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Tuning the Photovoltaic Performance of Benzocarbazole Based Sensitizers for Dye-Sensitized Solar Cells - A Joint Experimental and Theoretical Study of the Influence of #-spacers. Mahalingavelar Paramasivam, Ramesh Kumar Chitumalla, Surya Prakash Singh, Ashraful Islam, Liyuan Han, Jayathirtha Rao Vaidya, and Kotamarthi Bhanuprakash J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04629 • Publication Date (Web): 07 Jul 2015 Downloaded from http://pubs.acs.org on July 11, 2015

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The Journal of Physical Chemistry

Tuning the Photovoltaic Performance of Benzocarbazole Based Sensitizers for Dye-Sensitized Solar Cells — A Joint Experimental and Theoretical Study of the Influence of π-spacers Mahalingavelar Paramasivam, a, b Ramesh Kumar Chitumalla,a Surya Prakash Singh,a Ashraful c c b, d a, d Islam, Liyuan Han, V. Jayathirtha Rao* and K. Bhanuprakash * a Inorganic and Physical Chemistry division, CSIR- Indian Institute of Chemical Technology, Hyderabad, 500007, India. b Crop Protection Chemicals division, CSIR- Indian Institute of Chemical Technology, Hyderabad, 500007,India. c Photovoltaic Materials Unit, Environment and Energy Materials Division, National Institute for Materials Science, 1-2-1 Sengen,Tsukuba, Ibaraki 305-0047, Japan. d CSIR-Network of Institutes for Solar Energy, New Delhi, India.

ABSTRACT

Tuning the parameters to enhance the efficiencies of a novel set of metal free sensitizers for dye sensitized solar cells (DSSC) is carried out by varying the πconjugated spacers which link the donor benzocarbazole to the acceptor cyanoacrylic acid. The molecules are synthesized by different combinations of spacers,

namely

fluorene-thiophene (BFT),

fluorene-furan (BFF),

fluorene-

phenyl (BFB) and thiophene-phenyl (BTB). The molar extinction coefficients of all the dyes are high which is attributed to benzocarbazole, but it is higher in the dyes in which fluorene is one of the spacers. But interestingly in the photovoltaic device when the non fluorene dye, BTB is the sensitizer, red shifted and broader incident photon-to-current efficiency (IPCE) curves are obtained leading to larger short circuit current density, Jsc - almost double when compared to BFB based cell. The efficiency of the device with this dye as the sensitizer is also the highest in this series. The reasons behind these observations are investigated using computational techniques.

Keywords: metal free organic dyes, DFT-TDDFT, DSSC, D-π-A, benzocarbazole, PDOS.

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ACS Paragon Plus Environment


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Introduction In the recent years, there has been an increased demand for alternative sources for energy generation mainly due to the rapid depletion of exploited natural resources like oil and coal, and high-risk involved in nuclear power technology.1,

2

This has led to identification of solar energy, harnessed by solar cells, as an alternate renewable energy source due to the promising pollution free and possible cheaper generation.3-6 Even though, conventional silicon based solar cells have reached remarkable efficiency (~25%), it presently renders them unfeasible for largescale power generation due to high cost and fabrication problems for large panels.1 This has paved the way for the cheaper organic dye sensitized solar cells (DSSC) based on ruthenium sensitizers as a promising alternative. An efficiency of ~12% under standard global AM 1.5G irradiation for these cells have been reported.2,

7-8

One problem associated with ruthenium dyes as sensitizers, though they have a broad absorption spectrum which is ideal for sensitization, is the smaller molar extinction coefficients which lead to lower conversion.9 The other issues involved, which hampers its wide application potential, is the scarcity of ruthenium metal and cumbersome purification techniques involved to obtain the pure dye.10 This in turn led to the rapid development of metal free organic dyes as sensitizers which overcome the aforementioned issues and has been demonstrated as useful strategy.11 Tremendous efforts have gone into developing pure organic sensitizers especially to attain high molar extinction coefficients with broad absorption spectrum.8,

11-13

Recently, an unprecedented impressive efficiency of 10.3% with

excellent stability for DSSC has been reported.5 This indicates that further scrutinized and systematic optimization of the absorption spectrum and molar extinction coefficient can improve the overall efficiency. 2


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When compared to the inorganic counterparts, these sensitizers can be easily synthesized and fabricated at a low cost.14 Additionally they also can be designed and the properties fine-tuned by suitable substitution. Thus, knowledge of the electronic structure and the structure activity relationship play a major role in development of these sensitizers. The individual units such as donor, π-spacer and anchoring part play a crucial role in the design of organic dyes for DSSC applications. Of prime concern is the judicious chemical modification of these individual units to enhance the photovoltaic performance, by tuning their photophysical properties. Recently, attempts have been made to the exploration of various donors in D-π-A dyes based on coumarin,15,16 indoline,17, 18 triphenylamine,19, 21

phenothiazine,14,

21

and fluorenylamine.12,

22

The elegant way to get optimal

efficiency is determined by the following factors 1) the augmentation of electron density on donor part which synergistically increases molar extinction coefficient along with stabilization of HOMO. This strategy is desirable to make thin film solar cells for efficient light-harvesting and charge separation along with slower recombination; 2) extension of π-conjugated bridge units is deemed to be important which diverse the position of HOMO and LUMO levels for efficient intramolecular charge separation.23 The π-linkers such as thiophene, furan and benzene play significant role on enhancement in spectral response of the sensitizers due to their unique properties such as resonance energy, thermal stability, etc.21, 24 Keeping these points in mind, we have chosen benzocarbazole moiety as the donor and cyanoacrylic acid as the acceptor and designed new sensitizers. In fact, benzocarbazoles have aroused great interest in OLED studies owing to the excellent hole transport and thermal stability but to the best of our knowledge no study using benzocarbazole as the donor has been reported for DSSC.25 For the spacers we 3



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have opted a combination of two aromatic/heterocyclic rings. One such ring is the fluorene moiety which has been thoroughly investigated recently and suggested for not only enhancement of the molar extinction coefficient of the dye but also for thermal stability and rigidity.26, 27 Combinations of this ring with a second spacer to increase the conjugation of the dye is carried out using thiophene (BFT), furan (BFF) and phenyl rings (BFB). A non fluorene dye with the combination of thiophene and phenyl ring as a spacer (BTB) has also been synthesized and the performance compared to the fluorene dyes. The structures of these molecules are shown in scheme 1. The introduction of 2-ethylhexyl group on benzocarbazole and two butyl groups on fluorene is to improve the solubility, hydrophobic nature also to decrease the intermolecular aggregation.

Computational details Gaussian 09 ab initio quantum chemical software package was used for Density Functional Theory (DFT) calculations.28 Geometrical optimization was performed in gas phase without any symmetry constraints at the B3LYP/6-311G (d, p) level of theory.29, 30 Subsequently, all the optimized structures were subjected to vibrational analysis to check for imaginary frequencies. For computational convenience, long alkyl chains such as 2-ethylhexyl on benzocarbazole and two butyl groups on fluorene were replaced by methyl groups.31 This replacement does not affect the results as shown in many earlier studies.32 The optimized geometries were then used to obtain frontier molecular orbitals (FMOs). To simulate experimental UV/Vis spectra of the dyes, these optimized geometries were subjected to single-point time-dependent DFT (TDDFT) studies (first 15 vertical singlet–singlet 4


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transitions). The integral equation formalism polarizable continuum model (PCM) within self-consistent reaction field (SCRF) theory, has been used to describe the solvation of the dyes.33 The TDDFT calculations were performed using various functionals like B3LYP, PBE0, M06-2X and CAM-B3LYP with a 6-311g (d, p) basis set.30, 34, 35 The software GaussSum 2.2.5 was utilized to simulate the major portion of absorption spectrum and to analyze the nature of transitions. The percentage contributions of individual moieties of the dyes to the respective molecular orbitals have also been calculated.36 For carrying out the DFT studies involving the semiconductor, we have used the (TiO2)16 cluster. This model has been shown to be sufficient for understanding the adsorption energies and binding of the sensitizer on the semiconductor surface.37 The geometry optimizations of the bare (TiO2)16 nanocluster and dyes on (TiO2)16 cluster have been performed using B3LYP functional with a 6-311g (d, p) basis set for the atoms C, H, N, S, O and standard LANL2DZ basis set for the Ti atom. The dyes may get anchored onto the TiO2 surface via the dissociative monodentate ester type or bidentate chelating or bridging mode.38 It is well documented that the dissociative bidentate bridging mode is the preferred binding mode.39, 40 This mode, in which the COOH proton has been transferred to nearby surface oxygen, is considered in this work. Binding energies (adsorption energy) of the dye on to the semiconductor surface have been computed using the formula given below41 = + − The total density of states (TDOS) and partial density of states (PDOS) of the dyes on semiconductor was calculated, using the geometry obtained by B3LYP optimization, by the software GaussSum 2.2.5.

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Fabrication and characterization of DSSCs A nanocrystalline anatase TiO2 photoelectrode with an area of 0.25 cm2 was prepared by screen printing on conducting glass substrate. The thickness of this electrode is 19 µm (nanoporous layer of 14 µm and scattering layer of 5 µm). The dye solution of 3 x 10-4 M in acetonitrile / tert-butyl alcohol (1/1, v/v) was prepared and deoxycholic acid (DCA) (20 mM) was added as a co-adsorbent to prevent the dye aggregation on TiO2. The TiO2 electrode was immersed into the dye solution and then kept at 25 °C for 30 h. Photovoltaic measurements were performed in a sandwich type solar cell in conjunction with an electrolyte consisting of a solution of 0.6M dimethylpropyl-imidazolium iodide (DMPII), 0.05M I2, 0.1M LiI and 0.5M tertbutylpyridine (TBP) in acetonitrile. The dye-adsorbed TiO2 electrode and a counter electrode (platinum-coated conducting glass) were assembled into the solar cell and separated by a 40 µm Surlyn spacer. The solar cell was sealed by heating the polymer frame. Photocurrent density-voltage (I-V) was measured under AM 1.5G simulated solar light at a light intensity of 100 mW cm-2 with a metal mask of 0.25 cm2. The photovoltaic parameters, i.e. short circuit current density (Jsc), open circuit photovoltage (Voc), fill factor (FF), and power conversion efficiency (η) were estimated from I-V characteristics under illumination.

Results and discussion Synthesis of dyes The target dyes BFT, BFF, BFB and BTB were synthesized by multistep synthetic pathways as depicted in scheme 2. Synthesis of 8-bromo-11Hbenzo[a]carbazole(1) was carried out in two steps by modified Bücherer carbazole 6


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synthesis i.e., the mixture of α-tetralone and 4-bromophenyl hydrazine hydrochloride were refluxed in ethanol in the presence of catalytic amount of acetic acid to get 8bromo-6,11-dihydro-5H-benzo[a]carbazole dehydrogenation

was

carried

out

in

using

almost chloranil

quantitative to

get

yield

and

8-bromo-11H-

benzo[a]carbazole(1) in 96% overall yield.42 In another sequence, commercially available fluorene was brominated to get dibromo derivative43 and it was then dialkylated with n-butyl bromide to obtain 2,7-dibromo-9,9-dibutyl-9H-fluorene according to modified literature procedures.44 The N-alkylation of 8-bromo-11Hbenzo[a]carbazole (2) by ethylhexyl bromide was carried out by following the same method. The alkylated bromo derivative of benzo[a]carbazole (2) was treated with n-butyl lithium to generate the corresponding aryl lithio derivative at -78°C, then 2isopropoxy-4, 4, 5, 5-tetramethyl-1,3,2-dioxaborolane was added to yield the corresponding

boronate

derivative

of

8-bromo-11-(2-ethylhexyl)-11H-benzo[a]

carbazole (3). The bromo derivatives, 4 and 5 were obtained by Suzuki coupling of the boronate derivative (3) with 2, 7-dibromo-9, 9’-dibutyl fluorene and 2, 5-dibromo thiophene. A Suzuki–Miyaura coupling protocol was developed to make key aldehyde intermediates, 6–8, in good yields by the reaction of bromo derivative 4 with various boronic acid derivatives such as 5-formyl thiophene-2-boronic acid, 5formyl furan-2-boronic acid, and 4-formyl benzene boronic acid . The key intermediate 9 was synthesized by following the same protocol using 4-formyl benzene boronic acid with the corresponding bromo derivative 5. Finally, the target dyes were obtained through Knoevenagel condensation of formylated intermediates with cyanoacetic acid in presence of catalytic amount of ammonium acetate in acetic acid medium. All these dyes were characterized by 1H NMR,

13

C NMR, IR, EI-MS

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and ESI-HRMS. The details of the experimental characterization along with theoretical data are available in the ESI.

Molecular Geometry and orbitals of the dyes Knowledge of the molecular geometries and electron density distribution in the organic dyes can be of great help to gain insight of the electronic and spectroscopic properties of the π-conjugated spacers-a requisite for understanding the photovoltaic performance. These can be obtained by optimizing the geometries of the molecules shown in scheme 1 using softwares based on quantum mechanical properties (DFT methods). Literature reports have supported that the tuning of photophysical and electrochemical properties of a molecule is crucial to achieve an optimized efficiency.8 Among that, degree of π-conjugation in the system has a significant impact on efficient charge transport which can be generally assessed by dihedral angle between the donor, the π-bridge units and terminal acceptor.22,

45

Keeping these points in view, the geometries of the molecules shown in scheme 1 have been optimized and the resulting geometries along with the dihedral angles are shown in Fig. 1. It is seen from the figure that insertion of fluorene spacer into D-π-A architecture renders a slight twist between benzocarbazole and the spacer. The dihedral angle between benzocarbazole donor and fluorene units is calculated to be around 39° in all fluorene dyes and the dihedral angle possessed between fluorene and the second aromatic ring i. e., thiophene, furan, benzene are computed to be 25.6°, 1.7° and 35.7° respectively. In the case of BTB the dihedral angles between benzocarbazole-thiophene entity and thiophene-phenyl entity are calculated to be 29° and 18.9° respectively. The terminal cyanoacrylic acid is found to be almost 8


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coplanar (0.1-0.9°) with the π-conjugated units. It should be mentioned that for these dyes that the geometries indicated in the figure will be only one of the many possible minima on the potential energy surface in the gas phase; slightly higher energy conformations due to the rotamers (rotation between the rings) are possible, particularly in the bulk media. The isodensity plots of frontier molecular orbitals and the energy levels of the isolated dyes are shown in Fig. 2. The HOMO-LUMO gap (HLG) are 2.61 to 2.74 eV. The HOMO of all the dyes is between 5.45- 5.56 eV. The HOMO of BTB (5.56 eV) is slightly lower than the other molecules. BFT and BFF lie almost at the same level. To gain further insight into the electron density distribution in each dye, partial electron density contribution of the donor (D), fluorene/thiophene (π1) heterocyclic spacers/benzene (π2), and anchoring group (A) in each dye from the respective frontier orbitals have been generated and depicted in Fig. 3. The HOMO of the fluorene dyes is largely localized over benzocarbazole and to a small extent over the fluorene-auxiliary π-spacer segments and LUMO is predominantly localized over auxiliary π-spacer segments and cyanoacrylic unit. In HOMO-1 of fluorene based dyes, fluorene has a large contribution. On the other hand, the HOMO of BTB is delocalized over the donor and the two spacers while the LUMO is populated over thiophene-phenyl and the acceptor groups. Optical Properties The UV-Visible absorption spectra of the dyes in THF solution (1 X 10-5 M) are shown in Fig. 4.The corresponding properties for the dyes are summarized in Table. From the spectra it is seen that these dyes exhibit two major prominent bands, the first peak at 275-355 nm, and second peak at 350–520 nm. The lower wavelength 9



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region absorption bands at 275– 355 nm is ascribed to a localized aromatic π–π* transition, while the bands in the range of 350−520 nm is mainly attributed to an intramolecular charge-transfer (ICT) transition from benzocarbazole donor to cyanoacrylic acid acceptor. The absorption maxima are almost similar for BFT (433 nm), 436 nm (BFF), and 427 nm ( BTB ) while for BFB it is around 389 nm. The fluorene based dyes, namely BFT, BFF and BFB exhibit higher molar extinction coefficient (ε ≈ 62,782-65893 M-1cm-1) than thiophene based analogue BTB (ε = 50,623 M-1cm-1). To understand this behaviour, particularly the higher intensity in the case of fluorene dyes, we have carried out TDDFT studies of these molecules using PCM model by incorporating the effects of solvent (THF). Though we have carried out with studies with three different functionals, only the M06-2X results are shown in Table 1, while the other results are given in supporting information. The TDDFT results are in nearly good agreement with the experimental observations. The oscillator strengths of absorption peaks obtained from TDDFT studies for all the dyes show similar trend as that of the experimental results.The transitions which make up the absorption maxima are shown in the same table. Clearly, for the fluorene dyes the major contribution to the low energy transition is HOMO-1 to LUMO, which is a charge transfer from fluorene moiety to the acceptor.46,

47

The contribution due to

charge transfer from HOMO to LUMO is smaller i.e, from benzocarbazole to cyanoacrylic acid. While for BTB, the non fluorene dye, the lowest energy transition is due to the charge transfer from HOMO to LUMO, which is basically from benzocarbazole to cyanoacrylic acid. Thus, larger intensity of absorption in the fluorene dyes when compared to the BTB dye is due to the intensity borrowing from HOMO-1 to LUMO transition which involves the fluorene moiety. The variation of λmax 10


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with respect to dihedral angle is shown in Fig. S5 (ESI). It can be seen that larger red shift is observed when thiophene is rotated with respect to the donor. The dipole moments given in Table 1 indicate a large charge separation in the ground state mainly in the case of BTB for the dipole moment is 9.8 D. The smallest dipole moment is obtained for BFB which is around 7.1 D. Apparently, these higher molar extinction coefficients of the dyes could be considered as one of the paramount factor in order to make thinner TiO2 films for efficient light harvesting. The absorption spectra of the dyes adsorbed on TiO2 film is shown in Fig. 4. An interesting change is seen. Here, BTB has a larger red shift and larger area under the curve compared to fluorene based dyes especially BFB. To investigate this behaviour, we have carried out DFT calculations of the dye adsorbed on the TiO2 surface. The optimized geometry of dyes on TiO2 for the four dyes are shown in Fig. 1 and detailed in Fig. S1 (ESI) while the molecular orbitals generated are shown in Fig. 2. Interestingly, fluorene dyes do not show any major change in geometry upon binding. This is also reflected in the HOMO energy level of the dye on the semiconductor surface which hardly changes from the isolated dye HOMO energy level. On the other hand, BTB dye tends to be more planar when bound on to the TiO2 surface (a twist from 18.9° to 8.3° between thiophene and benzene is seen). This can be also seen in changes in the HOMO level of BTB on TiO2 which stabilizes by 0.05 eV. The LUMO levels of all the four dyes stabilize when bound to the TiO2 surface.

Electrochemical Studies

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In order to evaluate electrochemical properties of these dyes, redox behavior was investigated by performing the cyclic voltammetric (CV) technique with a standard three-electrode configuration. HOMO levels of all the dyes are calculated from the onset oxidation potential according to an empirical formula [EHOMO = - (Eox + 4.4)]. The cyclic voltammograms observed for these dyes are displayed in Fig. 5 and the results tabulated in Table 2. All dyes show an oxidation potential in the range of 1.02 V-1.08 V which is attributed to the removal of an electron from benzocarbazole segment. A critical aspect to utilize these dyes as sensitizers in DSSCs, it is essential to ensure their energetic alignment of HOMO and LUMO levels are suitable for efficient electron injection from dye to TiO2 (-4.2 eV) and electron regeneration of dye cation from I-/I3- electrolyte redox couple (-5.1 eV).10 The onset oxidation potential (Eox), which reflects the highest occupied molecular orbital (HOMO) of the dyes BFT, BFF, BFB, BTB, are calculated to be -5.43 eV, -5.48 eV, -5.42 eV and 5.43 eV respectively. This indicates that estimated HOMO level of all dyes are sufficiently lower than the redox potential of I-/I3- redox couple for efficient regeneration. It has been noticed that the substitution of thiophene segment in lieu of fluorene did not produce any significant changes in oxidation potential. Excited state oxidation potential (E*ox) was determined from first oxidation potential (Eox) of the ground state and zero–zero transition energy (E0–0) according to following equation: ∗ = −

The optical band gap (E0–0) was derived from onset value of absorption spectra. The estimated values for the dyes BFT, BFF, BFB and BTB from the above equation are -1.47 V, -1.45 V, -1.78 V and -1.46 V respectively which implies that excited state 12


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oxidation potential of these sensitizers are more negative than conduction band edge energy level of TiO2. This predicts a facile electron injection from dye to the TiO2 electrode providing thermodynamic feasibility for electron injection. The dyes except BFB have almost similar excited state oxidation potentials. The reason for the larger excited state oxidation potential of BFB in spite of similar ground state oxidation state potential is due to E0-0 which is larger. It has been observed from previous reports that the use of benzocarbazole as donor lowers the HOMO level in comparison to that of carbazole dyes which is beneficial to improve the Voc along with slower recombination and efficient charge separation.31 The schematic diagram clearly illustrates the comparison of HOMO and LUMO energy levels of isolated dyes estimated from electrochemical, optical data along with energy levels of isolated dyes and dyes on TiO2 assembly obtained from theoretical data (Fig. S7, ESI).

Thermal Properties The

thermal

properties

of

the

four

dyes

were

investigated

by

thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10°C min-1 is shown in Fig. 6 The results, shown in Table 3, and fig 6 reveals that the 5% weight loss temperatures (Td) are in the range 295-321°C. It is also seen from the figures and table that the fluorene analogues exhibits more thermal stability than their thiophene counterparts owing to the presence of rigid fluorene unit.26

Photovoltaic Performance In order to investigate the effect of various π-spacers substitution on photovoltaic performance of the dyes, DSSCs were fabricated using these new 13



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sensitizers employing an electrolyte composed of iodine (0.05 M), lithium iodide (0.1 M), 1, 2-dimethyl-3-Propyl imidazolium iodide (0.6M), and 4-tert-butylpyridine (0.5 M).The photocurrent voltage (I-V) plots of fabricated DSSCs and the action spectra of incident photon-to-current conversion efficiency (IPCE) as a function of wavelength are shown in Fig. 7 and parameters measured under standard global AM 1.5 solar light (100 mW cm-2), are compiled in Table 3. The onset of IPCE spectra for these dyes were found to be 630 nm for BFT and BFF, 590 nm for BFB and 680 nm for BTB respectively. IPCE values around 80% are observed in the range from 360 to 630 nm for BFT and BFF with an optimal value of 82% at 480 nm. The photovoltaic parameters of the dyes BFT, BFF consist of fluorene-heterocyclic spacers such as thiophene, furan exhibit the Jsc of 8.11 mA.cm-2 and 7.89 mA.cm-2, Voc of 0.75 V and 0.77 V, FF of 0.76 and 0.77, and the power conversion efficiency (η) of 4.61 and 4.63 respectively. When the heterocyclic ring in the fluoreneheterocyclic spacers is replaced by a benzene unit (BFB), efficiency (ɳ) of BFB is drastically reduced from 4.63% to 3.38% due to narrow IPCE spectrum of around 72% in the short range of 360-590 nm. On the other hand fluorene moiety in the spacer has been replaced with thiophene (BFB to BTB) there is an increase in the short circuit current (Jsc) to 10.18 mAcm-2 from 5.67 mA cm-2. The open circuit voltage (Voc) of BTB is 0.73 V, fill factor (FF) is 0.77 and the power conversion efficiency (η) of 5.74%. To get a better understanding of the behavior of the sensitizers, we investigate the extent of π-spacers influence on electronic coupling between dyes and dyes on TiO2 assembly. Because modifying the dyes with varying each individual units such as donor, π-spacer and anchoring group may also affect the electronic coupling between the molecular orbital of the dyes with electronic bands of 14


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the substrate and potential of the semiconductor band itself.48-50 Thus, in order to gain further insight into the electronic coupling strength between the dye’s LUMO and semiconductor conduction band and the electron transfer features during photo excitation, we have calculated the binding energies of the bound dyes and generated the molecular orbitals, TDOS and PDOS of bare TiO2, isolated dyes and dyes on TiO2 assembly. The binding energy of the dye on the semiconductor surface (Eads) is shown in Table 3. The binding energy of the BTB dye on the semiconductor surface is 14.7 kcals/mol while for the fluorene based dyes, it is lower. Here, we also observe that the BFB has lowest binding energy of 10.9 kcals/mol. It is clear from the results that the binding of BTB dye on the semiconductor surface is quite strong compared to the other three dyes. This higher binding energy of the BTB dye on the semiconductor surface is probably due to larger electrostatic interactions. The partial density of states (PDOS) obtained from atomic orbital coefficients located on the atoms of the dyes from dyes on TiO2 assembly have been constructed through an artificial gaussian broadening of individual orbital contributions by an arbitrary factor of 0.3 eV which is shown in Fig. 8. The rate of electron injection which plays prominent role in photovoltaic performance of DSSC is directly proportional to spatial and energetic overlap between the LUMO of the dyes and TiO2.48,

51

If electronic coupling strength is strong, the LUMO would be more

broadened and downshifted towards the energy levels of TiO2 conduction band. The comparison of TDOS of isolated dyes with TDOS of bare TiO2 and dye on TiO2 interacting system is shown in Fig. 8. The inset view of PDOS expansion of the dyes from dyes on TiO2 assembly has been extracted from the Lorentzian distribution of adsorbate LUMO levels and is depicted for clarity. As displayed in the Fig. 8, the shape and energy levels of LUMO of the isolated dyes remains unchanged for all the 15



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Page 16 of 44

dyes. But the dyes after anchoring on TiO2, the PDOS contribution of fluorene dyes BFT, BFF and BFB shows only slight broadening when compared to their isolated counterparts and appear in the range of ~60 meV. From the PDOS contributions of BTB in BTB on TiO2 assembly, it has been observed that the broadening effect of LUMO is spread over a range of ca.110 meV which suggests that there is strong interfacial electronic coupling over the interface which favors efficient electron injection into the semiconductor.

Conclusions The molar extinction coefficients of the dyes are large due to the donor benzocarbazole. The larger intensity of absorption seen in fluorene dyes BFT, BFF and BFB when compared to the non fluorene dye BTB is attributed to the mixing of the lowest energy transition with HOMO-1 to LUMO, which is a fluorene to cyanoacetic acid transition. The enhancement of intensity seen in fluorene based dyes can thus be attributed to “Intensity borrowing”. Nevertheless the other properties like oxidation potentials and the absorption maxima are nearly same in the dyes. On the other hand when the BTB dyes are adsorbed on TiO2, a better conjugation because of more planarity is seen. This leads to stronger binding of the BTB dye on TiO2 surface and also red shifts the lowest energy transition. We observe from DFT calculations that the LUMO of BTB on TiO2 system is broadened indicating a better injection of the electron into the semiconductor. This is reflected in the device which shows that the device with BTB as sensitizer is almost 70% more efficient than BFB based device. The Jsc is almost double of Jsc in BFB. A structure-

16


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activity relationship of these spacers with benzocarbazole as donor has been developed.

Experimental Materials and Instruments Unless otherwise specified, all reactions have been carried out under nitrogen atmosphere with standard Schlenk techniques. All reagents are reagent/analytical grade and used without further purification. Sodium and benzophenone were used to distil the solvents under nitrogen atmosphere. All chromatographic separations were carried out on silica gel (60-120 mesh). Bruker Avance (400 MHz) spectrometer and Varian Inova (500 MHz) characterize 1H NMR and

13

were used to

C NMR spectra and CDCl3 and DMSO-d6

used as

solvents with TMS as standard in both cases. Mass spectra were obtained by using ESI-MS (Thermofinnigan), EI-MS, GCMS (VG70–70H) and High Resolution Mass Spectra(HRMS) were obtained from Sanzox spectrometer. UV–Vis absorption and Fluorescence spectra were recorded on a Jasco V-550 spectrophotometer and Fluorolog-3 spectrofluorometer (Spex model) respectively. IR spectra of the sensitizers were characterized on Perkin-Elmer Spectrum BX spectrophotometer at a resolution of 4 cm−1. TGA/SDTA 851e (Mettler Toledo) thermal analyzer was used to perform thermogravimetric analysis (TGA) with a heating rate of 10°C min-1 under nitrogen atmosphere in the temperature range of 33–550°C. Melting points were measured with an Electro thermal IA 9100 series digital melting point instrument and are uncorrected. Cyclic voltammetric measurements were performed on a PC-controlled 17



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Page 18 of 44

CH instruments model CHI 620C electrochemical analyzer, 0.5mM dye solution in dichloromethane (DCM) ata scan rate of50 mV/s was used with 0.1 M tetra butyl ammonium perchlorate (TBAP) as supporting electrolyte. The glassy carbon, standard calomel electrode (SCE) and platinum wire were used as working, reference and auxiliary electrodes respectively. All the potentials reported are against SCE.

8-Bromo-11H-benzo[a]carbazole (1) α-Tetralone (9.16g, 6.26 mmol) and 4-bromophenylhydrazine hydrochloride (10 g, 44.7 mmol) were dissolved in 300 mL of ethanol and then catalytic amount of acetic acid was added to the reaction mixture, refluxed for 3 hrs under nitrogen atmosphere. The reaction mixture was cooled to room temperature. The formed product was filtered, dried and used for next step without purification. The dried compound (9.29 g, 30.8 mmol) and tetrachloro-1-benzoquinone (10.6 g, 43.2 mmol) in xylene were refluxed under nitrogen atmosphere for 8 hrs, cooled to room temperature, NaOH (10%) and water were put into the reaction solution. The organic layer was extracted with ethyl acetate and dried over sodium sulphate. The reaction solution was concentrated, purified by column chromatography over silica gel and the purified compound was recrystalized from ethanol to give desired product as white crystals (8.8 g, yield 96%). 1H NMR (500 MHz, CDCl3): 8.24 (s, 1H), 8.237.99 (m, 3H), 7.68-7.44 (m, 5H).

13

C NMR (500 MHz, CDCl3): 137.5, 132.6, 128.8,

127.0, 125.5, 125.4, 122.3, 121.6, 120.0, 119.1, 112.7. GC/MS: m/z 296 (M+).

8-Bromo-11-(2-ethylhexyl)-11H-benzo[a]carbazole (2) 18


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8-Bromo-11H-benzo[a]carbazole (5 g, 16.89 mmol), 2-ethylhexyl bromide (4.24g, 21.9 mmol), 50% aq.NaOH and catalytic amount of tetrabutyl ammonium iodide (0.623 g. 10 mol %) were taken in a flask. The reaction mixture was heated to 70°C continuously for 8 hrs and then cooled to room temperature. The reaction mixture was extracted with hexane, washed with water and dried over anhydrous sodium sulphate. The solvent was removed under vacuum and the crude was purified by column chromatography over silica gel with n-hexane as the eluent to give the desired product as white solid (6.67g, Yield 97%). 1H NMR (400 MHz, CDCl3): 8.39 (d, 1H, J = 8.3 Hz), 8.16 (d, 1H, J = 2.3 Hz), 7.99 (d, 2H, J = 8.3 Hz), 7.60-7.44 (m, 4H), 7.28 (t, 1H, J = 14.6 Hz), 4.41-4.37 (m, 2H, N-CH2), 2.17-2.13 (m, 1H, N-CH2-CH), 1.52-1.16 (m, 8H,), 0.91-0.78 (m, 6H).

13

C NMR (500 MHz, CDCl3):

139.6, 134.9, 133.8, 129.5, 127, 125.2, 124.8, 124.3, 122.1, 122, 121, 118.8, 118.3, 112.2, 111.2, 49.9, 39.5, 28.4, 23.8, 23, 14, 10.7. GC/MS: m/z 407 (M+).

11-(2-Ethylhexyl)-8-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-11H-benzo [a]carbazole (3) To a solution of 8-bromo-11-(2-ethylhexyl)-11H-benzo[a]carbazole (5 g, 12.3 mmol) in anhydrous THF (120 mL) at -78°C, 6.75 ml (13.51 mmol) of n-butyl lithium (2 M in hexane) was added drop wise to the solution under N2 atmosphere. The mixture was stirred at -78°C for 2 hr, then 3.25 ml (15.97 mmol) of 2-isopropoxy4, 4, 5, 5-tetramethyl-[1, 3, 2]-dioxaborolane was added rapidly to the solution, and the resulting solution was stirred for further 1hr at -78°C. The mixture was allowed to warm to room temperature and stirred during overnight. The reaction mixture was quenched with water and extracted with chloroform. The organic extracts were 19



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Page 20 of 44

washed with water, brine and dried over anhydrous sodium sulphate. The solvent was removed by rotary evaporation, and the residue was

purified by column

chromatography over silica gel with n-hexane: ethyl acetate (100:5) as the eluent to give the desired product as white solid (5.1g, Yield 91%). 1H NMR (400 MHz, CDCl3): 8.66 (s, 1H), 8.52 (d, 1H, J = 7.9 Hz), 8.24 (d, 1H, J = 3.9 Hz), 8.02 (d, 1H, J = 6.9 Hz), 7.92 (d, 1H, J = 7.9 Hz), 7.68 (d, 1H, J = 8.9 Hz), 7.58 (m, 3H), 4.724.62 (m, 2H, N-CH2), 2.28-2.24 (m, 1H, N-CH2-CH), 1.41(s, 12H), 1.37-1.16 (m, 8H,), 0.86-0.80 (m, 6H).

13

C NMR (500 MHz, CDCl3): 142.4, 133.8, 132.8, 130.1, 128.8,

126.4, 124.3, 123.6, 121.8, 121.6, 121.3, 120.3, 119, 118.4, 108.4, 82.7, 82.6, 49, 38.8, 29.7, 27.7, 24.3, 23, 22.2, 13.2, 10. GC/MS: m/z 455 (M+).

8-(7-Bromo-9,9-dibutyl-9H-fluoren-2-yl)-11-(2-ethylhexyl)-11H-benzo[a] carbazole (4) 11-(2-Ethylhexyl)-8-(4, 4, 5 ,5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-11Hbenzo[a] carbazole (1 g, 2.19 mmol), 2,7-dibromo-9,9-dibutyl-9H-fluorene (1.14 g, 2.64 mmol), tetrakis (triphenylphosphine) palladium dissolved in the mixture of toluene and

(127mg, 0.001 mmol) were

aqueous 2 M

potassium carbonate

solution (3:1, v/v) in a round-bottomed flask equipped with a reflux condenser and the reaction mixture was heated to 80°C for 24 hrs. After cooling the reaction mixture, it was poured into water and extracted with chloroform, then dried with anhydrous sodium sulphate. The solvent was removed by rotary evaporation and purified by column chromatography over silica gel with n-hexane as the eluent to give the desired product as white foamy solid. (1.1g, Yield 73%). 1H NMR (500 MHz, CDCl3): 8.48 (d, 1H, J = 8.3 Hz), 8.38 (d, 1H, J = 1.5 Hz), 8.23 (d, 1H, J = 9.1 Hz), 20


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7.98 (d, 1H, J = 8.31 Hz), 7.92 (d, 1H, J = 7.9 Hz), 7.74-7.63 (m, 4H), 7.57-7.42 (m, 5H), 4.60-4.45 (m, 2H, N-CH2), 2.27-2.22 (m, 1H, N-CH2-CH), 2.12-1.93 (m, 4H), 1.43-1.04 (m, 8H), 0.86-0.80 (t, 6H, J = 7.55), 0.72-0.67 (t, 6H, J = 8.31).

13

C NMR

(500 MHz, CDCl3): 153.2, 151.1, 141.6, 140.8, 140.1, 138.5, 135.2, 133.8, 133.1, 130.0, 127.6, 126.4, 126.2, 125.2, 124.6, 124.3, 123.4, 122.6, 122.2, 121.6, 121.0, 120.8, 120.7, 120.1, 119.7, 119.2, 117.9, 110.2, 55.5, 50.2, 40.3, 39.7, 30.6, 28.6, 26.0, 23.9, 23.1,14.1, 13.9, 10.8. MS (EI): m/z 684.2 (M+).

8-(5-Bromothiophen-2-yl)-11-(2-ethylhexyl)-11H-benzo[a]carbazole (5) 11-(2-Ethylhexyl)-8-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-11Hbenzo[a] carbazole (1g, 2.19 mmol), 2, 5-dibromothiophene (0.638g, 2.64 mmol) were dissolved in 15 ml of toluene and taken in a round-bottomed flask equipped with a reflux condenser. After adding 8 ml of aqueous 2M K2CO3 solution, the reaction mixture was degassed under nitrogen atmosphere for 15 minutes; tetrakis(triphenylphosphine)palladium (127mg, 0.001 mmol) was added to the reaction mixture and then heated at 80°C for 24 hrs. After cooling the reaction mixture, it was poured into water and extracted with chloroform, then dried with anhydrous sodium sulphate. The solvent was removed by rotary evaporation and purified by column chromatography over silica gel with n-hexane as the eluent to give the desired product as yellowish white solid (816mg, Yield 76%). 1H NMR (400 MHz, CDCl3): 8.45 (d, 1H, J = 7.9 Hz), 8.82 (s, 1H), 8.12 (d, 1H, J = 8.9 Hz), 8.1 (d, 1H, J = 7.9 Hz), 7.66 (d, 1H, J = 7.9 Hz), 7.61 (m, 3H), 7.38-7.35 (m, 1H), 7.07 (s, 2H), 4.49-4.37 (m, 2H, N-CH2), 2.26-2.20 (m, 1H, N-CH2-CH), 1.42-1.29 (m, 8H), 0.87-0.85 (t, 6H, J = 6.99).

13

C NMR (500 MHz, CDCl3): 147.4, 140.8, 135.1, 133.8, 21



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Page 22 of 44

130.8, 129.6, 125.3, 125.2, 124.7, 123.1, 122.7, 122.4, 122.0, 119.4, 119.0, 116.5, 110.3, 109.8, 49.9, 39.6, 30.8, 28.5, 23.9, 23.1, 14.2, 10.9. MS (EI): m/z 490.5 (M+).

5-(9, 9-Dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)-9H-fluoren-2yl) thiophene-2-carbaldehyde (6) A mixture of 8-(7-bromo-9, 9-dibutyl-9H-fluoren-2-yl)-11-(2-ethylhexyl)11H-benzo[a] carbazole (1 g, 1.4 mmol), 5-formyl thiophene-2-boronic acid (318 mg, 2.1 mmol), PdCl2(dppf) (54 mg, 5 mol %), K2CO3 (807mg, 5.8mmol) were dissolved in dry toluene/methanol mixture in the ratio of (3:1) and degassed with nitrogen for 15 minutes. The reaction mixture was heated to reflux for 12h, then cooled to room temperature. The reaction mixture was filtered through celite pad to remove the inorganic residues. The filtrate was extracted by chloroform, washed with water and brine. The organic layer was dried with anhydrous sodium sulphate and solvent was removed by rotary evaporation. The residue was purified by column chromatography on silica gel using n-hexane: ethyl acetate in the ratio of (95 : 5, v/v) as the eluent to give the desired product as a greenish yellow foamy solid (723 mg, 69%). 1H NMR (500 MHz, CDCl3): 9.88 (s, 1H), 8.55 (d, 1H, J = 8.9 Hz), 8.42 (s, 1H), 8.27 (d, 1H, J = 8.9 Hz), 8.25 (d, 1H, J = 7.9 Hz), 7.96-7.51(m, 12H),

7.44 (d, 1H, J = 3.9 Hz),

4.72-4.61 (m, 2H, N-CH2), 2.30-2.29 (m, 1H, N-CH2-CH), 2.22-2.01 (m, 4H), 1.481.21 (m, 6H), 1.17-1.10 (m, 4H), 0.89-0.83 (m, 6H), 0.73-0.70 (m, 6H).

13

C NMR

(500 MHz, CDCl3): 182.8, 155.3, 152.1, 152.0, 142.8, 1412.0, 141.9, 140.9, 138.7, 137.7, 135.2, 133.9, 133.0, 131.5, 129.8, 126.6, 125.2, 124.7, 124.4, 123.8, 123.5, 122.6, 122.3, 121.7, 121.0, 120.7, 120.6, 120.4, 119.7, 119.3,118.0, 110.3, 55.5,

22


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50.1, 40.4, 39.8, 30.7, 28.7, 26.2, 24.0, 23.2,23.1, 14.2, 14.0, 10.9. MS (EI): m/z 716.03 (M+). IR (KBr, cm-1): 2857 (νH-C=O); 1668 (νC=O).

5-(9, 9-Dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)-9H-fluoren-2yl) furan-2-carbaldehyde (7) This was synthesized by a procedure similar to that of 6 except that 5-formyl furan-2-boronic acid was used in lieu of 5-formyl thiophene-2-boronic acid (744 mg, 73%). 1H NMR (500 MHz, CDCl3): 9.70 (s, 1H), 8.58 (d, 1H, J = 8.9 Hz), 8.49 (s, 1H), 8.32 (d, 1H, J = 7.9 Hz), 8.1 (d, 1H, J = 7.9 Hz), 7.93(s, 1H), 7.85-7.72 (m, 9H), 7.65-7.57 (m, 3H),

7.35- 7.3 (d, 1H, J = 3.9 Hz), 6.88- 6.87 (d, 1H, J = 2.9 Hz),

4.70-4.60 (m, 2H, N-CH2), 2.32-2.18 (m, 5H), 1.52-1.2 (m, 10H), 0.93-0.76 (m, 18H). 13

C NMR (500 MHz, CDCl3): 177.0, 160.6, 152.0, 151.9, 151.8, 142.9, 141.9, 140.8,

138.7, 135.2, 133.8, 133.0, 129.7, 127.3, 126.5, 125.2, 124.8, 124.7, 124.3, 123.4, 122.6, 122.3, 121.6, 120.9, 120.5, 120.5, 120.1, 119.7, 119.5, 119.2, 117.9, 110.3, 107.6, 55.5, 50.1, 40.4, 39.7, 30.6, 28.6, 26.1, 23.9, 23.1, 14.1, 13.9, 10.8. MS (ESI): m/z 699.96 (M+H). IR (KBr, cm-1): 2854 (νH-C=O); 1672 (νC=O).

4-(9, 9-Dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)-9H-fluoren-2yl) benzaldehyde (8) This was synthesized by a procedure similar to that of 6 except that 4-formyl benzene boronic acid was used in lieu of 5-formyl thiophene-2-boronic acid. light greenish solid (687mg, Yield 66%). 1H NMR (500 MHz, CDCl3): 10.06 (s, 1H), 8.57 (d, 1H, J = 8.3 Hz), 8.44 (s, 1H), 8.29 (d, 1H, J = 8.3 Hz), 8.06 (dd, 3H, J = 15.86 23



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Page 24 of 44

Hz), 7.77-7.51(m, 13H), 4.78-4.62 (m, 2H, N-CH2), 2.31-2.11 (m, 5H), 1.62-1.10 (m, 16H), 0.92-0.68 (m, 12H).

13

C NMR (500 MHz, CDCl3): 191.4, 151.3, 151.2, 147.1,

141.1, 138.3, 137.5, 134.6, 134.4, 133.2, 129.8, 129.1, 127.1, 125.9, 125.8, 124.6, 124.1, 123.7, 122.8, 121.9, 121.6, 121.1, 120.3, 119.8, 119.6, 119.1, 118.6, 117.3, 109.7, 54.8, 49.6, 39.8, 39.2, 30.1, 28.1, 25.6, 23.3, 22.5, 13.5, 13.4, 10.3. MS (EI): m/z 710.00 (M+). IR (KBr, cm-1): 2858 (νH-C=O); 1699 (νC=O).

4-(5-(11-(2-Ethylhexyl)-11H-benzo[a]carbazol-8-yl) thiophen-2-yl) benzaldehyde (9) Under

N2

atmosphere,

a

mixture

of

8-(5-bromothiophen-2-yl)-11-(2-

ethylhexyl)-11H-benzo[a]carbazole (5) (1g, 2 mmol), 4-formyl benzene boronic acid (427mg, 2.9 mmol), PdCl2(dppf) (75 mg, 5mol%), K2CO3 (1.12g, 8.1mmol) were dissolved in dry toluene/methanol mixture in the ratio of 3:1 and degassed with nitrogen for 15 minutes. The reaction mixture was heated to reflux for 12hrs then cooled to room temperature. The reaction solution was filtered through celite pad to remove inorganic residues. The filtrate was extracted with chloroform, then washed with water and brine. The organic layer was dried over anhydrous sodium sulphate and the solvent was removed by rotary evaporation. The silica gel coated residue was purified by column chromatography on silica gel using n-hexane:ethyl acetate in the ratio of 95 : 5 (v/v) as the eluent to give the desired product as a yellowish foamy solid (836 mg, Yield 79%). 1H NMR (500 MHz, CDCl3): 9.96(s, 1H), 8.49 (d, 1H, J =8.3 Hz), 8.33 (s, 1H), 8.12 (d, 1H, J = 1.5 Hz), 8.19 (d, 1H, J = 8.5 Hz), 8.05 (d, 1H, J = 8.12 Hz), 7.86 (d, 2H, J = 8.3), 7.76-7.66 (m, 4H), 7.61-7.42 (m, 2H), 7.35 (d, 1H, J = 3.7), 4.65-4.50 (m, 2H, N-CH2), 1.60-1.45 (m, 1H, N-CH2-CH), 1.42-1.18 24


Page 25 of 44

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(m, 8H), 0.88-0.83 (m, 6H).

13

C NMR (500 MHz, CDCl3): 190.9, 147.1, 140.5, 139.9,

139.8, 134.8, 134.3, 133.4, 130.0, 129.8, 129.2, 127.4, 125.7, 125.1, 124.9, 124.8, 124.8, 122.8, 122.7, 122.4, 122.0, 121.7, 120.6, 119.0, 118.6, 116.3, 109.8, 49.7, 39.2, 30.1, 28.1, 23.4, 22.5, 13.5, 10.4. MS (EI): m/z 515.7 (M+). IR (KBr, cm-1): 2852 (νH-C=O); 1696 (νC=O).

(Z)-2-Cyano-3-(5-(9, 9-dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)9H-fluoren-2-yl) thiophen-2-yl) acrylic acid (BFT) A mixture of 5-(9,9-dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)-9Hfluoren-2-yl) thiophene-2-carbaldehyde(6) (500 mg, 0.7 mmol), cyanoacetic acid (237 mg, 2.7 mmol), and ammonium acetate (161mg, 2.1 mmol) were dissolved in glacial acetic acid (5 mL) and stirred at 80°C for 10 hrs. After the mixture was cooled to room temperature, water was added to quench the reaction. The precipitated solid was filtered out and extracted with dichloromethane, then washed with water and brine. The organic extracts were collected, dried over anhydrous sodium sulphate, filtered, and evaporated. The resulting crude solid was purified by column chromatography on silica gel with n-hexane and ethyl acetate as the eluent (10:3 v/v,) to afford as an orange solid. (435 mg yield 87%). 1H NMR (500 MHz, CDCl3): 8.59 (d, 1H, J = 8.5 Hz), 8.44 (d, 1H, J = 1.13), 8.31 (d, 1H, J = 2.6 Hz), 8.28 (s, 1H), 8.07 (d, 1H, J = 6.9 Hz), 7.85-7.5 (m, 13H),

7.34 (s, 1H), 4.83-4.70 (m, 2H, N-

CH2), 2.35-2.30 (m, 1H, N-CH2-CH), 2.13-2.11 (m, 4H), 1.47-1.11 (m, 12H), 0.930.84 (m, 6H), 0.73-0.68 (m, 10H).

13

C NMR (500 MHz, CDCl3): 164.3, 154.7, 151.6,

151.5, 145.9, 142.4, 141.4, 140.4, 138.8, 138.1, 134.7, 134.2, 133.3, 132.5, 130.9, 129.2, 126.0, 125.3, 124.9, 124.3, 123.9, 123.7, 122.9, 122.1, 121.8, 121.2, 120.5, 25



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Page 26 of 44

120.1, 119.9, 119.2, 118.8, 117.4, 116.3, 109.9, 98.0, 55.0, 49.8, 38.8, 39.6, 39.3, 30.2, 28.1, 25.6, 23.4,22.6, 13.7, 13.5, 10.5. HRMS calcd for C53H54N2O2S [M-H]+ m/z 782.3906, found 781.3834, m.pt:312.5°C. IR (KBr, cm-1):

2219, 1678, and

1577.

(Z)-2-Cyano-3-(5-(9, 9-dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)9H-fluoren-2-yl) furan-2-yl) acrylic acid (BFF) A procedure similar to that for the dye 10, but with compound 7 (500 mg, 0.71 mmol) instead of compound 6, was performed to give 11 as a red solid (452 mg, yield 83%). 1H NMR (500 MHz, CDCl3): 8.54 (d, 1H, J = 8.9 Hz), 8.42 (s, 1H), 8.27 (d, 1H, J = 7.9), 8.04 (d, 1H, J = 7.9 Hz), 7.98 (s, 1H), 7.89 (s, 1H), 7.85 (d, 1H, J = 6.9 Hz), 7.81-7.78 (m, 3H), 7.75-7.69 (m, 2H),

7.34 (s, 1H), 7.67 (d, 1H, J =

8.9 Hz), 7.59-7.57 (m, 2H), 7.54 (t, 1H, J = 14.9 Hz), 7.38 (s, 1H), 6.94 (d, 1H, J = 3.9 Hz), 4.67-4.63 (m, 2H, N-CH2), 2.33-2.28 (m, 1H, N-CH2-CH), 2.14-2.11 (m, 4H), 1.48-1.34 (m, 8H), 1.25-1.11 (m, 4H), 0.89-0.82 (m, 6H), 0.71-0.68 (m, 10H).

13

C

NMR (500 MHz, CDCl3): 164.3, 154.7, 151.6, 151.5, 145.9,142.4, 141.4, 140.4, 138.8, 138.2, 134.7, 134.2, 133.3, 132.5, 130.9, 129.2, 126.0, 125.3, 124.9, 124.3, 123.9, 123.7, 122.9, 122.1, 121.8, 121.1, 120.5, 120.1, 119.9, 119.2, 118.8, 117.4, 116.3, 109.9, 98.1, 55.0, 49.8, 39.9, 39.6, 39.3, 30.2, 28.1, 25.6, 23.4, 22.6, 13.7, 13.5, 10.5. HRMS calcd for C53H54N2O3 [M-H]+ m/z 766.4134, found 765.4061; m.pt:309.1°C; IR (KBr, cm-1): 2217, 1682, 1592.

(Z)-2-Cyano-3-(4-(9, 9-dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)9H-fluoren-2-yl) phenyl) acrylic acid (BFB)

26


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A procedure similar to that for the dye 10, but with compound 8 (500 mg, 0.70 mmol) instead of compound 6, was performed to give 12 as a yellow solid (408 mg, yield 75%). 1H NMR (500 MHz, CDCl3): 8.59 (d, 1H, J = 8.3 Hz), 8.44 (d, 1H, J = 1.5 Hz), 8.30 (t, 2H, J = 8.3 Hz), 8.13-8.04 (m, 3H), 7.85-7.72 (m, 5H), 7.69-7.64 (m, 2H), 7.62-7.52 (m, 4H), 4.82-4.66 (m, 2H, N-CH2), 2.35-2.30 (m, 1H, N-CH2-CH), 2.15-2.1 (m, 4H), 1.46-1.11 (m, 8H), 0.93-0.90 (m, 4H), ), 0.88-0.83 (m, 6H), ), 0.740.69 (m, 10H).

13

C NMR (500 MHz, CDCl3): 163.6, 153.2, 151.2, 145.4, 140.9,

140.8, 140.0, 138.1, 137.2, 134.4, 132.9, 132.3, 130.9, 129.6, 128.9, 126.9, 125.6, 125.5, 124.5, 123.9, 123.6, 122.6, 121.8, 121.5, 120.9, 120.7, 120.1, 119.6, 119.4, 118.9, 118.4, 117.1, 115.5, 109.5, 102.4, 54.6, 49.5, 29.8, 29.9, 27.8, 25.4, 23.2, 22.3, 13.3, 13.1, 10.1. HRMS calcd for C55H56N2O2 [M-H]+ m/z 776.4342, found 775.4252, m.pt:321°C; IR (KBr, cm-1): 2225, 1696, 1587.

(Z)-2-Cyano-3-(4-(5-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl) thiophen-2-yl) phenyl) acrylic acid (BTB) A procedure similar to that for the dye 10, but with compound 9 (500 mg, 0.70 mmol) instead of compound 6, was performed to give 13 as a red shiny solid (484 mg, yield 86%). 1H NMR (500 MHz, CDCl3): 8.56 (d, 1H, J =8.5 Hz), 8.44 (s, 1H), 8.28 (t, 2H, J = 15.8 Hz), 8.07 (d, 2H, J = 8.3 Hz), 7.92 (s, 1H), 7.83-7.50 (m, 7H), 7.49-7.16 (m, 2H), 4.75-4.73 (m, 2H, N-CH2), 1.43-1.21 (m, 10H), 0.89-0.80 (m, 6H).

13

C NMR

(500 MHz, CDCl3): 164.3, 153.3, 147.3, 140.8, 140.1, 138.7, 135.0, 133.6, 131.6, 129.9, 129.5, 126.1, 125.3, 125.3, 124.7, 123.3, 122.9, 122.7, 122.2, 122.0, 120.9, 119.2, 118.9, 116.5, 116.3, 110.4, 110.3, 49.9, 30.5, 28.3, 23.7, 22.9, 13.9, 10.7. HRMS calcd for C38H34N2O2S [M-H]+ m/z 582.2341, found 775.4252, m.pt:295.3°C; IR (KBr, cm-1): 2234, 1687, 1582. 27



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Page 28 of 44

ASSOCIATED CONTENT Supporting Information Electron density distribution of the isolated dyes, geometrical coordinates, absorption and fluorescence spectra in different solvents, comparison of TDDFT simulated spectra of isolated dyes and dyes on (TiO2)16 and spectral characterization of all the compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Phone: +91-40-27191429), [email protected] (Phone: +91-40-27193933). Acknowledgements MP thanks UGC for the fellowship. We acknowledge funding from NWP-0054 project. Note The authors declare no competing financial interests. References (1) Nozik, A. J.; Miller, J., Introduction to Solar Photon Conversion. Chem. Rev. 2010, 110, 6443-6445. (2) O'Regan, B.; Gratzel, M., A Low-Cost, High-Efficiency Solar Cell Based on DyeSensitized Colloidal TiO2 Films. Nature. 1991, 353, 737-740. (3) Brédas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V., Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691-1699.

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(25) Chen, J. P.; Tanabe, H.; Li, X.-C.; Thoms, T.; Okamura, Y.; Ueno, K., Novel Organic Hole Transport Material with Very High Tg for Light-Emitting Diodes. Synth. Met. 2003, 132, 173-176. (26) Baheti, A.; Singh, P.; Lee, C.-P.; Thomas, K. R. J.; Ho, K.-C., 2,7Diaminofluorene-Based Organic Dyes for Dye-Sensitized Solar Cells: Effect of Auxiliary Donor on Optical and Electrochemical Properties. J. Org. Chem. 2011, 76, 4910-4920. (27) Baheti, A.; Tyagi, P.; Thomas, K. R. J.; Hsu, Y.-C.; Lin, J. T. s., Simple Triarylamine-Based Dyes Containing Fluorene and Biphenyl Linkers for Efficient Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2009, 113, 8541-8547. (28) Frisch, M. J.; Trucks, G. W. ; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A. ; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al., Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT 2010. (29) Becke, A. D., Density‐Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (30) Becke, A. D., Density‐Functional Thermochemistry. IV. A New Dynamical Correlation Functional and Implications for Exact‐Exchange Mixing. J. Chem. Phys. 1996, 104, 1040-1046. (31) Paramasivam, M.; Gupta, A.; Raynor, A. M.; Bhosale, S. V.; Bhanuprakash, K.; Jayathirtha Rao, V., Small Band Gap D-π-A-π-D Benzothiadiazole Derivatives with Low-Lying Homo Levels as Potential Donors for Applications in Organic Photovoltaics: A Combined Experimental and Theoretical Investigation. RSC Adv. 2014, 4, 35318-35331. (32) Pastore, M.; Mosconi, E.; De Angelis, F.; Grätzel, M., A Computational Investigation of Organic Dyes for Dye-Sensitized Solar Cells: Benchmark, Strategies, and Open Issues. J. Phys. Chem. C. 2010, 114, 7205-7212. (33) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J., Ab Initio Study of Solvated Molecules: A New Implementation of the Polarizable Continuum Model. Chem. Phys. Lett. 1996, 255, 327-335. (34) Bryantsev, V. S.; Diallo, M. S.; van Duin, A. C. T.; Goddard, W. A., Evaluation of B3LYP, X3LYP, and M06-Class Density Functionals for Predicting the Binding Energies of Neutral, Protonated, and Deprotonated Water Clusters. J. Chem. Theory Comput. 2009, 5, 1016-1026. 31



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(45) Akhtaruzzaman, M.; Seya, Y.; Asao, N.; Islam, A.; Kwon, E.; El-Shafei, A.; Han, L.; Yamamoto, Y., Donor-Acceptor Dyes Incorporating a Stable Dibenzosilole πConjugated Spacer for Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 10771-10778. (46) Sitha, S.; Srinivas, K.; Raghunath, P.; Bhanuprakash, K.; Jayathirtha Rao, V., Linear Allenic Linkage for Nonlinear Optics: A Computational Study of the Role of Mutually

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33



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Page 34 of 44

Graphical Abstract

Changing of fluorene unit to thiophene as π-spacer leads to stronger binding of the dye and broadening of LUMO on TiO2 which doubles the short circuit current density (Jsc) and increases the power conversion efficiency (η) by 70%.

34


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Figures and Tables

Scheme 1 Chemical structures of the dyes

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Br2, FeCl3

n-BuBr

Br

Br

Br

CHCl3

Page 36 of 44

Br

TBAI , 50% NaOH

80 C O

H N

AcOH(Cat)

Br

Br

NHNH2.HCl EtOH, 80 C

H N

N

Br

4

O B O

n-BuLi, THF

-78 C 2h,

Br

then RT

2

80 C

PhCH3/ 2M K2CO3

N

Br

Br

1

Pd(PPh3)4 ,80 C

O B O O

Br

TBAI , 50% NaOH

m-Xylene, 140 C

Br

N

Br

Chloranil

3

S

Br

N

Br

S

Pd(PPh3)4 , 80 C

5

PhCH3/ 2M K2CO3

N

Br

(OH)2B

Ar CHO

COOH

NC

Ar CHO

N

Ar

N

NH4OAc, AcOH

PdCl2(dppf), K2CO3, PhCH3 / MeOH, 80 C

COOH CN

120 C,

6-8

10 - 12

Ar : S

O

6, 10 - BFT

(HO)2B

N

S

NC

CHO

N

Br

S

PdCl2(dppf), K2CO3, PhCH3 / MeOH, 80 C

7, 11- BFF

8, 12 - BFB

COOH

N

CHO

COOH

S

NH4OAc, AcOH

CN

120 C

9

13

BTB

Scheme 2 Outline of synthetic scheme for the dyes

36


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Figure 1 Geometry optimized structures and the dihedral angles between neighboring segments of the isolated dyes (dihedral angle values of dyes on (TiO2)16 are given in parenthesis).

37



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Page 38 of 44

Figure 2 Schematic representation of electronic distributions observed in frontier orbitals of the dyes using DFT/B3LYP/6311G (d, p) with band gap values. 38


Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Figure 3 Percentage contributions of the orbital density of the individual groups in HOMO-1, HOMO and LUMO of the dyes.

39



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Page 40 of 44

Figure 4 Absorption spectra of the dyes recorded in tetrahydrofuran (1 x 10-5 M) and on nanocrystalline TiO2 film.

Figure 5 Cyclic voltammograms of the dyes in DCM.


40

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Figure 6 TGA thermograms measured at a heating rate of 10 °C/min under N2 atmosphere.

Figure 7 Action spectra of incident photon-to-current conversion efficiency (IPCE) and photocurrent–voltage characteristic curves of the dyes under AM (1.5) irradiation.


41


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Page 42 of 44

Figure 8 Comparison of calculated TDOS of isolated dyes and bare (TiO2)16 with TDOS and PDOS of dyes on (TiO2)16 system. Inset shows the expansion of LUMO of the dyes described from PDOS contributions for clarity.


42

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Table 1 Comparison of absorption properties of the dyes with computed excitation energy, oscillator strength, composition in terms of molecular orbitals, and dipole moments.

λmax a Dye

a b

λmax b

ε -1

-1

(nm)

(M cm )

BFT

433 319

65,893 51,329

BFF

436 325

60,782 55,046

BFB

389 315

BTB

427 321

(nm)

c

M06-2X (THF) λ max (nm)

f

469

410 (3.02eV)

1.96

470

415 (2.99eV)

1.94

61,252 58,789

434

368 (3.37eV)

1.99

50,623 38,175

483

408 (3.04eV)

1.69

Composition

d

μ (Debye)

HOMO->LUMO (28%) HOMO-1->LUMO (37%) HOMO-2->LUMO (25%)

9.0


9.2


7.1


9.8

-5

absorption spectra were measured in THF in the concentration of 1x10 M at ambient temperature. absorption maxima on TiO2 film.

c

obtained with 6-311G(d, p) basis set.

d

dipole moment of the isolated dyes obtained using B3LYP functional.


43


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Page 44 of 44

Table 2 Electrochemical data estimated from Experiment.

Eox

a

Ered

a

Dye

BFT BFF BFB BTB a

HOMOb

E0-0

c

E ox

*

d

LUMOe

(V)

(V)

(eV)

(eV)

(V)

(eV)

1.03

-0.84

-5.43

2.50

-1.47

-2.93

1.08

-0.82

-5.48

2.53

-1.45

-2.95

1.02

-0.78

-5.42

2.80

-1.78

-2.62

1.03

-0.69

-5.43

2.49

-1.46

-2.94

measured in CH2Cl2 with 0.1 M Tetrabutylammonium perchlorate (TBAPC) as supporting electrolyte with a scan -1

rate of 50 mVs . b

deduced from the ground state oxidation potential using the formula HOMO = -(4.4 + Eox)

c

estimated from the onset absorption spectra measured in THF.

d

E ox = Eox- E0-0.

e

estimated by subtracting E0-0 from the HOMO.

*

Table 3 Photovoltaic performance, thermal properties and adsorption energy of the dyes. Dye

Jsc

Voc -2

FF

η (%)

Td

a

°

(mA.cm )

(mV)

BFT

8.11

747

0.760

BFF

7.89

767

BFB

5.67

BTB

10.18

Eads

b

( C)

(kcal/mol)

4.61

313

11.5

0.765

4.63

309

11.9

760

0.783

3.38

321

10.9

733

0.769

5.74

295

14.7

a

Td - decomposition temperature (corresponding to 5% weight loss).

b

Eads- Adsorption energy of the dyes on TiO2 calculated from the formula ( = + − )


44

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