9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole—A new donor building-block in the design of sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2019.112333

The novel donor building-block - 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole was designed and employed in the synthesis of dye-sensitized solar cell (DSSCs). An effective, high-yielding synthesis of 4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole from 1,2,3,4,4a,9a-hexahydrocarbazole was realized. Three new metal-free organic sensitizers, containing the new donor building block were prepared by a stepwise approach from 4,7-dibromobenzo[c][1,2,5]chacogenadiazoles. A 2,1,3-Benzothiadiazole dye containing hexahydrocarbazole donor, thiophene as π-spacer and cyanoacrylate as anchoring electron acceptor showed photovoltaic properties higher than the well-known WS-2 sensitizer with PCE = 5.86 %. Although benzoxa- and –selenadiazole dyes have a bathochromic shift (24?30 nm) in the UV–vis spectra, and smaller energy gap E_g (about 0.1 eV), they have lower photovoltaic parameters, including PCE of 1.5–2.3 %. Introducing a new donor 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole into the construction of the DSSCs has broadened possibilities for the optimization of their photovoltaic properties.

Full text of DOI:10.1016/j.jphotochem.2019.112333

Journal Pre-proof
9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole – a new donor
building-block in the design of sensitizers for dye-sensitized solar cells
Maxim S. Mikhailov (Investigation), Nikita S. Gudim (Investigation),
Ekaterina A. Knyazeva (Formal analysis)
(Visualization)<ce:contributor-role>Writing–original draft), Ellie
Tanaka (Investigation), Lu Zhang (Investigation), Ludmila V.
Mikhalchenko (Investigation), Neil Robertson (Conceptualization)
(Supervision)<ce:contributor-role>Writing–review and editing), Oleg
A. Rakitin (Conceptualization) (Project administration)
(Supervision)<ce:contributor-role>Writing–original draft)
PII:
S1010-6030(19)31295-X
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112333
JPC 112333
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
30 July 2019
16 December 2019
18 December 2019
Please cite this article as: Mikhailov MS, Gudim NS, Knyazeva EA, Tanaka E, Zhang L,
Mikhalchenko LV, Robertson N, Rakitin OA, 9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole
– a new donor building-block in the design of sensitizers for dye-sensitized solar cells, Journal
of Photochemistry and amp; Photobiology, A: Chemistry (2019),
doi: https://doi.org/10.1016/j.jphotochem.2019.112333

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9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole – a new
donor building-block in the design of sensitizers for dye-
sensitized solar cells
Maxim S. Mikhailova, Nikita S. Gudimb, Ekaterina A. Knyazevaa,b, Ellie Tanaka,c Lu Zhang
c, Ludmila V. Mikhalchenkob, Neil Robertsonс,*, Oleg A. Rakitina,b,*
aNanotechnology Education and Research Center, South Ural State University, 454080
Chelyabinsk, Russia
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russia
cEaStCHEM School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, UK
Graphical abstract

2
Highlights
 A series of novel D-A-π-A metal-free sensitizers were developed for DSSCs.
 Novel effective donor building-block - 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-
carbazole was designed.
 The PCE of MAX114 (5.86%) is higher than for the well-known WS-2 sensitizer.
 Dye with benzothiadiazole internal acceptor showed better PCE than the
corresponding benzooxa- and selenadiazole dyes.

3
Abstract
The novel donor building-block - 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole was
designed and employed in the synthesis of dye-sensitized solar cell (DSSCs). An effective,
high-yielding synthesis of 4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-
1,2,3,4,4a,9a-hexahydrocarbazole from 1,2,3,4,4a,9a-hexahydrocarbazole was realized. Three
new metal-free organic sensitizers, containing the new donor building block were prepared by
a stepwise approach from 4,7-dibromobenzo[c][1,2,5]chacogenadiazoles. A 2,1,3-
Benzothiadiazole dye containing hexahydrocarbazole donor, thiophene as π-spacer and
cyanoacrylate as anchoring electron acceptor showed photovoltaic properties higher than the
well-known WS-2 sensitizer with PCE = 5.86%. Although benzoxa- and –selenadiazole dyes
have a bathochromic shift (24-30 nm) in the UV-vis spectra, and smaller energy gap Eg (about
0.1 eV), they have lower photovoltaic parameters, including PCE of 1.5-2.3%. Introducing a
new donor 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole into the construction of the
DSSCs has broadened possibilities for the optimization of their photovoltaic properties.
Corresponding author. Tel.: +7-499-135-53-27; fax: +7-499-135-53-28; e-mail:
orakitin@ioc.ac.ru; neil.robertson@ed.ac.uk.
Keywords: dye-sensitized solar cells; 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-
carbazole; power conversion efficiency; 2,1,3-benzochalcogenadiazoles
1.
Introduction
In the last couple of decades, dye-sensitized solar cells (DSSCs) have attracted worldwide
research attention due to their potential for low cost production, easy fabrication and relatively
high power conversion efficiency [1-3]. A dye sensitizer, a photoanode, a redox electrolyte and
a counter electrode are the essential components for a typical DSSC. Among other components
of DSSCs, the dye sensitizer is responsible for harvesting sunlight, initiating charge transfer,
and injecting electrons into the photoanode. Efficient sensitizers have been developed based
on ruthenium polypyridine complexes, zinc porphyrins [4-6] and metal-free organic dyes,
among which the latter have been most intensively investigated [7-11]. Most typical organic
dyes are designed with a donor-π-bridge-acceptor (D-π-A) configuration due to easy synthesis
and efficient intramolecular charge transfer (ICT) properties [12-15]. The electronic interaction
between donor (D) and acceptor (A) results in strong charge-transfer absorption bands that

4
harvest sunlight for photon-to-electron conversion. Recently, Zhu and Tian proposed a new
concept of the D–A–π–A motif for designing a generation of stable and efficient organic dyes
[16-18]. Compared to D–π–A dyes, the properties of D–A–π–A dyes can be readily adjusted
by incorporation of an auxiliary acceptor between the donor and π-bridge, which broadens the
absorption and enhances efficient ICT for high performance DSSCs [19]. Some electron-
withdrawing heterocyclic systems, such as diketopyrrolopyrrole, benzotriazole,
benzothiadiazole, quinoxaline, benzoselenadiazole, pyridazinothiadiazole and many others
have been used to design a D-A-π-A framework [20-25]. It was reported that the auxiliary
acceptor is beneficial for extending absorption wavelength and enhancing the electron
coupling, thus greatly improving photovoltaic properties and the stability of organic sensitizers
[18]. Furthermore, the evolution of the π-bridge unit is essential in the molecular engineering
of sensitizers, due to enhanced light absorption and photoinduced charge separation, as well as
delayed charge recombination [26]. The most common electron withdrawing group widely
employed as terminal acceptor is cyanoacrylic acid due to its strong binding on the surface of
mesoporous TiO2 through carboxylic acid anchoring groups [27]. Various other molecular
engineering methods to improve the light harvesting efficiency of organic dyes involve an
increase in the electronic richness of the donor moieties [28]. Introduction of electron-rich
moieties on the donor segment helps to broaden the absorption spectra, and attachment of
energy delocalizing chromophores causes prolongation of excited state lifetime and facilitates
the electron injection into the conduction band of TiO2 [29]. In general, triarylamine, indoline,
tetrahydroquinoline, phenothiazine, phenoxazine are routinely used as donors [30]. 4-(p-
Tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole donor has been shown to be one of the
most effective donors [20,22]. A few attempts to modify this moiety with a change of p-tolyl
to other aryl groups have been made [31-39], but they have not achieved significant success.
In this work we aimed to design a new donor building block - 9-(p-tolyl)-
2,3,4,4a,9,9a-hexahydro-1H-carbazole, which is varied from 4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole by one extra CH2 group, and to investigate the influence of this
group on the photovoltaic properties of the organic sensitizers. Three novel dyes with various
chalcogen atoms (oxygen, sulfur and selenium) in 2,1,3-benzochalcogenadiazole internal
acceptor were synthesized and studied. The optical, electrochemical and photophysical
properties of these dyes were investigated to disclose important design criteria for the discovery
of further new organic dyes.
2.
Experimental details

5
2.1. Materials
The reagents were purchased from commercial sources and used as received. Solvents were
purified by distillation from the appropriate drying agents.
2.2.Synthesis and characterization of compounds:
2.2.1. Synthesis
of
4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-
1,2,3,4,4a,9a-hexahydrocarbazole 4.
2,9-(p-Tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole 2.
In 50 ml round-bottom flask, 1,2,3,4,4a,9a-hexahydrocarbazole 1 (520 mg, 3 mmol), p-
bromotoluene (513 mg, 3 mmol), cesium carbonate (1.36 g, 4.2 mmol), palladium acetate (3.5
mg, 15μmol), tris-tert-butylphosphine (72 μl, 0.3 mmol) and 10 ml dry xylene were added.
After boiling for 20 h under argon, the mixture was diluted with petroleum ether and purified
on a silica gel column to obtain colorless oil, 2 (715 mg, 80%). 1H NMR (300 MHz, δ, ppm):
7.24 (m, 4H), 7.20 (d, J=2.8 Hz, 1H), 7.10 (t, J=7.6 Hz, 1H), 6.82 (t, J=7.6 Hz, 1H), 6.80 (d,
J=7.6 Hz, 1H), 4.10 (m, 1H), 3.28 (m, 1H), 2.43 (s, 3H), 1.91-1.72 (m, 4H), 1.63-1.57 (m, 2H),
1.50-1.42 (m, 2H). 13C NMR (75 MHz, δ, ppm): 149.36, 141.05, 134.91, 133.04, 129.87,
126.98, 123.18, 123.02, 118.62, 109.02, 64.66, 40.64, 28.25, 25.89, 22.92, 21.30, 20.95.
HRMS-ESI (m/z): [M]+ calcd for (C19H21N) 263.1667, found 263.1670, [M+H]+ calcd for
(C19H22N) 264.1747, found 264.1744. IR, ν, cm-1: 3404, 2939, 2905, 2846, 1579, 1470, 1435,
1311, 1273, 1233, 1047, 971, 862, 798, 586, 485. Rf=0.41 (petroleum ether/ ethylacetate =
25:1).
3,6-Bromo-9-(p-tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole 3.
To a solution of 2 (450 mg, 1.7 mmol) and DMSO (133μl, 1,9 mmol) in 15 ml ethylacetate was
added dropwise hydrobromic acid (48%, 0.45 ml, 3.76 mmol) at 60 ºC. After 5 minutes K2CO3
was added (300 mg) and the mixture was stirred another 1 h. After cooling down to room
temperature the mixture was diluted with ethyl acetate, washed with water, and evaporated
under reduced pressure. The organic layer was evaporated, the residue was purified on a silica
gel column with petroleum ether to obtain colorless oil, 3 (494 mg, 85%). The sample was used
in the next step as it is.
4,6-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-1,2,3,4,4a,9a-
hexahydrocarbazole 4.
In 25 ml round-bottom flask compound 3 (480 mg, 1.4 mmol), B2Pin2 (540 mg, 2.14 mmol),
and potassium acetate (480 mg) were dissolved in 10 ml of dioxane. The solution was degassed

6
for 20 minutes with a stream of argon, Pd2dba3 (9.5 mg, 10 μmol) and X-Phos (20 mg, 40 μmol)
were added simultaneously. The reaction was then brought to 80 °C for 8 h, and then diluted
with ethylacetate and filtered through a thin pad of celite. The mixture was evaporated and the
compound purified on a silica gel column with a ethylacetate/petroleum ether mixture 1/50 as
eluent to obtain a pale yellow oil, 4 (450 mg, 82%). 1H NMR (300 MHz, δ, ppm): 7.66 (s, 1H),
7.63 (d, J=8 Hz, 1H), 7.23 (q, J=8.5 Hz, 4H), 6.77 (d, J=7.9 Hz, 1H), 4.12 (m, 1H), 3.28 (m,
1H), 2.42 (s, 3H), 1.91-1.72 (m, 4H), 1.63-1.57 (m, 2H), 1.41 (m, 14H). 13C NMR (75 MHz,
δ, ppm): 152.15, 140.28, 135.04, 134.01, 133.54, 129.93, 129.49, 123.37, 108.14, 83.23, 64.77,
40.31, 27.97, 25.89, 25.00, 24.83, 22.75, 21.22, 20.98. HRMS-ESI (m/z): [M+H]+ calcd for
(C25H32NBO2) 390.2603, found 390.2603. IR, ν, cm-1: 3237, 2929, 2858, 1603, 1515, 1460,
1382, 1233, 1198, 815, 739, 642. Rf = 0,34 (petroleum ether/ ethylacetate = 10:1).
2.2.2. General
procedure
for
the
cross-coupling
of
4,7-
dibromobenzo[c][1,2,5]chalcogenadiazoles 5 and boronic ester 4.
In 50 ml round-bottom flask, 4 (450 mg, 1.2 mmol) and 4,7-dibromobenzo-1,2,5-
chalcogenadiazole 5 (1.2 mmol) were dissolved in 15 ml of dioxane, and 2 M K2CO3 (10 ml)
was added. The mixture was degassed for 20 minutes with a stream of argon, after which time
Pd(PPh3)4 (67 mg, 60 μmol, 5%) was added. After refluxing for 10 h, the mixture was extracted
with ethylacetate; organic solvent was then removed under reduced pressure. The residue was
purified on a silica gel column with eluent ethylacetate/petroleum ether = 1:25.
4-Bromo-7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]thiadiazole (6a)
Orange solid with mp 127-129 ºC (330 mg, 58%). 1H NMR (300 MHz, δ, ppm): 7.88 (d, J=7.6
Hz, 1H), 7.73 (s, 1H), 7.65 (d, J=8.3 Hz, 1H), 7.52 (d, J=7.7 Hz, 1H), 7.22 (m, 4H), 6.86 (d,
J=8.3 Hz, 1H), 4.17 (m, 1H), 3.35 (m, 1H), 2.39 (s, 3H), 1.97-1.87 (m, 1H), 1.84-1.72 (m, 3H),
1.62-1.41 (m, 4H). 13C NMR (75 MHz, δ, ppm): 154.1, 153.5, 150.1, 140.3, 135.5, 134.8,
133.7, 132.5 130.0, 128.7, 126.9, 126.7, 124.2, 123.2, 111.1, 108.8, 64.8, 40.6, 28.3, 25.9, 22.8,
21.2, 21.0. HRMS-ESI (m/z): [M+H]+ calcd for (C25H22BrN3S) 476.0791, found 476.0792.
UV-Vis (CH2Cl2, λmax, nm/logε): 276/4.38, 415/3.83. IR, ν, cm-1: 2923, 2852, 1605, 1513,
1466, 1377, 1328, 1256, 883, 807, 506. Rf = 0.37 (15 petroleum ether/1 ethylacetate).
4-Bromo-7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-yl)benzo[c][1,2,5]oxadiazole
(6b)
Orange solid with mp 142-144 ºC (328 mg, 65%). 1H NMR (300 MHz, δ, ppm): 7.80 (s, 1H),
7.73 (d, J=8.4 Hz, 1H), 7.64 (d, J=7.5 Hz, 1H), 7.35 (d, J=7.5 Hz, 1H), 7.22 (q, J=8.5 Hz, 4H),

7
6.82 (d, J=8.3 Hz, 1H), 4.18 (m, 1H), 3.34 (m, 1H), 2.40 (s, 3H), 1.98-1.88 (m, 1H), 1.84-1.72
(m, 3H), 1.62-1.41 (m, 4H). 13C NMR (75 MHz, δ, ppm): 150.9, 150.2, 149.0, 139.9, 135.8,
134.7, 134.0, 130.1, 128.2, 125.8, 124.8, 123.4, 112.1, 108.9, 104.8, 64.9, 40.5, 28.2, 25.9,
22.7, 21.2, 21.1. HRMS-ESI (m/z): [M+H]+ calcd for (C25H22BrN3O) 462.1000, found
462.0989. UV-Vis (CH2Cl2, λmax, nm/logε): 310/4.21, 480/4. IR, ν, cm-1: 2925, 2853, 1606,
1514, 1482, 1380, 1269, 806. Rf=0,46 (10 petroleum ether/1 ethylacetate).
4-Bromo-7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]selenadiazole (6c)
Orange solid with mp 178-180 ºC (410 mg, 65%) 1H NMR (300 MHz, δ, ppm): 7.80 (d, J=7.5
Hz, 1H), 7.64 (s, 1H), 7.57 (d, J=8.2 Hz, 1H), 7.35 (d, J=7.5 Hz, 1H), 7.22 (m, 4H), 6.85 (d,
J=8.2 Hz, 1H), 4.17 (m, 1H), 3.34 (m, 1H), 2.39 (s, 3H), 1.93-1.71 (m, 4H), 1.63-1.53 (m, 2H),
1.47-1.39 (m, 2H). 13C NMR (75 MHz, δ, ppm): 158.8, 158.7, 149.9, 140.3, 136.4, 135.2,
133.5, 132.5, 129.9, 128.9, 127.5, 126.7, 124.4, 123.1, 113.8, 108.6, 64.7, 40.5, 28.2, 25.8,
22.8, 21.1, 20.9. HRMS-ESI (m/z): [M+H]+ calcd for (C25H22BrN3Se) 524.0234, found
524.0224. UV-Vis (CH2Cl2, λmax, nm/logε): 323/4.56, 495/3.99. IR, ν, cm-1: 2927, 2853, 1605,
1513, 1481, 1380, 1268, 806. Rf=0,58 (5 petroleum ether/1 ethylacetate).
2.2.3. General procedure for the cross-coupling reaction of mono-adducts 6a-c and tert-
butyl
2-cyano-3-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-
yl)acrylate 7.
In 50 ml round-bottom flask, mono-adduct 6a-c (0.45 mmol) and ester 7 (220 mg, 0.61 mmol)
were dissolved in 10 ml of dioxane, and then 2M K2CO3 (10 ml) was added. The mixture was
degassed for 20 minutes with a stream of argon, Pd(PPh3)4 (30 mg, 26 μmol, 5%) was added.
After refluxing for 12 h, the mixture was extracted with ethylacetate and solvent was removed
under reduced pressure. The residue was purified on a silica gel column with eluent
ethylacetate/petroleum ether = 1:20.
tert-Butyl
2-cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)acrylate 8a.
Dark red solid with mp 194-196 ºC (135 mg, 41%). 1H NMR (300 MHz, δ, ppm): 8.28 (s, 1H),
8.26 (d, J=4.2 Hz, 1H), 8.06 (d, J=7.6 Hz, 1H), 7.89 (d, J=4.1 Hz, 1H), 7.82 (s, 1H), 7.76 (d,
J=8.6 Hz, 1H) 7.71 (d, J=7.6 Hz, 1H), 7.23 (s, 4H), 6.87 (d, J=8.3 Hz, 1H), 4.19 (m,1H), 3.36
(m, 1H), 2.39 (s, 3H), 1.97-1.90 (s, 1H) 1.85-1.75 (m, 3H), 1.62 (s, 9H), 1.58-1.40 (m, 4H).
13C NMR (75 MHz, δ, ppm): 161.8, 154.0, 152.7, 150.2, 149.1, 145.4, 140.1, 137.8, 135.8,
135.4, 133.6, 129.9, 129.0, 128.0, 127.9, 127.0, 125.9, 124.2, 123.2, 123.0, 116.3, 108.7, 100.1,

8
83.4, 64.8, 40.5, 28.2, 28.0, 25.8, 22.7, 21.1, 20.9. HRMS-ESI (m/z): [M]+calcd for
(C37H34N4O2S2) 630.2118 , found 630.2112. UV-Vis (CH2Cl2, λmax, nm/logε): 318/4.41,
414/4.31, 523/4.44. IR, ν, cm-1: 2931, 2855, 2180, 1709, 1605, 1585, 1510, 1458, 1363, 1248,
1152, 808. Rf = 0.44 (petroleum ether/ ethylacetate = 5:1).
tert-Butyl
2-cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]oxadiazol-4-yl)thiophen-2-yl)acrylate 8b.
Dark purple solid with mp 120-122 ºC (127 mg, 46%). 1H NMR (300 MHz, δ, ppm): 8.27 (s,
1H), 8.24 (d, J=4.1 Hz, 1H), 7.90 (s, 1H), 7.83 (m, 3H), 7.57 (d, J=7.5 Hz, 1H), 7.23 (q, J=8.5
Hz, 4H), 6.85 (d, J=8.3 Hz, 1H), 4.19 (m, 1H), 3.37 (m, 1H), 2.40 (s, 3H), 1.97-1.90 (s, 1H)
1.85-1.75 (m, 3H), 1.62 (s, 9H), 1.58-1.40 (m, 4H). 13C NMR (75 MHz, δ, ppm): 161.5, 151.0,
149.0, 148.0, 147.2, 145.2, 139.7, 138.7, 135.7, 135.7, 134.0, 131.3, 130.0, 129.4, 128.9, 128.5,
125.1, 124.9, 123.3, 118.6, 116.1, 108.8, 100.8, 83.6, 64.9, 40.3, 28.1, 28.0, 25.8, 22.6, 21.0,
21.0. HRMS-ESI (m/z): [M+H]+ calcd for (C37H34N4O3S) 615.2424, found 615.2411; [M+Na]+
calcd for (C37H34N4O3SNa) 637.2244, found 637.2245. UV-Vis (CH2Cl2, λmax, nm/logε):
310/4.30, 405/4.18, 550/4.41. IR, ν, cm-1: 2927, 2854, 2216, 1702, 1605, 1578, 1513, 1482,
1370, 1268, 807, 654. Rf = 0.52 (petroleum ether/ ethylacetate = 5:1).
tert-Butyl
2-cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]selenadiazol-4-yl)thiophen-2-yl)acrylate 8c.
1
Dark purple solid with mp 98-100 ºC (192 mg, 49%). H NMR (300 MHz, δ, ppm): 8.28 (s,
1H), 8.15 (d, J=4.1 Hz, 1H), 8.00 (d, J=7.5 Hz, 1H), 7.91 (d, J=4.1 Hz, 1H), 7.74 (s, 1H), 7.67
(d, J=8.3 Hz, 1H), 7.58 (d, J=7.5 Hz, 1H), 7.22 (s, 4H), 6.86 (d, J=8.3 Hz, 1H), 4.18 (m, 1H),
3.36 (m, 1H), 2.40 (s, 3H), 1.97-1.88 (s, 1H) 1.84-1.72 (m, 3H), 1.61 (s, 9H), 1.58-1.40 (m,
4H). 13C NMR (75 MHz, δ, ppm): 162.0, 159.7, 158.3, 150.2, 149.6, 145.7, 140.3, 137.5, 137.4,
136.7, 135.3, 133.7, 130.0, 129.3, 128.4, 127.9, 126.3, 124.8, 124.6, 123.2, 116.5, 108.7, 99.9,
83.5, 64.8, 40.6, 28.3, 28.1, 25.9, 22.7, 21.2, 21.0. HRMS-ESI (m/z): [M+H]+ calcd for
(C37H34N4O2SSe) 679.1643, found 679.1628. UV-Vis (CH2Cl2, λmax, nm/logε): 326/4.57,
405/4.33, 550/4.55. IR, ν, cm-1: 2929, 2853, 2215, 1719, 1601, 1511, 1470, 1370, 1278, 1160,
840, 733. Rf = 0.38 (petroleum ether/ ethylacetate = 5:1).
2.2.4. General procedure for hydrolyses of ethers 8a-c.
In a 25 ml round-bottom flask, 8a-c (0.15 mmol, 96 mg, 92 mg and 104 mg respectively) was
dissolved in a 5 ml solution of 1/3 CF3COOH/CHCl3. After stirring for 4 h under argon, the
mixture was diluted with dichloromethane, washed with water, and removed under reduced

9
pressure. The residue was purified on
methanol/dichloromethane/acetic acid = 10:50:1.
a
silica gel column with eluent
2-Cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)acrylic acid (MAX114)
Dark red solid with mp >300 ºC (79 mg, 91%). 1H NMR (300 MHz, DMSO-d6, δ, ppm): 8.19
(m,2H), 8.10 (s, 1H), 7.88 (s, 1H), 7.85 (d, J=7.7 Hz, 1H), 7.79 (m, 2H), 7.23 (m, 4H), 6.80 (d,
J=8.3 Hz, 1H), 4.21 (m, 1H), 2.32 (s, 3H), 1.90-1.82 (m, 1H) 1.74-1.59 (m, 3H), 1.47-1.33 (m,
4H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 162.9, 153.6, 152.5, 149.5, 143.5, 140.2, 139.9,
138.7, 135.4, 135.1, 133.9, 133.2, 130.4, 129.2, 127.7, 127.6, 127.3, 126.7, 124.5, 123.5, 122.8,
120.0, 111.5, 108.6, 64.2, 27.9, 25.7, 22.5, 21.1, 21.0. MS-MALDI (m/z): [M]+ calcd for
(C33H26N4O2S2) 574.1497, found 574.2726. UV-Vis (CH2Cl2/MeOH (2%), λmax, nm/logε):
365/4.10, 482/4.19. IR, ν, cm-1: 2924, 2853, 2211, 1720, 1610, 1578, 1515, 1474, 1377, 1268,
806. Rf = 0.47 (ethylacetate/ methanol = 2:1).
2-Cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]selenadiazol-4-yl)thiophen-2-yl)acrylic acid (MAX115)
1
Dark purple solid with mp >300 ºC (83 mg, 88%). H NMR (300 MHz, DMSO-d6, δ, ppm):
8.13 (d, J=4.1 Hz, 1H), 8.08 (m, 2H), 7.81 (s, 1H), 7.77 (d, J=4.1 Hz, 1H), 7.68 (m, 2H), 7.23
(m, 4H), 6.78 (d, J=8.4 Hz, 1H), 4.20 (m, 1H), 2.32 (s, 3H), 1.90-1.82 (m, 1H) 1.74-1.59 (m,
3H), 1.47-1.33 (m, 4H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 163.3, 158.5, 157.32, 148.7,
144.0, 140.0, 139.8, 138.1, 135.2, 134.5, 134.2, 132.5, 129.9, 129.0, 127.8, 127.2, 127.0, 126.0,
124.7, 124.4, 122.2, 119.4, 109.9, 107.9, 63.7, 27.4, 25.2, 22.0, 20.6, 20.4. MS-MALDI (m/z):
[M]+ calcd for (C33H26N4O2SSe) 622.0944, found 622.2408. UV-Vis (CH2Cl2/MeOH (2%),
λmax, nm/logε): 375/4.14, 512/4.12. IR, ν, cm-1: 2924, 2853, 2214, 1718, 1604, 1582, 1511,
1466, 1378, 1271, 1151, 809. Rf = 0.49 (ethylacetate/ methanol = 2:1).
2-Cyano-3-(5-(7-(9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazol-6-
yl)benzo[c][1,2,5]oxadiazol-4-yl)thiophen-2-yl)acrylic acid (MAX123)
1
Dark purple solid with mp >300 ºC (77 mg, 92%). H NMR (300 MHz, DMSO-d6, δ, ppm):
8.17 (s, 1H), 8.06 (d, J=3.9 Hz, 1H), 7.94 (d, J=7.5 Hz, 1H), 7.88 (s, 1H), 7.81 (m, 2H), 7.75
(d, J=7.6 Hz, 1H), 7.21 (q, J=8.4 Hz, 4H), 6.77 (d, J=8.4 Hz, 1H), 4.21 (m, 1H), 2.32 (s, 3H),
13
1.91-1.82 (m, 1H) 1.74-1.64 (m, 2H), 1.62-1.53 (m, 1H), 1.47-1.33 (m, 4H). C NMR (75
MHz, DMSO-d6, δ, ppm): 164.5, 150.1, 148.9, 148.0, 142.8, 141.3, 140.0, 138.1, 136.5, 135.7,
133.4, 130.4, 129.6, 129.2, 128.6, 128.2, 126.6, 124.9, 123.3, 122.8, 119.2, 118.4, 109.4, 108.6,
64.2, 27.8, 25.6, 22.4, 21.0. MS-MALDI (m/z): [M]+ calcd for (C33H26N4O3S) 558.1725, found

10
558.2721. UV-Vis (CH2Cl2/MeOH (2%), λmax, nm/logε): 367/3.66, 506/3.85. IR, ν, cm-1: 2930,
2855, 2214, 1715, 1586, 1512, 1482, 1370, 1266, 1153, 808. Rf = 0.51 (ethylacetate/ methanol
= 2:1).
Scheme 1. Chemical structures of MAX114, MAX123 and MAX115.
Scheme 2. Synthesis of new donor building block 1. The reaction conditions: (i) Pd(OAc)2,
PtBu3, Cs2CO3, xylene; (ii) HBr, DMSO, ethylacetate (iii) B2Pin2, KOAc, Pd2dba3, X-Phos,
dioxane.

11
Scheme 3. Synthesis of sensitizers. The reaction conditions: (i) Pd(PPh3)4, K2CO3, dioxane;
(ii) CF3COOH, CHCl3
3. Results and Discussion
3.1 Synthesis and characterization
The molecular structures of the dyes, containing the novel donor building-block - 9-(p-tolyl)-
2,3,4,4a,9,9a-hexahydro-1H-carbazole, are shown in Scheme 1. 4,6-(4,4,5,5-Tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole 4 was prepared in four
steps from 1,2,3,4,4a,9a-hexahydrocarbazole 1 Scheme 2. Buchwald-Hartwig arylation of
hexahydrocarbazole with p-iodotoluene in the presence of Pd(OAc)2, PtBu3 and Cs2CO3 in
xylene afforded the p-tolyl derivative in good yield (80%). Bromination of the aromatic ring
with a mixture of HBr and DMSO followed by Miayura borylation with 4,4,4',4',5,5,5',5'-
octamethyl-2,2'-bi(1,3,2-dioxaborolane) in the presence of palladium catalyst Pd2dba3, ligand
X-Phos and KOAc in 1,4-dioxane gave key the donor component in good yield.
The general strategy for the synthesis of dyes with D-A1-π-A2 motif follows a stepwise
approach shown in Scheme 3. Internal acceptors A1 - 4,7-dibromobenzo[c][1,2,5]thiadiazole

12
5a
[41],
4,7-dibromobenzo[c][1,2,5]oxadiazole
5b
[42]
4,7-
dibromobenzo[c][1,2,5]selenadiazole 5b [43] and π-spacer-acceptor moiety - tert-butyl (2-
cyano-3-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)acrylate [44] were
prepared
by
known
procedures.
The
cross-coupling
reactions
of
4,7-
dibromobenzo[c][1,2,5]chalcogenadiazoles (5) with donor boronic ester 4 in the presence of
Pd(PPh3)4 as a catalyst and aqueous solution K2CO3 in THF successfully gave mono-adducts
6a-c
in
moderate
yields.
The
second
cross-coupling
of
7-
bromobenzo[c][1,2,5]chalcogenadiazoles 6a-c with acceptor tert-butyl ester gave bis-adducts
8a-c in high yields. Final hydrolysis of compounds 8a-c with CF3CO2H resulted in the
formation of the target MAX dyes in high yields. All dyes were purified by column
chromatography before measurement of the physical and electrochemical properties as well as
solar cell device fabrication.
3.2 Photophysical and electrochemical properties
The response region in sunlight for DSSCs is determined primarily by the UV-Vis absorption
of the sensitizer. Therefore, we initially characterized the spectral response of the MAX series
in DCM/MeOH (V/V = 98/2) at 4×10-5 mol L-1 (Fig. 1). The absorption peaks (λmax) and their
corresponding molar absorption coefficients (ε) are listed in Table 1. As shown in Fig. 1, all
these dyes exhibit two absorption regions. The absorption peaks between 350-400 nm mainly
correspond to the π-π* electron transition. The broad absorption bands in the 450-550 nm
region are assigned to an intramolecular charge transfer (ICT) process between the donor and
anchor/acceptor group, which produces the efficient charge-separated excited state. Despite the
fact that the extinction coefficient of the sulfur-containing derivative MAX114 is as close as
possible to the same value of the previously-described WS-2 dye (15.5 × 103 M-1·cm-1 and 16.7
× 103 M-1·cm-1, correspondingly) [16] ), the second absorption band suffers strong
hypsochromic shift from λmax = 533 nm for WS-2 to 482 nm for MAX114. Thus, a small
change in the donor fragment (addition of one CH2 group in the indoline ring) has a significant
effect on the second absorption maximum, which indicates a high sensitivity of intramolecular
electron transfer to even small variations in the structure of the donor part of the molecules.
We found that the nature of the chalcogen in the internal acceptor affects the long-wave
absorption maximum. MAX dyes show a significant bathochromic shift during the replacement
of the sulfur atom in the 1,2,5-chalcogenadiazole ring on the more electronegative oxygen atom
(24 nm), and the more electropositive atom of selenium (30 nm). At the same time, the short-

13
wave absorption maximum (about 370 nm) is practically independent of the chalcogen atom in
the heterocycle: the deviation is no more than 10 nm. Probably, such a nonlinear dependence
can be explained, on the one hand, by a decrease in the electronegativity of sulfur in comparison
with oxygen, which reduces the efficiency of intramolecular charge transfer by reducing the
electron acceptor effect of the chalcogenadiazole fragment. On the other hand, an increase in
the radius of the atom in the transition from sulfur to selenium seems to contribute to the ICT
process due to the presence of d-orbitals in the selenium atom that can participate in the
conjugation. The extinction coefficient was also dependent on the nature of the chalcogen: the
smallest extinction was observed for the oxygen-containing compound MAX123 (6.8 × 103 M-
1·cm-1). However, in this case, the dye based on thiadiazole showed values that do not
correspond to the linear change in the sequential change of chalcogen atoms: the extinction
coefficient for the sulfur-containing dye (MAX114) was 15.5 × 103 M-1·cm-1, while the
selenium analogue (MAX115) showed a value of 13.0 × 103 M-1·cm-1. Thus, the maximum
conjugation has dyes based on sulfur and selenium, and the introduction of an electronegative
oxygen atom with a minimum radius probably alters the geometry of the molecule, reducing
the conjugation. The calculated Eg opt for known dye WS-2 is much less compared to the dyes
of the MAX series. It is surprising that the oxygen and selenium dyes (MAX115 and MAX123)
showed better results, compared with the sulfur-containing compound MAX114, which is the
closest analog of WS-2 (Table 1).

14
Fig. 1 UV-Visible absorption (a) in DCM/MeOH (V/V = 98/2) at 4×10-5 mol L-1 of MAX dyes.
Table 1. UV-Visible absorption properties of the MAX series in DCM/MeOH (V/V = 98/2)
solutions.
Dye
εmax1 × 103,
[M-1·cm-1][a]
12.5
λmax1
,
λmax2
,
εmax2 × 103, λonset
,
Eg, opt, eV[b]
[nm][a]
[nm][a]
[M-1·cm-1][a] [nm][a]
365
375
367
482
512
506
533
595
2.087
1.975
1.962
1.840
MAX114
MAX115
MAX123
WS-2 [c]
15.5
629
13.8
4.4
13.0
633
6.8
675
16.7
[a] Absorption peaks (λmax, λonset) and molar extinction coefficients (ε) in DCM/MeOH (V/V =
98/2) [b] Calculated by 1242/λonset [с] [16]
To estimate the energy value of the frontier orbitals, the potential values of the first
oxidation and reduction peaks on the cyclic voltammetry curves of the investigated compounds
in DMF were obtained. Figure 2 shows a cyclic voltammogram of the new sensitizers obtained

15
versus ferrocene/ferrocenium (Fc/Fc+) at a scan rate of 100 mVs−1. Electroreduction peaks, as
seen in Fig.1, are reversible at low potential sweep rates. Electrooxidation of all compounds is
irreversible even at a sweep rate of 10 Vs-1. Relevant electrochemical data for the compounds
are presented in Table 2. Assuming a value of -5.1 eV [45,46] for the absolute potential of the
Fc/Fc+ couple in non-aqueous electrolytes, EHOMO and ELUMO were calculated using equations
(1) and (2):
EHOMO (eV) = - |e|(E ox
+ 5.1)
(1)
(2)
Fc/Fc+
ELUMO (eV) = - |e|(E red Fc/Fc+ + 5.1)
As a result, ELUMO of MAX114 (-3.48 eV), MAX115 (-3.57 eV) and MAX123 (-3.63 eV)
are higher in energy than the conduction band edge of nanocrystalline TiO2 (-4.2 eV) [47],
indicating that the electron injection process from the excited dye molecules to the TiO2
conduction band is energetically permitted. However, in the series of three dyes, there is no
linear dependence of these values in the transition from an oxygen-containing analogue
MAX115 to a selenium-containing one MAX123. The minimum value of ELUMO, is seen for
the dye based on thiadiazole MAX114. Since the energy of the LUMO depends on the degree
of conjugation of the molecule [48,49], it can be assumed that an increase in the
electronegativity of the central chalcogen atom by transition from thiadiazole to oxadiazole
leads to too strong charge separation, while the transition to a more voluminous selenium atom
leads to a reduction in conjugation due to an increase in the radius of the central atom and
changes in the geometry of the molecule.
The EHOMO level of MAX114 (-5.39 eV), MAX115 (-5.34 eV) and MAX123 (-5.44
-
eV) show more negative values than the energy level of I-/I3 redox (-5.2 eV) [50], therefore,
dye regeneration should be thermodynamically favorable. Since the HOMO energy is
dependent mostly on the nature of the donor fragment of the molecules, it is not surprising that
the values of EHOMO for all three dyes differ by no more than 0.1 eV.
Comparison of the ELUMO level of MAX114 with the corresponding values of the
known analogue WS-2 (Table 2, [52]), differing in the size of the alicycle in the donor part of
the molecule, leads to the conclusion that the replacement of the cyclopentane fragment in the
donor in WS-2 to cyclohexane in MAX114, apparently leads to a decrease in the degree of
conjugation of the molecule. The decrease in the EHOMO level of MAX114 compared to WS-2
by 0.38 eV indicates an increase in the degree of delocalization of the donor fragment
containing the cyclohexane ring.

16
CV
The electrochemical gap energy Eg for each dye is calculated from the difference
between their EHOMO and ELUMO. All Eg values (Table 2) satisfy the design expectation that this
value for dye-sensitized solar cells should be less than 2.5 eV [51]. As a result, all dyes have
enough energetic driving force for efficient DSSCs using a nanocrystalline titania photocatalyst
−
and the I−/I3 redox couple.
(a)
(b)
(c)
Fig. 2. Cyclic voltammogram showing reduction (a) and oxidation (b) of MAX 114 (1),
MAX 115 (2), MAX 123 (3). Scan rate 100 mVs−1, electrolyte 0.1 M Bu4NClO4 in DMF.
−
(c) Schematic diagram of energy levels of TiO2 conduction band, dyes and I−/I3 redox
couple.
Table 2. Electrochemical properties of the dyes in DMF solutions.
[b]
[b]
Compound Ered (vs Fc/Fc+)[a], ЕLUMO
Eox
Fc/Fc+)[a],V
(vs ЕHOMO
eV
Еg[c], eV
,
,
V
eV
-1.62
-1.53
-1.47
-3.48
-3.57
-3.63
0.29
-5.39
1.91
1.77
1.80
MAX 114
MAX 115
MAX 123
0.24
-5.34
0.34
-5.44

17
-1.33[d]
-3.77
0.67[d]
-5.77
2.00[52]
WS-2
a
Here Eox
and Ered
are oxidation and reduction peak potential relative to Fc/Fc+
onset
onset
respectively
b Energies of frontier orbitals were calculated according to equations (1) and (2)
c Еg= ЕLUMO -ЕHOMO
Unsurprisingly the Eg data of the MAX dyes obtained from the CV data agree well with
the absorption onset. Thiadiazole dye (МАХ114) has a bit higher Eg than its selenium and
oxygen counterparts. This enhancement can lead apparently to a longer lifetime of the first
excited state of MAX114, which should have a positive impact on the photovoltaic efficiency
of solar cells made on its basis. These data once again prove the better photovoltaic properties
of fused thiadiazoles compared to oxa - and selenadiazole analogs, which leads to nonlinear
changes in the properties of a number of chalcogen atoms. The Eg value for the known dye
WS-2 is close to its structural analogue MAX114, which should lead to close parameters of
solar cells based on these compounds.
3.3 DSC performance
To study the influence of the chalcogen atom in chalcogenadiazole ring of the central acceptor
unit of the dye on the photovoltaic efficiency, we constructed DSSCs based on the dye series
MAX. Commercially available ruthenium dye N719 was chosen as a reference dye. The J-V
curves are shown on Fig. 3 and the results are summarized in Table 3. Fill factors (FF) have
similar values for all dyes in the MAX series, from 0.61 to 0.64, therefore PCE values will
depend mainly on open-circuit voltage (Voc) and short-circuit current density (Jsc). Voc values
for all three dyes of the MAX series are approximately equal to each otheroc The three dyes
possess similar structures such that suppression of electronic recombination is likely to be
similar in each case. To confirm this, the electron lifetime of the conduction band electrons in
the TiO2 (= electron lifetime) was analyzed by electrochemical impedance spectroscopy. Fig.
S4 shows the electron lifetime plot against VOC with the MAX DSSCs. The values were derived
from the peak frequency in the Bode plots. In comparison with the reference N719, the electron
lifetime trends are similar among the series, ranging around 2-5 ms at 0.8-1.0 sun. The results
are coherent with the similar VOC values achieved by the three dyes. 2
The Jsc of the benzothiadiazole dye MAX114 issignificantly higher than the oxygen
analogue MAX123. The selenadiazole MAX115 sits between MAX114 and MAX123. This is
consistent with the fact that the dye MAX114 has the highest extinction coefficient but is
contrary to the values of long-wavelength absorption maxima. Since all three dyes have similar
HOMO values (the difference does not exceed 0.1 eV), it is likely that the oxidized state,

18
formed after electron injection, in all three dyes is easily regenerated to the neutral state. The
interesting fact is that the values of LUMO energy level of synthesized dyes change nonlinearly
when moving from oxygen-containing MAX123 through sulfur- (MAX114) to selenium-
analogues (MAX115). Thiadiazole derivative MAX114 has the higher LUMO level whilst the
values of LUMO energy level of MAX115 and MAX123 are, respectively, 0.09 and 0.15 eV
lower. The energy gap values of MAX115 and MAX123 dyes obtained by cyclic voltammetry
are 0.14 and 0.11 eV, respectively, less than the Eg of the MAX114 dye. The energy gap values
obtained from the optical data are consistent with the EgCV values: the difference between the
values exceeded 0.1 eV. The combination of these factors leads to relatively low values of the
photovoltaic efficiency of oxygen-containing dye in comparison with the sulfur and shows that
the performance of sulfur-based dyes should be superior than oxygen and selenium-based dyes.
The dark current in DSSCs based on N719 dyeis less than that for the MAX series dyes,
which means less effective suppression of charge recombination from TiO2 to the electrolyte
for the latter.
It is interesting to note that the current-voltage characteristics for DSSCs based on the
dye WS-2, performed under similar conditions [52], are lower than the similar dye MAX114,
differing by one CH2 group in the donor alicyclic fragment. The most important difference for
WS-2 and MAX114 was found for Jsc and consequently for PCE; 5.07 and 5.86%, respectively.
The incident photon-to-current efficiency curves (Fig. S25) also support the superior JSC of
MAX114 to WS-2. Unlike the UV-Visible absorption in solution (Fig. 1), the optical
absorption of MAX114 is efficient throughout the visible spectrum. The poorer results for
MAX115 and MAX123 are consistent with our predictions above.
Table 3. J-V characteristics for DSSCs based on the dyes MAX series and N719[a].
Voc (mV) Jsc (mA cm-2) FF
η (%)
5.86
5.78
4.66
7.54
Dye
0.64
0.65
0.64
0.78
14.75
13.75
11.90
15.32
0.62
0.64
0.61
0.63
MAX114
MAX115
MAX123
N719

19
0.59
11.8
0.63
5.07
WS-2
[a] The data given for MAX dyes and N719 were obtained with 8 µm (4 µm transparent + 4 µm
scattering) TiO2 films and CDCA additive. The data for WS2 was taken from the paper [52]
and were obtained with 12 µm (8 µm transparent + 4 µm scattering) TiO2 films and no CDCA
additive.
Fig. 3. J-V characteristic for DSSCs based on the dyes MAX series and N719
4
Conclusions
In summary, we have synthesized 4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-
tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole - a new donor building-block for DSSCs. Three novel
D-A-π-A metal-free organic sensitizers were obtained using this donor. 2,1,3-Benzothiadiazole
sensitizer MAX114 based on 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole showed
higher
PCE
by
comparison
to
the
well-known
4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indolyl building block, which is varied from extensively investigated
4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole by one excessive CH2 group in
indoline cycle (see, for example, dye WS-2 and other similar dyes). Interestingly these higher
PCE values for MAX114 were achieved despite significant hypsochromic shift in the UV-vis
spectrum and a slightly higher Eg than the WS-2 dye. This can be explained by the changes of
the ELUMO and EHOMO levels, which depend on the donor strength of the condensed alicyclic
fragment, affecting the degree of conjugation of the molecules and also on molecular geometry,
changing in the transition from cyclopentene to cyclohexene derivative, which affects the
delocalization of the charge. Surprisingly, replacement of the chalcogen atom in the central
acceptor 2,1,3-benzochalcogenadiazole block from the sulfur atom to a more electronegative
oxygen atom or a more voluminous selenium atom leads to nonlinear dependence with

20
bathochromic shift (24-30 nm) in the UV-vis spectra, and bigger energy gap Eg of about 0.1
eV. Nevertheless, the exchange of oxygen- and selenium atoms in benzochalcogenadiazole to
sulfur atom leads to a sharp increase in photovoltaic parameters, including PCE. In summary,
the 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole donor building block opens new
possibilities to improve the photovoltaic efficiency of organic sensitizers in DSSCs and
demonstrates the importance of tuning also the non-conjugated aliphatic part of the donor
group.
Author statement
Maxim S. Mikhailov - Investigation
Nikita S. Gudim - Investigation
Ekaterina A. Knyazeva - Formal analysis, Visualization, Roles/Writing – original draft,
Ellie Tanaka - Investigation
Lu Zhang - Investigation
Ludmila V. Mikhalchenko - Investigation
Neil Robertson – Conceptualization, Supervision, Writing – review & editing
Oleg A. Rakitin – Conceptualization, Project administration, Supervision, Roles/Writing –
original draft
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge financial support from the Russian Science Foundation (grant no.
15-13-10022). We would also like to thank the Centre for Plastic Electronics (CPE) for support
with the IPCE measurements. ET thanks JASSO for a PhD studentship.
Supplementary data
Supplementary data associated with this article can be found, in the online version, at …
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21
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