Tuning optical absorption in pyran derivatives for DSSC

Article abstract of DOI:10.1016/j.jphotochem.2016.01.018

Four nobel metal free dyes for application as photosensitizers in DSSC have been synthesized and their chemical-physical properties characterized by UV-vis absorption spectroscopy, cyclic voltammetry and DFT theoretical computations. The dyes are based on a pyran core, functionalized with different electron acceptor groups, symmetrically linked to carbazole electron donor mojeties. The four dyes have different optical absorption properties, affording a range of colors ranging from orange to blue, covering most of the visible spectral region. All the dyes feature high molar extinction coefficient, up to a value of 1.0 × 10⁵ L mol^-1 cm^-1. The dyes were used as photosensitizers in DSSC and the photovoltaic characterization of these devices was performed under simulated solar radiation. The cell performance is higher for chromophores featuring higher LUMO energies as a consequence of a better electron injection in TiO₂ oxide. A maximum power conversion efficiency of 1.9% has been achieved.

Full text of DOI:10.1016/j.jphotochem.2016.01.018

Accepted Manuscript
Title: Tuning optical absorption in pyran derivatives for DSSC
Author: C. Maglione A. Carella R. Centore S. Fusco A.
Velardo A. Peluso D. Colonna A. Di Carlo
PII:
S1010-6030(16)00021-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.01.018
JPC 10117
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
27-7-2015
9-12-2015
16-1-2016
Please cite this article as: C.Maglione, A.Carella, R.Centore, S.Fusco, A.Velardo,
A.Peluso, D.Colonna, A.Di Carlo, Tuning optical absorption in pyran derivatives
for DSSC, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.01.018
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Tuning optical absorption in pyran derivatives for DSSC
C. Maglionea, A. Carellaa* antonio.carella@unina.it, R. Centorea, S. Fuscob, A. Velardob, A.
Pelusob, D. Colonnac, A. Di Carlod
aDepartment of Chemical Sciences, University of Napoli Federico II, Complesso Universitario
Monte Sant’Angelo, Via Cintia, 80126, Napoli, Italy
bDepartment of Chemistry and Biology, University of Salerno, via Giovanni Paolo II, Fisciano, I-
84084, Italy
cDYEPOWER, Viale Castro Pretorio 122, 00185 Roma, Italy
dCenter for Hybrid and Organic Solar Energy (CHOSE), University of Rome “Tor Vergata”, Via
Giacomo Peroni 400/402, 00131 Roma
*Corresponding author.
1

Graphical Abstract
2

Highlights
We synthesized four novel metal free photosensitizers for DSSC
Dyes are based on a pyran acceptor core simmetrically linked to carbazole donor groups
Dyes show colors ranging from orange to blue, covering the most of visible spectrum
The electronic structure of the dyes were investigated at DFT level
DSSC based on the reported chromophores show power conversion efficiency up to 1.9%
3

Abstract
Four novel metal free dyes for application as photosensitizers in DSSC have been synthesized and their
chemical-physical properties characterized by Uv-vis absorption spectroscopy, cyclic voltammetry and DFT
theoretical computations. The dyes are based on a pyran core, functionalized with different electron
acceptor groups, symmetrically linked to carbazole electron donor mojeties. The four dyes have different
optical absorption properties, affording a range of colors ranging from orange to blue, covering most of the
visible spectral region. All the dyes feature high molar extinction coefficient, up to a value of 1.0∙105 L∙mol-
1∙cm-1. The dyes were used as photosensitizers in DSSC and the photovoltaic characterization of these
devices was performed under simulated solar radiation. The cell performance is higher for chromophores
featuring higher LUMO energies as a consequence of a better electron injection in TiO2 oxide. A maximum
power conversion efficiency of 1.9 % has been achieved.
Keywords: Synthesis; dyes; pyran; DFT; DSSC.
4

1. Introduction
The intensive exploiting of the fossil fuels reserves, due to the increasing world energy consumption, along
with the environmental issues related to the combustion of these energy sources, has fueled in the recent
years a growing interest in photovoltaic technology.1 Solar energy represents the most abundant
renewable and clean energy: the coverage of 0.4 % of the world’s surface area with photovoltaic modules
of 15 % efficiency has been considered sufficient to meet the world’s energy needs. While solar energy
market is nowadays dominated by silicon, the high production and energy cost of this class of photovoltaics
has fostered the development of organic solar cells which could represent a cost-effective viable
alternative; moreover, the peculiar mechanical features of organic materials open the way to a range of
novel possible applications, not affordable for the rigid and heavy inorganic materials. In this context, dye
sensitized solar cells (DSSCs) have attracted considerable attention in the last 20 years2 and very promising
efficiency, up to 11% 3 have been obtained using ruthenium(II)-polypyridyl complexes as photosensitizers.
Ruthenium complexes are, anyway, intrinsically expensive and, moreover, feature moderate molar
absorption coefficient (around 20000 L mol-1 cm-1). For these reasons, recent years have seen a growing
interest in metal free organic dyes for application in DSSCs, typically constituted by a push-pull molecular
structure:4 the advantages of this kind of system rely on their high molar absorption coefficient and on the
tunability of their absorption and electronic properties achievable through a suitable molecular design.
Most of synthetic organic dyes used in DSSCs are based on heterocycles.4a,b These moieties, in fact, have
proven to be very efficient building blocks for the synthesis of active molecules in many fields of advanced
materials spanning from liquid crystals 5 and optoelectronics 6 to organic semiconductors 7. In the field of
DSSCs, heterocycles can be used, depending on their character, as electron donor or acceptor groups 4a,b, as
pi-conjugated bridges 4a,b or even as acidic groups for anchoring to the inorganic semiconductor.8
Pyran containing dyes, in particular, represent a class of compounds deeply investigated in the field of
organic electronics and photonics; the versatility of the starting building block of this kind of compounds (i.
e. 2,6-dimethyl--pyrone) has allowed the synthesis of several different molecular structures, affording
materials characterized by different interesting properties, such as fluorescence,9 second order nonlinear
5

optical activity,10 two photon absorption,11 photovoltaic activity.12 More recently, the use of this class of
compounds in DSSCs was investigated.13
We report here on novel organic photosensitizers based on an electron acceptor pyran core symmetrically
linked, through ethylene bridges, to donor carbazole moieties. Their chemical diagrams are reported in
Figure 1.
Carbazole ring is a building block widely investigated in organic electronics, mainly because of its strong
electron donor ability and low oxidation potential.14 Several studies on DSSC photosensitizers containing
carbazole mojeties in their molecular structure are reported in the literature.15 In the present work,
carbazole groups are functionalized with two acrylic acid moieties for allowing binding to the TiO2 surface.
The enhanced anchoring properties of the dyes due to the presence of two carboxyl groups are expected to
increase the binding strength of the photosensitizers to TiO2, possibly resulting in an enhancement of
charge transfer and cell performance, as shown in previous papers.16 The pyran core is instead
functionalized with electron acceptor groups of different strengths: as also reported in previous works,17
the functionalization of a same molecular fragment with electron acceptor groups of different strength, is
expected to result in the tuning of the optical absorption of the dyes so that different parts of the visible
spectrum can be covered.
The synthesized chromophores have been characterized by optical and electrochemical measurements;
their electronic structure has been analysed by Density Functional Theory (DFT) and their efficiencies in
DSSC tested under simulated sun radiation.
2. Materials and methods
2.1 Synthesis
All the intermediates used in the synthetic procedures described below, have been purchased by Sigma
Aldrich, TCE Europe or Acros Organics, and used without further purification. The only exceptions are 3-
6

dicianovinylindan-1-one and 2-(1,1-dioxidobenzo[b]thiophen-3(2H)-ylidene)malononitrile (Sandoz group)
that have been prepared according, respectively, to references 18 and 19.
2-(2,6-dimethyl-4H-pyran-4-ylidene)malononitrile (A1)
2,6-dimethyl--pyrone (12.50 g, 0.100 mol) and malononitrile (6.90 g, 0.104 mol) were dissolved in 50 mL of
acetic anhydride; the solution was refluxed, under nitrogen flux, for 5 h. The system was then slowly cooled
down to room temperature and the crystallization of a brown compound occurred. This compound was
recovered by suction filtration and recrystallized by chloroform-hexane. Light brown needle-shape crystals
were obtained. The yield was 60 %.
1H-NMR (CDCl3, 200 MHz); = 2.31 (s, 6H, -CH3); 6.53 (s, 2H, pyran CH).
Mp: 192 °C.
2-(2,6-dimethyl-4H-pyran-4-ylidene)-thiobarbituric acid (A2)
The same synthetic strategy used for the preparation of A1 was followed; the only difference is that
thiobarbituric acid was used instead of malonitrile in the condesantion reaction with the pyrone derivative.
Yellow needles were recovered after crystallization in CHCl3-hexane. The yield was 68 %.
1H-NMR (CDCl3, 200 MHz); = 1.30 (t, 6H, J=, -CH3);2.46 (s, 6H, -CH3), 4.57 (q, 4H, J=, -CH2); 8.82 (s, 2H, pyran
CH).
Mp: 195°C.
2-(2-(2,6-dimethyl-4H-pyran-4-ylidene)-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (A3)
The compound was synthesized following the same procedure used for A1 synthesis, except that 3-
dicianovinylindan-1-one was used instead of malononitrile. A brown crystalline solid was obtained after
recrystallization (CHCl3-hexane). The yield was 58 %.
7

1H-NMR (CDCl3, 400 MHz); = 2.50 (s, 6H, -CH3), 7.50 (broad, 1H); 7.60 (m, 3H,Ar-H); 7.74 (d, 1H, J=7.0 Hz);
8.44 (d, 1H, J=7.0 Hz, Ar-H).
Mp: 212 °C
2-(2-(2,6-dimethyl-4H-pyran-4-ylidene)-1,1-dioxidobenzo[b]thiophen-3(2H)-ylidene)malononitrile (A4)
The compound was synthesized following the same procedure used for A1 synthesis, except that Sandoz
group was used instead of malononitrile. A reddish crystalline solid was obtained. After recrystallization
(CHCl3-hexane) The yield was 62 %.
1H-NMR (CDCl3, 400 MHz); = 2.52 (s, 6H, -CH3), 7.05 (broad, 1H, Ar-H); 7.73 (m, 3H, Ar-H); 8.09 (m, 1H, Ar-
H); 8.68 (m, 1H, Ar-H).
Mp: 258 °C
9-(2-ethylhexyl)-9H-carbazole (1)
Finely powdered KOH (34.00 g, 0.61 mol) were suspended in DMF (150 mL). The system was kept under
stirring at room temperature for 15 min and then, carbazole (16.10 g, 0.0960 mol) was added. After 1 h, 2-
ethylhexylbromide (25.28 g, 0.130 mol) was poured in the system and the temperature was raised to
100 °C. After 6 h, the system was cooled down to room temperature and the solid inorganic part filtered
off. The filtered solution was poured in 300 mL of water and extracted with hexane (3x100 mL). The organic
phase was then washed with brine and water, dried over Na2SO4 and the solvent removed at reduced
pressure. The resulting crude oil was purified by silica gel liquid chromatography (hexane). A colorless oil
was recovered. The yield was 75 %.
1H-NMR (CDCl3, 400 MHz); = 0.90 (m, 6H, -CH3); 1.27-1.40 (m, 8H, -CH2); 2.07 (broad, 1H,-CH); 4.15 (d, 2H,
J=8.0 Hz); 7.23 (t, 2H, J=8.0 Hz, Ar-H); 7.40 (d, 2H, J=8.0 Hz, Ar-H); 7.47 (d, 2H, J=8.0 Hz, Ar-H), 8.11(d, 2H,
J=8.0 Hz, Ar-H) .
3-bromo-9-(2-ethylhexyl)-9H-carbazole (2)
8

Compound 1 (23.9 g, 0.0850 mol) was solved in dry DMF; a solution of N-bromosuccinimide (15.22 g,
0.0850 mol) in 25 mL of dry DMF was then poured dropwise in the carbazole solution. The system reacted
overnight at room temperature, in the dark. The solution was then poured in 200 mL of water and
extracted with diethyl ether (3x100 mL). The organic phase was then washed with brine and water, dried
over Na2SO4 and the solvent removed at reduced pressure. The resulting crude oil was purified by silica gel
liquid chromatography (hexane/ethyl acetate 80/20 v/v). A yellow oil was recovered. The yield was 75 %.
1H-NMR (CDCl3, 400 MHz); = 0.87 (t, 3H, J=7.2 Hz, -CH3); 0.90 (t, 3H, J=7.2 Hz, -CH3); 1.24-1.40 (m, 8H, -
CH2); 2.07 (broad, 1H,-CH); 4.15 (d, 2H, J=8.0 Hz); 7.22 (d, 2H, J=8.0 Hz, Ar-H); 7.37-7.55 (m, 3H, Ar-H); 8.03-
8.20 (m, 2H Ar-H).
3-bromo-9-(2-ethylhexyl)-6-formyl-9H-carbazole (3)
Dry DMF (20.9 mL, 19.73 g, 0.27 mol) was solved in 25 mL of dry 1,2-dichloroethane (DCE) and kept under
nitrogen at 0 °C, in an ice water bath. POCl3 (25.1 mL, 41.4 g, 0.27 mol) was then added dropwise to the
system that, after the addition, was warmed to room temperature. Then compound 2 (15.0 g, 0.0420 mol),
solved in 20 mL of dry DCE, was added dropwise and the temperature raised to 90 °C. The system was
allowed to react at that temperature overnight and, after being cooled down to room temperature, poured
in 400 mL of water. The pH was raised to about 5 by adding 5 % wt NaOH water solution. The obtained
emulsion was extracted with chloroform (3x250 mL). The organic phase was washed with brine and water,
dried over Na2SO4 and the solvent removed at reduced pressure. The resulting crude oil was purified by
silica gel liquid chromatography (hexane/ethyl acetate 80/20 v/v). A yellow oil was recovered. The yield was
65 %.
1H-NMR (CDCl3, 200 MHz); = 0.84 (t, 3H, J=7.2 Hz, -CH3); 0.91 (t, 3H, J=7.2 Hz, -CH3) 1.23-1.38 (m, 8H, -CH2);
2.00 (q, 1H,J=6.4 Hz,-CH); 4.18 (d, 2H, J=8.0 Hz); 7.31 (d, 1H,J=8.7 Hz, Ar-H); 7.46 (d, 1H, J=8.4 Hz, Ar-H);
7.60 (dd, 1H, J1=2.0 Hz, J2=8.4 Hz, Ar-H); 8.03 (dd, 1H, J1=1.4 Hz, J2=8.0 Hz, Ar-H); 8.26 (d, 1H, J=2.0 Hz, Ar-
H); 8.55 (d, 1H, J=1.4 Hz, Ar-H); 10.09 (s, 1H, CHO).
9

(E)-3-(9-(2-ethylhexyl)-6-formyl-9H-carbazol-3-yl)acrylic acid (D)
Compound 3 (2.000 g, 0.0052 mol) and acrylic acid (3.5 ml, 3.68 g, 0.051 mol) were dissolved in 10 mL of
dry DMF; the solution was degassed with nitrogen for 15 minutes and then palladium acetate ( 0.26 mmol,
58.3 mg), tri-o-tolylphosphine (0.157 g, 0.52 mmol) and triethylamine (1.8 mL, 1.307 g, 0.013 mol) were
added. The system was kept at 100 °C for 24 h, stirred under nitrogen. The reaction mixture was then
cooled down to ambient temperature and poured in 50 mL of water. The aqueous layer was extracted with
chloroform (3 x 50 mL). The organic phase is washed with brine and water, dried over Na2SO4 and the
solvent removed at reduced pressure. The resulting crude oil was purified by silica gel liquid
chromatography (chloroform-methanol 98/2 v/v). A yellow oil was recovered that slowly crystallized at
ambient temperature. The yield was 52 %.
1H-NMR (CDCl3, 400 MHz); = 0.84 (t, 3H, J=7.2 Hz, -CH3); 0.90 (t, 3H, J=7.2 Hz, -CH3); 1.25-1.36 (m, 8H, -
CH2); 2.00 (broad, 1H,-CH); 4.11 (d, 2H, J=7.6 Hz);6.44 (d, 1H, J=16.0 Hz, CH=CH); 7.35 (d, 1H, J=8.4 Hz, Ar-
H); 7.41 (d, 1H, J=8.4 Hz); 7.66 (d, 1H, J=8.0 Hz, Ar-H); 7.91 (d, 1H, J=16.0 Hz, CH=CH), 8.13 (s, 1H); 8.48 (s,
1H); 10 06 (s, 1H).
(2E,2'E)-3,3'-(((1E,1'E)-(4-(dicyanomethylene)-4H-pyran-2,6-diyl)bis(ethene-2,1-diyl))bis(9-(2-ethylhexyl)-
9H-carbazole-6,3-diyl))diacrylic acid (C1)
Compound A1 (0.151 g, 0.88 mmol) and D (0.730 g, 1.93 mmol), were dissolved in a solution constituted by
9 mL of CH3CN and 1 mL of pyridine. The solution, stirred in nitrogen atmosphere, was taken at reflux
temperature and then, five drops of piperidine were added. The solution was refluxed overnight and then
poured in 50 mL of MeOH. The precipitation of a compound occurred, that was then recovered by filtration
and recrystallized by THF-hexane. An amorphous orange solid was obtained. Yield was 62 %.
1H-NMR (DMSO, 500 MHz); = 0.74 (t, 6H, J=7.0 Hz, -CH3); 0.81 (t, 6H, J=7.0 Hz, -CH3); 1.10-1.29 (m, 16H, -
CH2); 1.92 (m, 2H,-CH); 4.19 (d, 4H, J=6.0 Hz); 6.51 (d, 2H, J=15.0 Hz, CH=CH); 6.64 (s, 2H, pyran CH); 7.25 (d,
10

2H, J=15.0 Hz, CH=CH); 7.54 (m, 4H, Ar-H); 7.74 (d, 2H, J=15.0 Hz, CH=CH); 7.78-7.80 (m, 6H, Ar-H), 8.42 (s,
2H); 8.59 (s, 2H); 12.23 (s broad, 1H).
13C-NMR (DMSO, 75 MHz): =167.9; 158.7; 155.4; 144.8; 142.0; 141.9; 138.2; 127.2; 126.5; 125.8; 122.6;
122.5; 120.7; 120.4; 116.1; 116.0; 115.7; 110.1; 109.8; 105.6; 55.4; 46.8; 30.1; 28.0; 23.7; 22.4; 13.7; 10.6.
Tg: 192 °C.
MALDI-TOF-MS: Found: 890.46; ‘Molecular formula C58H58N4O5 requires’: 890.44.
CHN elemental analysis: Found: C 78.24 H 6.63 N 6.21 %; ’Molecular Formula C58H58N4O5 requires’: C 78.18;
H 6.56; N 6.29 %.
(2E,2'E)-3,3'-(((1E,1'E)-(4-(1,3-diethyl-4,6-dioxo-2-thioxotetrahydropyrimidin-5(2H)-ylidene)-4H-pyran-2,6-
diyl)bis(ethene-2,1-diyl))bis(9-(2-ethylhexyl)-9H-carbazole-6,3-diyl))diacrylic acid (C2)
Compound A2 (0.185 g, 0.60 mmol) and D (0.500 g, 1.32 mmol), were dissolved in a solution constituted by
9 mL of CH3CN and 1 mL of pyridine. The solution, stirred in nitrogen atmosphere, was taken at reflux
temperature and then, five drops of piperidine were added. The solution was refluxed overnight and then
cooled down to room temperature. The precipitation of a compound occurred, that was then recovered by
filtration and recrystallized by THF-hexane. An amorphous purple solid was obtained. Yield was 60 %.
1H-NMR (DMSO, 500 MHz); = 0.73 (t, 6H, J=7.0 Hz, -CH3); 0.80 (t, 6H, J=7.0 Hz, -CH3); 1.14-1.29 (m, 20H, -
CH2 + CH3 TB ring); 1.88 (m, 2H,-CH); 4.12 (d, 4H, J=6.0 Hz, N-CH2 carbazole); 4.33 (q, 4H.N-CH2 TB ring), 6.54 (d, 2H,
J=15.0 Hz, CH=CH); 7.24 (d, 2H, J=15.0 Hz, CH=CH); 7.44-7.46 (m, 4H, Ar-H); 7.70-7.73 (m, 4H,Ar-CH); 7.81
(d, 2H, J=15.0 Hz, CH=CH), 7.88 (d, 2H, J=10.0 Hz, Ar-CH); 8.43 (s, 2H, Ar-H); 8.62 (s, 2H, Ar-H); 8.77 (s, 2H,
pyran CH).
13C-NMR (DMSO, 75 MHz): = 176.6; 168.5; 161.0; 160.5.3; 155.9; 142.1; 141.9; 138.7; 138.5; 126.6; 122.8;
116.8; 109.9; 109.7; 95.6; 45.4; 30.4; 28.1; 23.9; 22.4; 13.5; 10.5.
11

MALDI-TOF-MS: Found: 994.52 (deriving from the fragmentation of two methyl groups), 1024.47;
‘Molecular formula C63H68N4O7S requires’: 1024.48.
CHN elemental analysis: Found: C 73.95 H 6.62 N 5.35 %; ’Molecular Formula C63H68N4O7S requires’: C
73.80; H 6.69; N 5.46 O 10.92; S 3.13 %.
(2E,2'E)-3,3'-(((1E,1'E)-(4-(1-(dicyanomethylene)-3-oxo-1,3-dihydro-2H-inden-2-ylidene)-4H-pyran-2,6-
diyl)bis(ethene-2,1-diyl))bis(9-(2-ethylhexyl)-9H-carbazole-6,3-diyl))diacrylic acid (C3)
Compound A3 (0.181 g, 0.60 mmol) and D (0.500 g, 1.32 mmol), were dissolved in a solution constituted by
9 mL of CH3CN and 1 mL of pyridine. The solution, stirred in nitrogen atmosphere, was taken at reflux
temperature and then, five drops of piperidine were added. The solution was refluxed overnight and then
cooled down to room temperature. The precipitation of a compound occurred, that was then recovered by
filtration and recrystallized by THF-hexane. An amorphous violet solid was obtained. Yield was 64 %.
1H-NMR (DMSO, 500 MHz); = 0.76 (t, 6H, J=7.5 Hz, -CH3); 0.81 (t, 6H, J=7.5 Hz, -CH3); 1.15-1.30 (m, 16H, -
CH2); 2.05 (m, 2H,-CH); 4.04 (d, 4H, J=6.0 Hz, N-CH2); 6.53 (d, 2H, J=15.0 Hz, CH=CH); 7.25 (d, 2H, J=15.0 Hz,
CH=CH); 7.42 (d, 2H, J=10.0 Hz, Ar-H); 7.47-7.55 (m, 7H, Ar-CH); 7.73 (d, 2H, J=5.5 Hz, Ar-H), 7.76 (s, 2H,
pyran CH); 7.86 (s, 2H, pyran CH); 7.89 (d, 2H, J=5.5 Hz, Ar-H); 8.11 (d, 1H, J= 7.0 Hz, Ar- H); 8.40 (s, 2H, Ar-
H); 8.49 (s, 2H, Ar-H); 12.20 (s broad, 2H).
MALDI-TOF-MS: Found: 1018.48; ‘Molecular formula C67H62N4O6 requires’: 1018.46.
CHN elemental analysis: Found: C 79.11 H 6.19 N 5.44 %; ’Molecular Formula C67H62N4O6 requires’: C 78.95;
H 6.13; N 5.50; O 9.42 %.
(2E,2'E)-3,3'-(((1E,1'E)-(4-(3-(dicyanomethylene)-1,1-dioxidobenzo[b]thiophen-2(3H)-ylidene)-4H-pyran-
2,6-diyl)bis(ethene-2,1-diyl))bis(9-(2-ethylhexyl)-9H-carbazole-6,3-diyl))diacrylic acid (C4)
12

Compound A4 (0.201 g, 0.60 mmol) and D (0.500 g, 1.32 mmol), were dissolved in a solution constituted by
9 mL of CH3CN and 1 mL of pyridine. The solution, stirred in nitrogen atmosphere, was taken at reflux
temperature and then, five drops of piperidine were added. The solution was refluxed overnight and then
cooled down to room temperature. The precipitation of a compound occurred, that was then recovered by
filtration and recrystallized by THF-hexane. An amorphous blue solid was obtained. Yield was 65 %.
1H-NMR (DMSO, 500 MHz); = 0.76 (t, 6H, J=7.0 Hz, -CH3); 0.85 (t, 6H, J=7.0 Hz, -CH3); 1.16-1.32 (m, 16H, -
CH2); 1.97 (m, 2H,-CH); 4.29 (d, 4H, J=6.0 Hz, N-CH2); 6.53 (d, 2H, J=15.0 Hz, CH=CH); 7.11 (s, 2H, Ar-H); 7.41
(d, 2H, J=10.5 Hz, Ar-H); 7.54 (d, 2H, J=8.0 Hz, Ar-CH); 7.62 (d, 2H, J=8.0 Hz, Ar-H); 7.74 (d, 2H, J=15.0 Hz,
CH=CH); 7.78-7.83 (m, 4H, Ar-H); 7.96 (d, 2H, J=8.0 Hz, Ar-H); 8.04-8.07 (m, 3H, Ar-H); 8.48 (s, 2H, Ar- H);
8.53 (d, 1H, J=7.5 Hz, Ar-H); 8.67 (s, 2H, Ar-H); 12.17 (s broad, 2H).
13C-NMR (DMSO, 75 MHz): = 172.2; 167.9; 161.3; 145.3; 144.9; 142.4; 142.0; 140.6; 137.9; 134.0; 126.5;
126.0; 122.7; 122.5; 121.4; 121.1; 121.0; 117.2; 116.3; 116.0; 111.0; 110.3; 59.0; 46.8; 30.1; 28.0; 23.6;
22.5; 13.8; 10.6.
MALDI-TOF-MS: Found: 1054.78; ‘Molecular formula C67H62N4O6 requires’: 1054.43.
CHN elemental analysis: Found: C 75.34 H 5.85 N 5.19 %; ’Molecular Formula C66H62N4O7S requires’: C
75.12; H 5.92; N 5.31; O 10.61; S 3.04 %.
2.2.Chemical Physical characterization
1
All compounds were characterized by standard techniques. H NMR Spectra were recorded with Varian XL
200, Bruker 400 or Varian 500 MHz spectrometers. DSC analyses were performed in dry ultrapure nitrogen
flow using a DSC7 PerkinElmer apparatus at a scan rate of 10 K/min; DSC temperature calibration was made
using a Perkin Elmer indium sample having 99.999% purity.
13

Solution and thin film UV-Vis spectra were recorded on a spectrophotometer Jasco V-560 (scan rate 200 nm
min-1). Thin film of chromophores were prepared by spin coating of a DMF solution (10 mg∙mL-1) on glass
substrates, using a Laurell WS-650Mz-23NPP spin processor .
2.3. Electrochemical characterization
Electrochemical measurements were performed with a Metrohm 757 VA Computrace combined with a
AUTOLAB potentiostat-galvanostat. Voltammetric experiments were run under nitrogen on thin films of the
chromophores deposited on the working electrode, at a 100 mV/sec scan speed and at room temperature,
ranging between 0÷1.5 V. A single compartment three-electrode cell have been employed, with a glassy
carbon working electrode, using Ferrocene/Ferrocinum (Fc/Fc+) redox couple as internal reference as
suggested by IUPAC for organic systems. Tetrabutylammonium hexafluorophosphate (100 mM) served as
inert electrolyte.
2.4. Computational analysis
The electronic structures of the ground and the excited states were determined at density functional
theory (DFT) level, using the CAM-B3LYP functional and the 6-31+G(d,p) basis set. CAM-B3LYP was chosen
as it holds a high percentage of Hartree-Fock exchange, which is known to give more accurate results than
standard hybrid functionals for donor-acceptor conjugate dyes.20a-c Solvent effects, dimethylformamide,
have been included by using the Polarizable Continuum Model (PCM).20d All the alkyl substituents have
been replaced by methyl groups. Conformational analyses have been performed by using the Spartan
package and the Merck molecular force field.19e All computations have been performed by G09 package.20f
2.5. DSSC materials, assembly and measurement system
Titania pastes 18NR-T (transparent φ ≈ 21nm) and 18NR-AO (opaque φ ≈ 20-300nm) Dyesol were used as
received and screen printed on glass substrates (Pilkington). After relaxation the substrates were sintered
at 525 °C for 30 minutes. Counterelectrodes were made by screen printing the platinum paste (Kimet) and
fired at 425°C for 20 minutes. The sintered TiO2 were immersed in the different tetrahydrofuran solution of
dyes overnight (10-4 M, 14-16 hours) at room temperature. Substrates then have been rinsed with
14

tetrahydrofuran and dried. Surlyn 25 Solaronix is used to seal the photo and counter-electrodes and the
High Stability Electrolyte Dyesol is introduced within the electrodes.
Measurements have been carried out by means of a solar simulator ABET 2000 sun simulator supplied with
a filter AM1.5 under illumination at 0.1W/cm2.
3. Results and discussion
3.1. Synthesis
Commercial 2,6-dimethyl- -pyrone is the starting compound in the synthesis of all the reported
chromophores. Its ketone group can be easily functionalized by means of a condensation reaction with
compounds containing an acidic methylene group. In Scheme 2, the synthesis of four different derivatives,
representing the acceptor core of the designed chromophores, is reported; the reaction was performed in
acetic anydride and the pyrone molecule was reacted with groups featuring different electron-withdrawing
(EWD) strength.
The synthesis of the donor branches of the dyes is schematically shown in Scheme 3. The key reaction is the
Heck coupling between the bromo-formyl derivative of carbazole (3) and acrylic acid.
The obtained compound contains a formyl group that can react with the methyl groups of the acceptor
cores (An) in a Knoevenagel type condensation, affording the desired dyes (see Scheme 3).
All the prepared chromophores show good solubility in solvent like THF and DMF. They show a poor
tendency to crystallize and were generally recovered as amorphous powders. In one case, namely for C1,
DSC analysis makes it possible to measure the glass transition temperature (Tg = 210 °C).
3.2. Optical and electrochemical analysis
15

The optical properties of the dyes were investigated by UV-Vis analysis performed in DMF solution. The
strength of the EWD group functionalizing the pyran core has a marked effect on the absorption spectrum
of the chromophores: in particular, the stronger is the electron acceptor group, the more red-shifted is the
absorption spectrum (see Figure 1). A set of colors ranging from orange to blue is obtained that covers a
large part of the visible spectrum. All the chromophores show very high molar absorption coefficient,
ranging from 6·104 to 1·105 L∙mol-1∙cm-1, a highly desirable feature for their application as photosensitizers
in DSSC.
Because of the low tendency to crystallize, it was possible to prepare amorphous thin films of the
chromophores by spin coating from DMF solution. The UV-Vis absorption spectra of the films are shown in
Figure 2: for all the chromophores but C3, a slight bathocromic shift of the absorption features is observed
as compared to their behaviour in DMF solution. The optical bandgap of chromophores in thin films was
measured by applying the Tauc extrapolation to the absorption spectra,21 and range from 2.24 eV (C1) to
1.65 eV (C4). The optical properties of the dyes are summarized in Table 1.
The photoluminescence properties of the dyes were investigated in THF solution. C1 and C2 show a
photoluminescence quantum yield of, respectively, 2.1 and 0.7 %. The values were calculated using quinine
sulfate as fluorescence standard. Dyes C3 and C4 feature no fluorescence at all. The fluorescence spectra of
chromophores C1 and C2 are reported as supplementary data (Figure S2).
Oxidation potentials of the chromophores have been measured by means of cyclic voltammetry (Figure 3),
using the redox couple ferrocene/ferrocinium (Fc/Fc+) as internal standard. From the voltammograms,
shown in Figure 3, chromophore’s HOMO energies have been determined by applying the formula:
EHOMO  (Eonset,ox  EFc / Fc   5.1)
The HOMO energy slightly differs for each chromophores: except for C2, the observed trend seems to
suggest that, by increasing the strength of the EWD group functionalizing the pyran core, the HOMO energy
of the dye is stabilized. All the measured values are much lower than the oxidation potential of the redox
16

couple I-/I3- (-4.8 eV), generally used in DSSC devices, so that sensitizer regeneration during DSSC operation
should not pose problems.
LUMO energies of dyes have been obtained from the optical bandgaps, by applying:
ELUMO  EHOMO  Eg
LUMO energies of the dyes are reported in Table 1: during DSSC operation, electron injection from the
excited state of the dye to the conduction band of TiO2 has a driving force related to the energy difference
between dye’s LUMO and the edge of TiO2 conduction band (ECB). The commonly accepted values for the
latter, in DSSC literature, range from -4.25 to 3.95 eV.22 As observed from Figure 4, this band alignment
requirement is satisfied for chromophores C1 and C2, while dyes C3 and C4 have LUMO energies very close
to the ECB of TiO2, so that the electron injection process from the chromophore to the oxide could be less
efficient.
3.3. Computational Analysis
To gain information about the electronic structures of the ground and the excited states of the four dyes
we have carried out a computational analysis at density functional theory (DFT) level. DFT computations
were preceded by a full conformational analysis, focusing attention on the geometrical arrangements of
the carbazolic branches with respect to the pyranic core. The lowest energy conformations of all the
chromophores in the gas phase exhibit a s-trans arrangement of the conjugated C=C bonds of the pyran
ring and the ethylene bridge connecting it to the carbazole mojeties. Indeed the s-trans conformation
allows, in the gas-phase, the formation of a strong intramolecular hydrogen bond between the two
carbazolic branches. Apart from that, molecular mechanics computations predict that the s-trans
conformation is more stable than the s-cis one by ca. 1.6 kcal/mol, for each C=C double bond, and it will be
therefore retained even when the intramolecular H-bonds are disrupted. We have thus started all
electronic computations from trans conformations of both carbazolic branches.
17

The results of DFT computations are summarized in Table 2, where the predicted absorption wavelengths
and oscillator strengths have been reported, together with the dipole moments of the ground states and of
the first excited singlets.
Chromophores C1 and C2 can be classified as standard push-pull molecules inasmuch as the dipole moments
of the first excited state are significantly larger than the ground state ones. Indeed, computations predict
that excitation to the first excited singlet causes a transfer of electron density from the carbazolic moieties
towards the acceptor core, as shown in Figure 5, where the electron density changes upon transition to the
first excited state are reported for all the chromophores.
Chromophores C3 and C4 behave differently, their ground state dipole moments do not significantly
increase upon excitation, see Table 2, and the electronic transitions are no longer charge transfer
transitions; Figure 5 in fact shows that in the case of C3, and C4 electron density changes upon transition to
the first excited singlet are localized on the pyranic core and on the EWD group. Analysis of the electronic
distribution in the ground electronic state show that the net charge on the EWD group significantly
increases for C3 and C4 with respect to C1 and C2, being approximately -1 for C4, the strongest EWD group.
Thus by increasing the strength of the EWD group functionalizing the pyran core, the electronic structures
of the chromophores can be finely tuned from that well characteristic of a push-pull dye toward
zwitterionic structures.23
In Table 2 the predicted vertical absorption wavelengths are reported. Since the synthesized dyes consist
of symmetric heterotrimers, two transitions in the same spectral region occur because of exciton
coupling.24 The observed shifts at longer wavelengths, about 100 nm for each heterotrimer with respect to
the absorption wavelength of the individual components, has to be attributed either to the CT character of
the transitions, especially for C1 and C2, or to electronic interactions between the carbazolic and the
pyranic core in the ground state.
The computed oxidation potentials are reported in Table 2. All dyes are characterized by high oxidation
potentials, larger than carbazole one, because of the presence of strong electron acceptor groups. As
18

shown in figure 6, the electron hole of the oxidized species mainly resides on the carbazolic branches for all
the four chromophores.
3.4. Photovoltaic measurements
All the synthesized chromophores were used as TiO2 sensitizers in standard DSSCs (0.25 cm2). Two kinds of
photoanode were used to which we will refer as transparent titania, in which the average nanoparticle
diameter is 21 nm, and opaque titania, which is constituted by a mixture of nanoparticles of different size,
-
ranging from 20 to 350 nm. The redox couple used was I-/I3 . Details about cell fabrication are reported in
the Experimental Section.
J/V curves, recorded under AM1.5 G simulated solar irradiation (at incident power of 100 mW/cm2) are
reported in Figure 6, while the main electrical parameters of the fabricated cells, are summarized in Table
3.
DSSC efficiencies are related both to the dye and to the type of photoanode used: as far as the latter is
concerned, better results are obtained using the opaque titania. This is related to light trapping effect
occurring with this kind of photoanode, because of the incoherent light reflection inside the DSSC, that
results in an increased photon absorption and hence, an increased charge photogeneration.
Regarding the dyes, the best results are obtained with the sensitizers C1 and C2, which afford power
moderate conversion efficiency up to, respectively, 1.90 % and 1.70 %, notwithstanding the low electron
density around the carboxylic groups (anchoring points to the oxide) highlighted by DFT calculations: the
peculiar conformation adopted by the chromophores, emerged by DFT analysis, suggests an anchoring
mode on TiO2 in which the two carboxylic simultaneously link to the metal, allowing the electron rich area
of chromophore’s LUMO to approach the oxide surface, thus favouring the electron injection process.
The electrical parameters are characterized by the fact that the open circuit voltage (Voc) remains quite low
(600mV and less), affecting the PCE of the cells, and this is due to multiple reasons. It is known that the
19

maximum photovoltage (Vmax) obtained in the DSSC is attributed to the energy gap between the quasi-
Fermi level (approximately the energy level of the conduction-band edge) of the TiO2 and the redox
potential of the electrolyte. Moreover it is crucial to have complete titania covering by the dye in order to
avoid undesirable recombination, hence voltage drop, between the generated charge injected into the TiO2
network and the electrolyte solution .25 To this end the dye must be tailored to efficiently bond to the TiO2,
having minimal distance between adjacent molecules, minimum steric hindrance between them and so less
TiO2-electrolyte interface. In the present case it seems that the dye covering of the titania should be
enhanced and this can be done by using a proper solvent, 26 change the type of titania paste, i.e. porosity,
27
grain size, hence the dye coverage, , TiCl4 treatment 28 and so on. In the present study, we meant to
investigate the effectiveness of the novel synthesized photosensitizers in standard conditions, reserving the
possibility of optimize their performance in further studies. Indeed, despite the low voltage, there is charge
injection, hence a certain Jsc and this is due to the correct alignment of the HOMO level with respect to the
TiO2-CB, demonstrating the efficacy of the, at least, C1 and C2. For what concerns C3 and C4, these dyes give
rise to devices with poor efficiency. As previously anticipated, this can be related with a poor driving force
in the process of electron injection from the excited state of the chromophore (see Figure 4). It is known
from the literature that a poor efficiency in the electron injection process results in a diminished
photocurrent (low Jsc) and also negatively affects the cell Voc and FF, thus strongly limiting the PCE. The less
efficient photosensitizer is hence C4, which features a LUMO energy even lower that the conduction band
edge of TiO2.
4. Conclusions
We have reported the synthesis and characterization of four novel chromophores characterized by a donor-
acceptor-donor molecular structure and based on a pyran core symmetrically linked to two carbazole donor
moieties. All chromophores feature high extinction coefficients, up to 1.0∙105 L. mol-1∙cm-1. By
functionalizing the pyran core with groups characterized by different electron withdrawing (EWD) activity,
20

it was possible to tune the optical properties of the resulting dyes: in particular, the higher is the EWD
strength of the substituents, the more red shifted is the chromophore absorption. A set of colors ranging
from orange to blue was obtained, so that most of the visible spectrum was covered. Cyclic voltammetry
studies performed on the dyes in combination with optical measurement, gave some indication about the
energy of their frontier molecular orbitals: from this combined analysis, it came out that the LUMO energy
is deeply influenced by the EWD power of the group functionalizing the pyran core of the chromophores.
Chromophores C3 and C4, in particular, show very high electron affinity, with LUMO energies close to the
commonly reported value for ECB of titanium dioxide. The electronic properties of the dyes were
investigated at DFT level: for all the chromophores the electron density around the carboxylic groups is
depleted upon excitation and mainly shifted on the pyran core, so that the process of electron injection in
TiO2 conducting band could be negatively affected.
The chromophores were investigated as photosensitizers in DSSC: as expected, the performances of devices
based on C3 and C4 are rather low: the reason for low efficiency derived from the high electron affinity of
the dyes that negatively influenced the process of electron transfer in the conduction band of titania. A
moderate efficiency, close to 2 % was instead obtained for devices based on C1 and C2, notwithstanding the
low electron density around the carboxylic groups (anchoring points to the oxide) highlighted by DFT
calculations: we speculated on the possibility that, the conformation adopted by the chromophores,
suggested by DFT analysis, could result in a specific anchoring arrangement that might help the electron
transfer by placing the electron rich pyran core in the proximity of the oxide surface.
21

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24

Figure Captions
Scheme 1. Molecular structure of the synthesized dyes.
25

Scheme 2. Functionalization of the pyran core with different electron withdrawing groups
26

Scheme 3. Synthesis of the electron donor carbazolic branches and of the final chromophores
27

Figure 1. Normalized absorption spectra of the synthesized chromophores in DMF
28

Figure 2. Normalized absorption spectra of the synthesized chromophores as thin films spin coated on glass
substrates.
29