a three-neck flask. A catalytic amount of
molecular sieves and 5 equiv of trimethyl orthoformate (1.55 ꢁ
10ꢀ1 mol, 16.9 mL) were introduced in 35 mL of dry methyl
alcohol. The mixture was refluxed overnight under inert atmo-
sphere. Then, the mixture was allowed to cool to room tempera-
ture, and 200 mL of a solution of sodium hydroxide at 10ꢀ3 M
was added. The mixture was extracted with 200 mL of dichlor-
omethane. The organic phases were washed three times with
water (3 ꢁ 200 mL) and dried over anhydrous magnesium sulfate.
After the evaporation of the solvent, 7.18 g of crude acetal was
recovered. This was later dissolved in 105 mL of dry tetrahy-
drofuran, and the mixture was cooled to ꢀ78 °C under inert
atmosphere. To the mixture was added 1.1 equiv of n-BuLi(2.5Min
hexanes, 3.27 ꢁ 10ꢀ2 mol, 13.15 mL). The mixture was stirred
for 2 h under inert atmosphere at ꢀ78 °C. Then, 1.1 equiv of a
solution of trimethyltin chloride (1 M) in tetrahydrofuran (32.87
mmol, 32.9 mL) was added dropwise at ꢀ78 °C, and the mixture
was maintained at this temperature for 2 h. The mixture was
stirred overnight at room temperature. The crude product was
then washed with 200 mL of water and extracted with dichlor-
omethane. The organic phase was washed three times with water
(3 ꢁ 200 mL) and then dried over anhydrous magnesium sulfate.
After the evaporation of the solvent, 10.8 g of product was
recovered and used without any further purification (yield: 89%).
1H NMR (CDCl3, ppm): 7.26 (d, 1H, 3J = 4 Hz); 7.08 (d, 1H,
3J = 3.5 Hz); 7.06 (d, 1H, 3J = 3.5 Hz); 6.96 (d, 1H, 3J = 4 Hz);
5.61 (s, 1H); 3.38 (s, 6H); 0.38 (s, 9H); 0.09 (m, 9H). 13C NMR
(CDCl3, ppm): 142.8; 139.97; 137.9; 137.6; 135.8; 126.1; 124.97;
123.1; 99.9; 52.6; ꢀ8.22.
Tris[4-(5-formyl-2,20-bithiophen-5-yl)phenyl]amine (5). Un-
der inert atmosphere, 1.9 g of tris-(4-bromo)phenylamine (3.9 mmol)
was dissolved in a three-neck flask in 300 mL of dry toluene.
Then, 6 equiv of stannic compound 4 (23.7 mmol, 9.6 g) was
added. The mixture was degassed for 10 min using N2, and10mol%
of catalyst Pd(PPh3)4 was added. The mixture was then refluxed
overnight under inert atmosphere. Then, the mixture was allowed to
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cool to room temperature, and 200 mL of an aqueous solution of
sodium hydrogenocarbonate was added. The organic phase was
washed three times with water (3 ꢁ 200 mL) and then dried over
anhydrous magnesium sulfate. After the evaporation of the solvent,
the residue was purified by chromatography on silica gel, using a
mixture of dichloromethane and dichloromethane/ethyl acetate
95:5 as eluent. An orange powder was recovered (yield: 2.32 g,
72%). 1H NMR (CDCl3, ppm): 9.86 (s, 1H), 7.68 (d, 1H, 3J =
4 Hz), 7.53 (d, 2H, 3J = 8.5 Hz), 7.34 (d, 1H, 3J = 4 Hz), 7.26
(d, 1H, 3J = 4 Hz), 7.22 (d, 1H, 3J = 4 Hz), 7.17 (d, 2H, 3J = 8.5 Hz).
13C NMR (CDCl3, ppm): 182.5; 147.2; 146.8; 145.6; 141.4; 137.5;
134.6; 128.6; 127.2; 126.9; 124.5; 123.9; 123.6. MALDI-TOF
(calcd/found, [M+.], g molꢀ1): 821.031/821.062. HRMS (calcd/
found, [M+.], gmolꢀ1): 821.0315/821.0300. Elemental anal. (calcd/
found, %): C (65.82/65.89); H (3.95/3.76); N (1.57/1.54); S
(21.52/22.59). Tf (DSC, °C): 155. Td (TGA, °C): 211. IR (cmꢀ1):
1652 (—CdO).
VilsmeierꢀHaack formylation of bithiophene led quantita-
tively to 5-formyl-2,20-bithiophene. This later compound was
acetalized with trimethyl orthoformate in the presence of a
catalytic amount of p-toluene sulfonic acid and molecular sieves.
The acetal was used for the next step without further purification;
the addition of n-BuLi and trimethyltin chloride at ꢀ78 °C led to
the stannic derivative 4 in 89% overall yield.11 A Stille coupling
between compound 4 and trisbromophenylamine followed by a
hydrolysis with a NaHCO3 solution led to trialdehyde 5 in 72%
yield. Knoevenagel condensation of trialdehyde 5 and malono-
dinitrile gave the target BT compound in 57% yield. All
compounds were satisfactorily characterized by NMR and mass
spectrometry.
Spectroscopic Measurements. The optical and electroche-
mical properties of the compounds have been analyzed by
UVꢀvis absorption spectroscopy, fluorescence emission spec-
troscopy, and cyclic voltammetry in methylene chloride in the
presence of tetrabutylammoniumhexafluorophosphate (TBAHP)
as supporting electrolyte. The corresponding results are listed in
Table 1.
While the UVꢀvis absorption spectrum of compound 2
presents a single broad absorption band with a maximum at
402 nm, the spectra of all other compounds that contain electron-
withdrawing end groups exhibit two absorption bands. Examina-
tion of the data in Table 1 shows that replacing the carboxalde-
hyde end group with the stronger electron-acceptor dicyanovinyl
leads to a bathochromic shift of the first and second absorption
maximum of 64 and 87 nm between 3 and TV, respectively, and
of 61 and 83 nm between 5 and BT, respectively.
On the other hand, comparison of the data for the three target
compounds T, TV, and BT shows that, as expected, the extension
of the conjugation length leads to a red shift of the absorption
maxima (see Figure 1). However, although BT possesses, in principle,
the longest topological conjugation length, TV absorbs at the
longest wavelength and thus presents the most extended electron
delocalization. This result can be explained by the combined
effects of the lower overall aromatic character of thienyleneviny-
lene systems and by the limitation of inter-ring rotational disorder
inherent to oligothiophenes.12
Tris[4-(5-dicyanomethylidenemethyl-2,20-bithiophen-5yl)-
phenyl]amine (BT). At room temperature and under inert atmos-
phere, 150mgofaldehyde5(0.182 mmol) was dissolved in 150 mL
of chloroform. Then, 10 equiv of malonodinitrile (1.82 mmol, 121 mg)
and four drops of triethylamine were added. The mixture was refluxed
overnight. The mixture was allowed to cool to room temperature,
and 50 mL of chloroform was added. The organic phase was washed
three times with 150 mL of an aqueous solution of sodium hydro-
xide (10% molar) and washed once with water, and then it was
dried over anhydrous magnesium sulfate. After the filtration and
evaporation of the solvent, the residue was purified by chroma-
tography on silica gel, using dichloromethane as eluent, leading
to 100 mg of a purple powder (yield: 57%). 1HNMR(CDCl3, ppm):
7.75 (s, 1H); 7.63 (d, 1H, 3J = 4 Hz); 7.51 (d, 2H, 3J = 8.5 Hz);
7.41 (d, 1H, 3J = 4 Hz); 7.18 (m, 2H); 7.13 (d, 2H, 3J = 8.5 Hz).
13C NMR (CD2Cl2, ppm): 150.2; 149.1; 146.9; 142.9; 140.4;
134.9; 133.7; 133.2; 128.4; 127.9; 126.7; 126.5; 124.2; 124.1;
114.2; 113.5. MALDI-TOF (calcd/found, [M + H]+., g molꢀ1):
966.06/966.04. HRMS (calcd/found, [M+.], g molꢀ1): 965.06467/
965.0638. Tf (DSC, °C): 181. Td (TGA, °C): 201.
Computational Study. The geometries of the molecules
under study have been first optimized at the DFT (Density Functional
Theory) level with the B3LYP functional and the 6-31G(d,p) basis
set by imposing a C3 symmetry.8 On the basis of the optimized
geometries, absorption spectra have been calculated by the time-
dependent DFT (TD-DFT) formalism with the BHandHLYP
functional and a 6-31G(d,p) basis set. The use of the hybrid
BHandHLYP functional has been motivated by the presence of
electronic transitions with a strong intramolecular charge transfer
character to cope with the inherent deficiency of pure TD-DFT
in dealing with such excited states.9
To gain more detailed information on the nature of the two
optical transitions observed in the spectrum of the donorꢀacceptor
compounds, the UVꢀvis absorption spectra of compounds T,
TV, and BT have been recorded in a series of solvents, presenting
a large variation of polarity (toluene, THF, chlorobenzene, and
chloroform) and KamletꢀTaft π* constants.13 When a correla-
tion with the empirical Taft polarity scale is found, an almost
linear relationship is observed between the transition energy
(expressed in kcal molꢀ1) and π*, E = E° + Sπ*. The values of E°
and S give valuable information on the nature of electronic trans-
itions. E° corresponds to the transition energy in a nonpolar solvent
so that two compounds of comparable polarity present similar E°
values. S models the solvatochromic sensibility of the compound;
a negative value of S indicates a positive solvatochromism. The
absolute value of S increases with the polarizability of the
compound. Figure 2 shows the variation of the energy of the long
wavelength absorption band of the three compounds vs the
KamletꢀTaft constant. For all compounds, a negative S coeffi-
cient is observed, suggesting high polarizabilities and an internal
charge transfer character associated with the transition. The E°
’ RESULTS AND DISCUSSION
Syntheses. Compound T was prepared as already reported.5c,d
Tris(p-formylphenyl)amine (1) was obtained in two steps from
triphenylamine using a known procedure.10 A 3-fold WittigꢀHorner
olefination of compound 1, with an excess of the phosphonate
anion of thiophene, gave compound tris(4-(thiophen-2-ylvinyl)-
phenyl)amine (2) in 67% yield. Vilsmeier formylation of 2, using
POCl3/DMF in refluxing 1,2-dichloroethane, led to the corre-
sponding trialdehyde 3 in 30% yield. Compound TV was finally
obtained in 87% yield by Knoevenagel condensation of 3, with
malonodinitrile in the presence of triethylamine in chloroform at
room temperature (see Scheme 1).
values of T and BT are very close (62.0 and 61.8 kcal molꢀ1
respectively; see the Supporting Information), indicating analo-
gous polarities. The lower value of E° for TV (59.1 kcal molꢀ1
,
)
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Scheme 1
Table 1. Data of UVꢀVis Spectroscopy, Cyclic Voltammetry and Fluorescence Emission Spectroscopy for T, TV, BT, 2, 3, and 5
λmax
(nm)a
log
(ε)a
λmax
(nm)b
λem
(nm)d
Φem
(%)d
c
c
c
Epa1
Epa2
Epc
T
370ꢀ510
407ꢀ541
415ꢀ527
402
4.66ꢀ5.02
4.45ꢀ4.76
4.73ꢀ4.92
4.88
377ꢀ539
360ꢀ576
358ꢀ548
1.08
0.90
0.88
0.68
0.84
0.82
1.67e
1.40e
1.28
ꢀ1.2e
ꢀ1.1e
ꢀ1.1e
635/559
700/604
703/599
15/25
0.3/2
TV
BT
2
0.4/31
3
343ꢀ454
354ꢀ444
4.54ꢀ4.84
4.70ꢀ4.99
5
1.23
a 10ꢀ5 M in CH2Cl2. b In the solid state, spin-cast from CH2Cl2 2.5 mg/mL, 2000 rnd/min. c 0.10 M TBAHP methylene chloride, scan rate 100 mV sꢀ1
,
ref SCE. d In CH2Cl2/toluene against rhodamine B in ethanol as standard. e Nonreversible processes.
reflects a higher polarity. The S values of three compounds are
very close. Nevertheless, despite high standard deviations, one
can observe a slightly higher value for TV (S = ꢀ9.1) when
compared with T (S = ꢀ8.2) and BT (S = ꢀ8.8), which may
indicate a higher polarizability. These two observations may
indicate a better conjugation between the triphenylamine node
and the outer electron withdrawing moieties in TV. The same
measurements performed on the high energy transition band
lead to a weaker linearity of results and lower S coefficients, which
indicates a moderate effect of the solvent on this transition.
Thin films of the target compounds have been spun-cast on
glass from methylene chloride solutions. The UVꢀvis absorption
spectra of the films (Table 1, Figure 1) reveal bathochromic shifts
of the absorption maxima as a result of intermolecular interac-
tions and/or planarization of the conjugated backbone. T, BT,
and TV present a maximum of absorption at 540, 548, and
576 nm, respectively (Table 1).
The photoluminescence of the three compounds has been
investigated by fluorescence emission spectroscopy; the results
are reported in Table 1. In methylenechloride, only T presents a
decent fluorescent quantum yield, while the other two com-
pounds, TV and BT, only fluoresce scarcely. On the contrary, in
toluene, T, TV, and BT present fluorescent quantum yields
attaining 25, 31, and 32%, respectively. This solvent-dependent
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Figure 1. UVꢀvis absorption spectra of compounds T (long dash), TV (short dash), and BT (solid line). (Left) 10ꢀ5 M in methylene chloride. (Right)
In the solid states (spin-cast films).
Figure 2. Energy transitions of the low energy band for compounds T
(triangle), TV (square), and BT (circle) vs Taft constants of solvents.
behavior remains unexplained up to now and deserves to be
cleared up by further photophysical experiments. Analogously to
λmax, the λem values of compounds T, TV, and BT all present a
large sensibility to solvent polarity. For example, in toluene and
methylenechloride, derivative TV emits at 604 and 700 nm,
respectively. This behavior is characteristic of a photoinduced
intramolecular charge transfer. In the series, compound BT
presents larger Stokes' shifts than T and TV, probably because
of larger conformation changes between fundamental and ex-
cited states.
Electrochemical Measurements. The cyclic voltammograms
of compounds 2, 3, 5, TV, and BT exhibit a first reversible one-
electron oxidation wave, corresponding to the formation of a
stable cation radical (Figure 3). The potential of the first anodic
peak (Epa1) depends on the extension of the conjugated system
as well as on the nature of the terminal substituent. Thus, the
introduction of substituents of increasing electron-acceptor
character produces a positive shift of Epa1 from 0.68 V for
compound 2 to 0.84 V for 3 and 0.90 V for TV. Conversely,
the lengthening of the conjugated branches produces the reverse
Figure 3. CV traces of compounds T (top), TV (middle), and BT
(bottom) in 0.10 M TBAHP/CH2Cl2; scan rate 100 mV sꢀ1
.
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Figure 4. DFT-calculated orbitals of compound TV: (a) HOMO, (b) HOMOꢀ1, (c) HOMOꢀ2, (d) LUMO, (e) LUMO+1, and (f) LUMO+2.
Table 2. DFT-Calculated Occupied (H: HOMO) and Unoccupied (L: LUMO) Molecular Orbital Energies of T, TV, BT, 2, 3, and 5
cmpd
Hꢀ3 (eV)
Hꢀ2 (eV)
Hꢀ1 (eV)
H (eV)
L (eV)
L+1 (eV)
L+2 (eV)
L+3 (eV)
T
ꢀ8.16
ꢀ7.79
ꢀ7.51
ꢀ7.22
ꢀ7.69
ꢀ7.36
ꢀ7.46
ꢀ7.09
ꢀ6.92
ꢀ6.33
ꢀ6.85
ꢀ6.68
ꢀ7.46
ꢀ7.09
ꢀ6.92
ꢀ6.33
ꢀ6.85
ꢀ6.68
ꢀ6.61
ꢀ6.40
ꢀ6.33
ꢀ5.51
ꢀ6.03
ꢀ6.00
ꢀ2.30
ꢀ2.42
ꢀ2.38
ꢀ0.63
ꢀ1.52
ꢀ1.49
ꢀ2.30
ꢀ2.42
ꢀ2.38
ꢀ0.06
ꢀ1.52
ꢀ1.49
ꢀ2.10
ꢀ2.25
ꢀ2.31
ꢀ0.18
ꢀ1.24
ꢀ1.34
ꢀ0.42
ꢀ0.94
ꢀ1.00
0.70
TV
BT
2
3
0.01
5
ꢀ0.24
effect as a result of the extended charge delocalization. Thus,
at 1.08 V, compounds TV and BT show Epa1 values at 0.90 and
while T, which contains the shortest conjugated branches, oxidizes
0.88 V, respectively.
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Table 3. Experimental (exptl) and Theoretical (theor) Transition Energies for the Lowest Two Absorption Peaks of Compounds
T, TV, BT, 2, 3, and 5a
1st peak(exptl) (eV)
2nd peak(exptl) (eV)
1st peak(theor) (eV)
2nd peak(theor) (eV)
ΔE(exptl) (eV)
ΔE(theor) (eV)
T
2.43
2.29
2.35
3.07
2.72
2.79
3.34
3.07
2.99
3.94
3.61
3.51
2.98
2.73
2.74
3.37
3.12
3.12
3.95
3.63
3.54
4.41
4.12
3.99
0.91
0.78
0.64
0.87
0.89
0.72
0.964
0.90
0.80
1.04
1.01
0.87
TV
BT
2
3
5
a The experimental and theoretical energy difference between the two transitions is also reported.
observed experimentally. When comparing the energies of the
frontier orbitals of TV vs 3 as well as BT vs 5, we notice a
significant stabilization of the LUMO (0.9 eV in both cases) and a
weaker stabilization of the HOMO level (around 0.35 eV in both
cases) as a result of the strong withdrawing character of the cyano
groups.
The optimized geometry and shape of the frontier electronic
levels of TV are shown in Figure 4. As generally observed for TPA
derivatives, the three central NꢀC bonds are coplanar, which is
characteristic of enamines. Steric hindrance between the phenyl
rings grafted on the central nitrogen atom generates the typical
propeller shape of the molecules.14 In the case of T and BT, the
steric hindrances between the hydrogen atoms grafted on the
central phenyl ring and the lateral thiophene lead to a twist angle
of ca. 24°. On the contrary, in compound TV, which presents a
vinylene-linkage between the heterocycles, each conjugated
branch appears planar.
The HOMO is delocalized over the whole molecule with large
coefficients on the central node and a smaller weight on the
lateral dicyanovinyl systems (Figure 4a), while the LUMO
(LUMO+1, Figure 4dꢀe) is localized on two branches and
presents large coefficients on the lateral electron withdrawing
groups. Interestingly, the orbital features of TV described above
are reproduced for compounds T and BT, although the chemical
structures are different (see the Supporting Information).
UVꢀVis Spectra. Experimental and theoretical transition en-
ergies of the two main electronic transitions are collected in
Table 3. Experimental and theoretical results, though being
systematically shifted by ca. 0.5 eV, are very consistent. The
lengthening of the conjugated spacer going from T to TVꢀBT
leads, in both cases, to a decrease in the difference of energy
between the two transitions and to a bathochromic shift of the
low energy band. Moreover, the reinforcement of the acceptor
strength from aldehyde to dicyanovinyl group leads to a further
bathochromic shift of the lowest band by 0.4ꢀ0.5 eV.
Figure 5. Experimental (10ꢀ5 M in methylenechloride, top) and
theoretical (bottom) spectra for T (black), BT (blue), and TV
(red).
Figure 5 presents the calculated and experimental UVꢀvis
spectra of T, TV, and BT in electronvolts. The detailed descrip-
tion of the different transitions provided by the calculations is
given in the Supporting Information. In both cases, the low
energy band corresponds mainly to two degenerate excited states
mostly described by HOMOꢀLUMO and HOMOꢀLUMO+1
transitions, respectively. As seen from Figure 4, these transitions
correspond to charge transfer transitions, with both an intrab-
ranch character due to the different localizations of the HOMO/
LUMOꢀLUMO+1 levels on a given branch (vide supra) and
interbranch character (likely to dominate the solvatochromic
effects) due to the different spatial extent of the HOMO/
LUMOꢀLUMO+1 levels on the whole molecular backbone; this
fully rationalizes the solvatochromism observed experimentally.
Compound BT, which contains the longest conjugated branches,
undergoes a second reversible oxidation process to form a
dication at 1.28 V, while for T and TV the second oxidation process
is quasi-irreversible.
Theoretical Calculations. Electronic Structure. The calcu-
lated occupied and unoccupied molecular orbital energies in
the gas phase are collected in Table 2 for 2, 3, 5, T, TV, and BT.
As a consequence of the imposed C3 symmetry, the occupied
HOMOꢀ1 and HOMOꢀ2 and unoccupied LUMO and LUMO
+1 levels are degenerate. Compounds TV and BT do show an
increase (a decrease) in their HOMO (LUMO) energies com-
pared to T as a result of the increase in conjugation length, as
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ARTICLE
The second peak is described at the TD-DFT level by a large
mixing of transitions; the reduced solvatochromism might be
rationalized by the fact that the interbranch charge transfer
character is reduced by an increased intrabranch character (as a
result of a similar spatial localization of the orbitals involved in
the one-electron transitions) and/or by compensation effects
between the existing interbranch charge transfer processes
associated with the individual transitions.
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’ CONCLUSION
A new series of star-shaped chromophores constituted by a
TPA core substituted by conjugated branches of variable con-
jugation length and constitution bearing terminal electron-with-
drawing groups has been synthesized, and their electronic
properties have been analyzed by UVꢀvis spectroscopy and
cyclic voltammetry. The nature of the low energy optical transi-
tion has been investigated using TD-DFT calculations, confirm-
ing the charge transfer character of this transition. The synthesis
of more soluble analogues of this series of compounds is now in
progress and will be reported in due time.
’ ASSOCIATED CONTENT
S
Supporting Information. Taft constants of solvents, E°
b
and S values, absorption maxima, and corresponding transition
energy in kcal/mol. Theoretical description of the optical transi-
tions. Shape of the molecular orbitals for BT and T. This material
is available free of charge via the Internet at http://pubs.acs.org.
(7) (a) Zhang, J.; Deng, D.; He, C.; He, Y.; Zhang, M.; Zhang, Z.-G.;
Zhang, Z.; Li, Y. Chem. Mater. 2011, 23, 817–822. (b) Mikroyannidis,
J. A.; Kabanakis, A. N.; Sharma, S. S.; Sharma, G. D. Org. Electrochem.
2011, 12, 774–784. (c) Shan, H.; Fan, H.; Liu, Y.; Hu, W.; Li, Y.; Zhan, X.
Adv. Mater. 2011, 23, 1554–1557.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: philippe.leriche@univ-angers.fr. Phone: 0033-2-41-73-
50-10.
(8) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
*E-mail:jean.roncali@univ-angers.fr. Phone:0033-2-41-73-54-43.
(9) Dreuw, A.; Weisman, J. L.; Head-Gordon, M. J. Chem. Phys.
2003, 119, 2943–2946.
(10) Mallegol, T.; Gmouh, S.; Ait Amer Meziane, M.; Blanchard-
’ ACKNOWLEDGMENT
€
Desce, M.; Mongin, O. Synthesis 2005, 11, 1771–1774.
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The authors thank the French Research Minister for a grant
given to E.R. The work in Mons is supported by the Belgian
National Fund for Scientific Research (FNRS). Y.O. and J.C. are
FNRS research fellows.
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