Synthesis, optical, electrochemical properties and photovoltaic performance of novel panchromatic meso-thiophene BODIPY dyes with a variety of electron-donating groups at the 3,5-positions

Abstract

A series of novel BODIPY derivatives (BDP1–5) containing a thiophene motif at the meso-position and various electron-donating units at the 3,5-positions were designed and synthesized. With the extended D–A–D conjugated skeleton, these dyes exhibit effective panchromatic absorption with maximum extinction coefficients over 1.4 × 10⁵ M⁻¹ cm⁻¹ in solution state and dramatically higher values in the film state. Suitable HOMO/LUMO energy levels, around −5.4 eV and −3.8 eV, respectively, vs. PCBM combined with the good solubility and perfect film-forming ability make them attractive for BHJ solar cell applications. The photovoltaic device based on BDP2 and PC₆₁BM exhibits a potential power conversion efficiency of 2.12%, a moderate open-circuit voltage of 0.73 V and a relatively high short-circuit current density of 7.64 mA m⁻².

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Introduction

Absorption of incident light is the preliminary process in the function of an organic photovoltaic (OPV) device.^1,2 It can be said that the material's absorption ability significantly affects the device performance. A qualitative analysis of the solar irradiance spectrum reveals that more than 60% of the solar photon flux density lies between 300–900 nm of the visible spectrum.² Arguably, efficient light absorbers must therefore have absorption spectra that overlap with the solar irradiance spectrum in the visible region to harness most of the incident sunlight. An attractive strategy to obtain high-efficient OPV devices is designing panchromatic materials with reduced optical band gaps to encompass more of the visible and near IR region of the sun-light spectrum.^3–9 Another key factor for OPV donor materials is that they should possess commensurately aligned frontier molecular orbitals with acceptor materials to enable efficient charge separation at the donor–acceptor interface and provide large open circuit voltage.^1,4,10,11 However, the challenge is the fact that tuning of the optical band gap and the frontier molecular orbitals is not mutually exclusive. Consequently, to address this dichotomy is to construct materials based on building blocks that inherently possess deep HOMO energy levels such that link with other functional groups does not alarmingly raise the effective HOMO energy levels of the ensuing molecule.^12,13

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes attract considerable interest owing to a unique combination of facile synthesis, stability, high absorption coefficient, and high photoluminescence efficiency,^14–18 along with the spectroscopic and photophysical properties of BODIPYs can be fine-tuned by attachment of ancillary residues at the appropriate positions of the BODIPY core.^19,20 The BODIPY derivatives commonly possess solution-state absorption spectra with absorption maxima exceeding 500 nm, even up to 1000 nm, by a suitable modification.^20–23 In addition, BODIPY inherently bear deep HOMO energy levels (<5.2 eV).¹⁹ Therefore, the relatively high extinction coefficient of the BODIPY core combined with deep HOMO energy levels and propensity to π-stack in solid-state, make BODIPY-based materials attractive candidates for organic electronic applications such as light-emitting diodes and organic photovoltaic.^18,19,24,25

Inspired by the strategy for constructing low-band gap materials with conjugated donor–acceptor (D–A) system,¹ herein, the highly flexible BODIPY platform was selected for the development of D–A–D type small molecular-based panchromatic light-harvester. In this paper, we designed five novel dyes incorporating uncommon reported meso-thiophene BODIPY^26,27 as acceptor and different donors, including thiophene, bithiophene, carbazole, fluorine and triphenylamine, in the main backbone. We expect that these dyes would significant broadening of absorption and improving light-harvesting ability in the whole sunlight region. The photophysical properties and electrochemical behaviors of these dyes were studied systematically to reveal the relationships between the structures and the photoelectric characteristics. DFT theoretical calculations were performed to further understanding their structural and electronic features. Moreover, the photovoltaic performance was investigated to insight the potential applications of this kind of materials.

Results and discussion

Synthesis and characterization

For the construction of these novel D–A–D type panchromatic BODIPY dyes, we utilized the Suzuki–Miyaura and Stille cross-coupling reaction to connect two electron-donor moieties covalently to the 3,5-positions of meso-thiophene BODIPY nucleus, where a substituted dibromated BODIPY 4 should be prerequisite for the synthesis (Scheme 1). Thus, our synthetic approach started with the preparation of 4 from commercially available 5-octylthiophene-2-carbaldehyde through a four-step sequence involving condensation–bisbromination–oxidation–chelation reaction. The dipyrromethane 2 was prepared in a mild way by InCl₃ catalysis aldehyde 1 condensation with pyrrole at room temperature, which has been reported by our group.²⁸ Subsequently, bisbromination of 2 with NBS under −78 °C afforded compound 3 in 65% yield. Oxidation of 3 with 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ) and then treated with BF₃Et₂O to form the important intermediate BODIPY 4. This reaction sequence turns out to be mild and efficient, and the key intermediate 4 was obtained with high quality. On the other hand, the important electron-donor segments were prepared according to the literature via the brominate-Miyaura coupling synthetic route. The corresponding borates such as compounds 7 and 10 were readily afforded though this strategy. Finally, the conversion from intermediate 4 to the desired BODIPY dyes (BDP1–5) was executed by the coupling 4 with the corresponding donor moieties via Suzuki and Stille reaction. To be noted that all these intermediates and the final BODIPY conjugates BDP1–5 were readily available, and the success in the synthesis of these products was characterized by a range of spectroscopies including ¹H NMR, ¹³C NMR and MALDI-TOF-MS. Moreover, the target BODIPY dyes BDP1–5 were further accurately confirmed by HRMS.

Scheme 1 Synthetic routes of novel BODIPY dyes (BDP1–5).

Electronic absorption

UV-Vis absorption and normalized absorption spectra of these BODIPY conjugates BDP1–5 in dichloromethane solution (2 × 10⁻⁵ mol L⁻¹) are shown in Fig. 1 and 1 in ESI,† and the corresponding optical data are summarized in Table 1. It is a known fact that most of BODIPY's modification and functionalization have been undertaken at 1–3 and 5–7 positions, while the meso-position (8-position) has not been exploited fully in the π-extension of BODIPYs. The ever reported π-extended BODIPY chromophores have intrinsically two extinct intensive absorption bands,¹⁸ but there is a lack of significant absorption in the spectral region between these two features, which is harmful for the light-harvesting materials. Interestingly, this disadvantage can be compensated by fine-tune the meso-motifs of BODIPY. As shown in Fig. 1 and Scheme 1, meso-thiophene substituted BODIPY (4 and reference 4′) show two absorption bands. The more energetic one range of 400–500 nm can efficiently compensate the weak absorption range. Benefit for this feature, all these covalently assembled light harvesters (BDP1–5) showed panchromatic absorption with three major absorption bands (Fig. 1). The first one located at 250–400 nm due to the π–π*, S₀–S₁ transition derived from donor units, overlap with the S₀–S₂ of BODIPY.²⁹ The second band range from 400 to 550 nm correspond to the π–π*, S₀–S₁ transition originated from meso-thiophene. The third band locate at long wavelength regions of 520–780 nm, peak at 628, 696, 623, 601 and 670 nm, respectively, is attributed to the intensive intramolecular charge transfer (ICT) interactions between two donor moieties and BODIPY core overlap with the ICT interaction originate from meso-thiophene to BODIPY core. Compared to the reference 4′, all these dyes with different substituents significantly broaden the absorption spectra and shift the absorption onsets to longer wavelength (Fig. 1). As we can see, BDP2 and BDP5 showed more bathochromic-shift than other ones. This is likely due to the dithiophene and triphenylamine moieties possess stronger electron donating ability and their more excellent molecular configuration, which facilitate π-conjugation electronic transfer between the D–A units. On the other hand, all these dyes showed high extinction coefficient range from 4.2 × 10⁴ to 1.4 × 10⁵ cm⁻¹ M⁻¹ in solution-state, which ensured their excellent light harvesting ability.

Fig. 1 Normalized absorption spectra of the key intermediate 4 (above) and absorption spectra of BODIPY dyes BDP1–5 (below).

Table 1 The summary absorption data of BDP1–5

Entry	Solution^a				Film^b
	λmax (nm)	lg ε	λonset (nm)	Eg^c (eV)	λmax (nm)	λonset (nm)	Eg^c (eV)
a Recorded in CH2Cl2.b Deposited onto quartz substrate by the spin-coating technique from CH2Cl2 solution.c Eg = 1240/λonset.
BDP1	326, 471, 628	4.09, 4.11, 4.63	750	1.65	328, 474, 654	850	1.46
BDP2	392, 507, 696	4.61, 4.52, 4.96	780	1.59	424, 532, 752	950	1.31
BDP3	273, 478, 623	5.07, 4.50, 4.70	710	1.71	273, 488, 653	802	1.55
BDP4	305, 459, 601	4.48, 4.40, 4.62	685	1.81	307, 466, 625	870	1.43
BDP5	308, 526, 670	5.16, 4.72, 4.83	782	1.59	305, 533, 681	800	1.55

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In comparison with the absorption spectrum in CH₂Cl₂ solution, broader absorption bands with obvious bathochromic shifts were observed in the solid state (Fig. 2). This could be attributed to the molecular stacking in solid state, which suggests an increased conjugation length in the solid state due to the π–π stacking of the nearly planar conformations of BODIPY. Interestingly, BDP1 and BDP2 not only show broader absorption bands and bathochromic shifts but also exhibit a remarkable increase of the extinction coefficient (Fig. 2 and 2 in ESI†). It could be explain as BDP1-2 attachment with thiophene and dithiophene have the ability to form short S⋯S contact, which result in better intermolecular interactions and further increase conjugation length in solid.

Fig. 2 Normalized absorption spectra of dyes BDP1–5 in film (above) and the comparison of the absorption spectrum of BDP2 in solution state and in film state (below).

It should be noted that all these dyes displayed perfect solubility and film-forming ability. The desirable combination of attractive photophysical characteristics, good solubility, favorable film-forming ability and functional group tolerance indicate that these dyes are promising applying in solution-processed organic electron devices such as light-harvesting antennae, BHJ photovoltaic solar cells, and so on.

Photoluminescence

The photoluminescence (PL) of complexes BDP1–5 in different solvents were investigated at room temperature. Rhodamine B (=0.71) was used as standard in methanol to estimate the fluorescence quantum yield.³⁰ Their normalized emission spectra in CH₂Cl₂ solution at a concentration of 1 × 10⁻⁷ mol L⁻¹ are illustrated in Fig. 3. The emission band maxima and quantum yields are listed in Table 2. Excitations of these compounds at their two absorption bands maximum corresponding to donor units and BODIPY units, resulted in the same near-infrared and deep red emissive BODIPY dyes with emission maxima of 644, 716, 686, 648 and 801 nm, respectively. This can be explained on the basis of intramolecular energy transfer from the BODIPY π–π* excited state to the lower lying singlet excited state of the donor moieties and/or to the existence of new non-radiative pathways from the BODIPY π–π* excited state to the ground state. These BODIPY dyes with donor units at 3,5-positions (BDP1–5) present a red-shift emission compared to the reference 4′, which indicate that the substituents significantly affect the emission and an effective intramolecular energy transfer for the excitons in the D–A units is occurred. Moreover, remarkable Stokes shifts of these dyes range from 480 to 2686 cm⁻¹ were observed. Taking these features into account, we can assign the observed emission to the ICT emission. Due to the ICT process, BDP3 and BDP5 exhibit more significant bathochromic shift emissions than other BDPs, caused by the carbazole and triphenylamine substituents with stronger electron-donating ability. On the other hand, BDP1–5 gave low fluorescent quantum yield. These could be attributed to the increased internal conversion according to the energy gap law that states the non-radiative deactivation probability of S₀–S₁ increases as the energy gap of S₀–S₁ decreases in a highly extended conjugating system. Photoluminescence (PL) quenching provides the direct evidence for exciton dissociation, and thus efficient PL quenching is necessary to obtain efficient organic solar cells.

Fig. 3 Normalized fluorescence spectra of BDP1–5 recorded in CH₂Cl₂ solution (c = 1 × 10⁻⁷ mol L⁻¹).

Table 2 Summary of photoluminescence data of BDP1–5

Entry		λmax (nm)			Stocks shift (cm^−1)	ϕPL
		CH2Cl2	Et2OAc	CH3COH3	CH2Cl2
BDP1		648	645	644	491	0.06
BDP2		720	714	716	480	0.04
BDP3		715	671	686	2065	0.05
BDP4		646	642	648	1160	0.12
BDP5		817	770	801	2686	0.04

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Solvent dependence study was performed in solvents with various polarities, such as dichloromethane, acetone and ethyl acetate (Table 2 and Fig. 3 in ESI†). The minor solvatochromic effect observed for BDP1-2 and BDP4 indicate that these emissions were not caused by solvent effect and/or aggregation effect. The results further confirmed the ICT emission features of these dyes. To be noted that the emission bands of the dyes BDP3 and BDP5 bathochromically shift to longer wavelengths in more polar solvents.

The drastic positive solvatochromic effect observed should be attributed to the N contained electron-donating groups (carbazole and triphenylamine), which enhance electron transfer from the symmetrical peripheral chromophores to the BODIPY motif.

Electrochemical properties

In order to investigate the electrochemical behaviors of BDP1–5, the electronic states of these dyes were studied through cyclic voltammetry in CH₂Cl₂ solution at a scan rate of 100 mV s⁻¹ using n-Bu₄NPF₆ as supporting electrolyte. Potentials were standardized with ferrocene/ferrocenium (Fc/Fc⁺) couple as internal reference vs. Ag/AgCl. The cyclic voltammograms of BDP1–5 are illustrated in Fig. 4, and their electrochemical properties are summarized in Table 3. As shown in Fig. 4, dyes BDP1, BDP2 and BDP4 present reversible onset oxidation potential around 1.1 V, while BDP3 and BDP5 exhibit reversible onset oxidation potential around 0.9 V. It indicated that the aromatic substituents have obvious effects on the E^ox_onset. The reduction potentials (E_red) calculated from E^ox_onset − E_g were approximately matched with the experimental values obtained from reduction waves. The corresponding highest occupied molecular orbital (HOMO) energy level of BDP1–5 were calculated (5.36, 5.40, 5.15, 5.41 and 5.17 eV, respectively) from the equation E_HOMO = [(−E^ox_onset − 0.52)4.8] eV, where 0.52 V is the value for ferrocene versus Ag/AgCl and 4.8 eV is the energy level of Fc/Fc⁺ relative to the vacuum energy level. These dyes were actually located at very low energy level, which is attractive property in bulk-heterojunction (BHJ) solar cell device. The important factor for this result was that the strong electron donating group would cause an intensive ICT process. By neglecting any entropy change during light absorption, the reduction potential versus NHE (E_red), which corresponds to the lowest unoccupied molecular orbital (LUMO versus NHE), can be obtained from E^ox_onset and E_g, where E_g is estimated from the absorption thresholds from absorption spectra in solution. The results were shown in Table 3. It should be noted that HOMO/LUMO levels of these dyes are match with the p-type semiconductor such as PCBM well, which suggest that they are capable of organic BHJ solar cells applications.

Fig. 4 Cyclic voltammograms of BDP1-2 (above) and BDP3–5 (below) in CH₂Cl₂, containing 0.1 M n-Bu₄NPF₆ as the supporting electrolyte recorded at a scan speed of 100 mV s⁻¹, Fc/Fc⁺ refers to the ferrocene/ferricinium couple used as internal reference.

Table 3 Solution electrochemical properties of BDP1–5

Entry	Eg^a (eV)	E^oxonset^b (eV)	E^oxp^c (v)	Ered^d (v)	HOMO (eV)	LUMO^e (eV)
a Eg, estimated from the absorption spectra of dyes absorbed in CH2Cl2 solution, Eg = 1240/λonset.b E^oxonset, onset oxidation potential.c E^oxp, oxidation peak potential.d Ered, the reduction potential, was calculated from E^oxonset − Eg.e EHOMO = [(−E^oxonset − 0.52)4.8] eV, ELUMO = EHOMO + Eg eV.
BDP1	1.65	1.10	1.18	−0.47	−5.36	−3.81
BDP2	1.59	1.14	1.23	−0.45	−5.40	−3.81
BDP3	1.69	0.89	0.98	−0.80	−5.15	−3.46
BDP4	1.81	1.15	1.24	−0.66	−5.41	−3.60
BDP5	1.59	0.91	0.99	−0.68	−5.17	−3.58

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Theoretical calculations

For a better understanding of the structural and electronic features of these novel dyes, the DFT theoretical calculations were further performed, and the optimized structures and the electronic distribution in HOMO and LUMO levels are presented in Fig. 5 and 6. All calculations were carried out with the Gaussian 09 program suite by using the B3LYP method and 6-31 G* basis set. From the optimized ground-state geometries of the dyes in Fig. 5, we can see that the dihedral angle between the donor units and the BODIPY framework are computed to be 14.1°, 15.7°, 52.5°, 40.3° and 30.2°, respectively, for BDP1–5. This calculated results indicated that the BDP1 and BDP2 produce a higher degree of molecular coplanarity than BDP3 and BDP4, which is in accordance with the absorption spectra observed in solution. While the noticeable bathochromic shift in BDP5 could be attributed to a π–π* mixed twisted intramolecular charge transfer (TICT) transition originated from the appended strong electron-donating triphenylamine group to the central BODIPY component.³¹ The dihedral angle between meso-thiophene and BODIPY core are around 46°, which amenable to further extending π-conjugation as observed in Fig. 1.

Fig. 5 Optimized structures of dyes BDP1–5.

Fig. 6 Electron distribution in HOMO and LUMO levels of dyes BDP1–5.

The electron distributions of BDP1–5 indicated that π-electrons in the HOMO are delocalized over the entire molecule backbone, offering effective orbital interactions among the stacked π-systems, while their LUMOs are mainly localized on the BODIPY moieties. Therefore, the HOMO–LUMO transition that is the dominant contributing configuration to the S₁ state in BDP1–5 should be ascribed to a mixture of π–π* and ICT characters, which is consistent with their absorption assignments. It should be noted that their LUMOs were also locate at the meso-thiophene motif, which could not been seen in commonly reported meso-phenyl BODIPY derivatives,^32,33 indicate that effective π-conjugation was formed between the BODIPY framework and meso-thiophene unit. The extended π-conjugation is helpful for broadening absorption response ranges, and this have be attested by the UV-Vis absorption. On the other hand, the calculated results clearly demonstrate that strong electron-donating substituent with more coplanar geometry, such as dithiophene in BDP2, increases the energy level of HOMO more than that of LUMO, thus the HOMO–LUMO gap decreases, causing a bathochromic shift of the π–π*/ICT band. This trend follows that observed from the UV-Vis absorption measurement. Fig. 6 presented the HOMO–LUMO energy differences (energy band gaps, calculated E^cal_g) of BDP1–5. We can see that the trend of the predicted E^cal_g values (1.18–1.52 eV) is in consistent with the estimated E^opt_g (calculated from the ground-state absorption, 1.59–1.81 eV) in Table 1.

Photovoltaic performance

In recent years, small molecule-based organic OPV attract desirable attention due to their special advantages.^34–41 BODIPY derivatives are promising compounds to be used in the active layer of OPV devices.^18,25 However, the examples were rather limited. It is imperative to develop novel structures based on BODIPY for OPV investigations. The above discussed results indicate that the photophysical and electrochemical properties of dyes BDP1–5 were excellent and their performance as electron donor in BHJ solar cells would be feasible.

We expect that they would become valuable candidates to the application of BHJ solar cells as a series of new organic small molecule-based dyes. Herein, BDP2 was chosen to construct BHJ solar cell device for insight the potential applications of this kind of materials.

The BHJ solar cell device was fabricated using BDP2 as the electron donor material and PC₆₁BM as the electron acceptor material through the conventional spin-coating process from CHCl₃ solution with a concentration of 15 mg mL⁻¹ at room temperature. The cell structure was ITO/PEDOT : PSS (30 nm)/donor : acceptor (1 : 3, 100 nm)/LiF (0.7 nm)/Al (100 nm). The effective device area was 1 × 10⁻² cm², as defined by a shadow mask. Fig. 7 shows typical current density–voltage (J–V) characteristic of optimized device at a ratio of 1 : 3 between BDP2 and PC₆₁BM under 1000 W m⁻² air mass 1.5 global (AM 1.5 G) illumination. Device parameters such as short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), fill factor (FF), and solar energy-to-electricity conversion yields (PCE) were deduced from the J–V characteristics and summarized in Table 4. A PCE of 2.12% with a V_oc of 0.73 V, a J_sc of 7.64 mA cm², and a FF of 38% was achieved in the BDP2: PC₆₁BM-based device. To be noted that the PCE of BHJ solar cell was obtained without any thermal annealing or high boiling temperature additive in the solutions. As we expected, the device based on BDP2 afford relatively higher J_sc value, higher than most of analogous device based on other BODIPY derivatives,^19,25 which is mainly attributed to the panchromatic effective absorption. However, FF value of this device is relatively low, which suggest that the charge transfer within the cell was hindered once the charges in the active layer are formed.^4,42 To further get some information, the morphology of blend film of BDP2/PC₆₁BM was tested using atomic force microscopy (AFM), and is shown in Fig. 8. The film surface was relative flat with a root-mean-square (RMS) roughness of 0.989 nm, and relative small phase domains were observed. This indicated that BDP2 have a good miscibility with PC₆₁BM, while fail to form a well-interpenetrating network with a suitable phase separation scale in device, which lead to the low FF. Further optimization of device structure and fabrication process and determination the photovoltaic performances of other dyes (BDP1 and BDP3–5) were undergoing.

Fig. 7 Current density–voltage (J–V) characteristics of the device at a 1 : 3 weight ratio between BDP2 and PC₆₁BM.

Table 4 Photovoltaic performance of the BHJ device fabricated with BDP2/PC₆₁BM

Donor	Acceptor	Jsc (mA cm^−2)	Voc (V)	FF	PCE %
BDP2	PC61BM	7.64	0.73	0.38	2.12

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Fig. 8 AFM height image of BDP2/PC₆₁BM blend film.

Experimental

Materials and characterization

Compound 5-octylthiophene-2-carbaldehyde (1), 9-octyl-9H-carbazole (5), 4-(2-methylheptan-2-yl)-N-(4-(2-methylheptan-2-yl)phenyl)-N-phenylaniline (8), tributyl(thiophen-2-yl)stannane (11), [2,2′-bithiophen]-5-yltributylstannane (12), 2-(9,9-dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (13) were synthesized according to the literature procedures.^43–45 THF and toluene were dried and distilled over sodium and benzophenone. Dichloromethane, ethyl acetate, methanol were dried under 4A molecular sieves. Pyrrole was distilled from CaH₂ before use. All other chemicals with reagent-grade were purchased and used without further purification unless otherwise specified. Column chromatography was performed on silica (300–400 mesh). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance spectrometer (600 and 151 MHz, respectively) with tetramethylsilane (TMS) as the internal standard. Matrix-assisted laser desorption/ionization time of flight mass spectra (MALDI-TOF-MS) were recorded on Bruker ultraflex-II spectrometer, matrix-assisted laser desorption ionization mass spectrometry using an α-cyano-4-hydroxy-cinnamic acid (α-CHCA) matrix. High resolution mass spectra (HRMS) were recorded on an Agilent 6530 Accurate Mass Q-TOF LC-MS instrument. Absorption spectra were collected on an SHIMADZU UV-2550 UV-Vis spectrophotometer and emission spectra were recorded by using a Hitachi F-4500 instrument. Excitation and emission spectra were fully corrected by reference to a standard lamp. Cyclic voltammetric (CV) was carried out on a CHI660E electrochemical workstation utilizing the three electrode configuration consisting of a gold electrode (working electrode), platinum wire (auxiliary electrode) and Ag/AgCl electrode (reference electrode). The experiment was performed in dry dichloromethane solution using 0.1 M n-Bu₄NPF₆ as supporting electrolyte. The solution was deoxygenated at room temperature under nitrogen protection.

Synthesis

2,2′-((5-Octylthiophen-2-yl)methylene)bis(1H-pyrrole) (2). 5-octylthiophene-2-carbaldehyde 1 (1.5 g, 6.7 mmol) and pyrrole (35 mL, 530 mmol) were added to a 50 mL single-neck flask containing a magnetic stir bar. After the solution was degassed with a stream of argon for 10 min, InCl₃ (40 mg, 0.20 mmol) was added, and the mixture was stirred under argon at room temperature for 3 h. Then, powdered NaOH (0.2 g, 5.0 mmol) was added and the reaction mixture was stirred for another 30 min. The mixture was filtered and the filtrate was concentrated by the vacuum distillation. Trace of pyrrole were removed by entrainment with hexanes, then the resulting yellow residue was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent afforded 2 as an orange solid (1.2 g, yield 53%). ¹H NMR (600 MHz, CDCl₃) δ 7.74 (s, 2H), 6.54 (s, 1H), 6.53 (d, J = 2.8 Hz, 1H), 6.51 (s, 2H), 6.07 (s, 2H), 5.95 (s, 2H), 5.48 (s, 1H), 1.64–1.56 (m, 2H), 1.34–1.16 (m, 10H), 0.85 (t, J = 6.8 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 145.29, 142.93, 132.38, 125.14, 123.47, 117.52, 108.38, 107.12, 39.34, 32.01, 31.76, 30.36, 29.47, 29.38, 29.35, 22.83, 14.30.

5,5′-((5-Octylthiophen-2-yl)methylene)bis(2-bromo-1H-pyrrole) (3). According to a modified literature, a solution of 2,2′-((5-octylthiophen-2-yl)methylene)bis(1H-pyrrole) 2 (2.0 g, 5.9 mmol) in dry THF (40 mL) was purged with argon, and cooled to −78 °C. Then, a solution of NBS (2.2 g, 12.4 mmol) in dry THF (20 mL) was added drop-wise to the stirred mixture for three portions at this temperature, showing full conversion on TLC. The crude mixture was poured into ethyl acetate (150 mL), washed with water and brine. The organic layer was dried over anhydrous MgSO₄, evaporated to dryness. The crude residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate, 10 : 1) to give the desired compound 3 as brown solid (1.86 g, yield 65%). ¹H NMR (600 MHz, CDCl₃) δ 7.96 (s, 2H), 6.66 (d, J = 3.4 Hz, 1H), 6.61 (d, J = 3.3 Hz, 1H), 6.08 (d, J = 3.1 Hz, 2H), 5.95 (d, J = 3.0 Hz, 2H), 5.51 (s, 1H), 2.74 (t, J = 7.6 Hz, 2H), 1.67–1.59 (m, 2H), 1.36–1.23 (m, 10H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 146.00, 140.97, 132.84, 125.47, 123.58, 110.60, 109.02, 97.49, 39.46, 31.87, 31.56, 30.23, 29.32, 29.23, 29.20, 22.68, 14.14. MALDI-TOF-MS, m/z: calcd for C₂₁H₂₆Br₂N₂S [M]⁺: 498.016, found: 498.199.

3,7-Dibromo-5,5-difluoro-10-(5-octylthiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (4). To a solution of compound 5,5′-((5-octylthiophen-2-yl)methylene)bis(2-bromo-1H-pyrrole) (1.0 g, 2 mmol) in dry CH₂Cl₂ (50 mL) was added TCQ (590 mg, 2.4 mmol). The mixture was stirred at room temperature for 8 h. Then transfer it under nitrogen flow, Et₃N (5.6 mL, 40 mmol) and BF₃Et₂O (5.4 mL, 43 mmol) were added orderly and the reaction was allowed to proceed at room temperature for an additional 3 h. The reaction mixture was washed with aqueous NaOH (0.1 M) and water. The organic layer was dried over anhydrous Na₂SO₄, filtered and concentrated under vacuum. The residue was purified by silica gel column chromatography using petroleum ether/ethyl acetate (8 : 1, v/v) as eluent, afforded the important intermediate 4 as red solid (820 mg, yield 75%). ¹H NMR (600 MHz, CDCl₃) δ 7.34 (d, J = 3.5 Hz, 1H), 7.18 (d, J = 4.1 Hz, 2H), 6.94 (d, J = 3.4 Hz, 1H), 6.56 (d, J = 4.2 Hz, 2H), 2.90 (t, J = 7.6 Hz, 2H), 1.76–1.71 (m, 2H), 1.44–1.24 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 154.09, 136.14, 134.71, 133.57, 131.53, 130.49, 125.91, 122.43, 31.84, 31.49, 30.42, 29.26, 29.18, 29.17, 22.66, 14.12. MALDI-TOF-MS, m/z: calcd for C₂₁H₂₃BBr₂F₂N₂S [M]⁺: 543.999, found: 544.156.

3-Bromo-9-octyl-9H-carbazole (6). Treated 9-octyl-9H-carbazole 5 (2.8 g, 10 mmol) in dry DMF (50 mL) at −20 °C under argon atmosphere with NBS (1.96 g, 11 mmol), and the mixture was stirred at this temperature for 12 h. Then, the reaction mixture was brought to room temperature, and poured into water and extracted with ethyl acetate. The organic layer was dried over anhydrous Na₂SO₄, and evaporated to dryness. The residue was purified by silica gel column chromatography using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent, afforded 6 as white solid (2.54 g, yield 71%). ¹H NMR (600 MHz, CDCl₃), δ: 8.21 (s, 1H), 8.05 (d, 1H), 7.53 (d, 1H), 7.48 (d, J = 7.2 Hz, 1H), 7.40 (d, 1H), 7.27 (t, J = 12.0 Hz, 2H), 4.26 (t, 2H), 1.84 (m, 2H), 1.25 (m, 10H), 0.85 (t, 3H).

9-Octyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (7). To the solution of 3-bromo-9-octyl-9H-carbazole 6 (2.0 g, 5.6 mmol) and 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane) (1.7 g, 6.7 mmol) in DMF (50 mL) was added Pd(dppf)Cl₂ (205 mg, 0.28 mmol) and potassium acetate (1.65 g, 16.8 mmol). The mixture was stirred at 90 °C under argon atmosphere for 8 h. The reaction mixture was cooled to room temperature and poured to water (200 mL), extracted with ethyl acetate, the organic layer was washed with brine for several times, dried over anhydrous Na₂SO₄, and evaporated to dryness. The residue purified by silica gel column chromatography using petroleum ether/ethyl acetate (20 : 1, v/v) as eluent, afforded 7 as white solid (1.7 g, yield 76%). ¹H NMR (600 MHz, CDCl₃), δ: 8.60 (s, 1H), 8.13 (d, J = 7.6 Hz, 1H), 7.92 (d, J = 8.1 Hz, 1H), 7.45 (d, J = 7.3 Hz, 1H), 7.40 (t, J = 9.5 Hz, 2H), 7.23 (d, J = 7.0 Hz, 1H), 4.30 (t, J = 6.9 Hz, 2H), 1.90–1.86 (m, 2H), 1.44 (s, 12H), 1.36–1.28 (m, 10H), 0.85 (t, J = 13.6 Hz, 3H). MALDI-TOF-MS, m/z: calcd for C₂₆H₃₆BNO₂ [M]⁺: 405.300, found 405.333.

4-Bromo-N,N-bis(4-(2-methylheptan-2-yl)phenyl)aniline (9). To the solution of 4-(2-methylheptan-2-yl)-N-(4-(2-methylheptan-2-yl)phenyl)-N-phenylaniline 8 (4.7 g, 10 mmol) in DMF (60 mL) was added NBS (1.96 g, 11 mmol), and the mixture was stirred at room temperature under argon atmosphere for 12 h. After the reaction was completed, to the resulting suspension was added ethyl acetate (200 mL). The mixture washed with water and brine for several times, the organic layer dried over anhydrous Na₂SO₄, and evaporated to dryness. The residue purified by silica gel column chromatography using petroleum ether/ethyl acetate (20 : 1, v/v) as eluent, afforded 9 as white solid (5.1 g, yield 93%). ¹H NMR (600 MHz, CDCl₃) δ 7.31 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.5 Hz, 4H), 6.99 (d, J = 8.6 Hz, 4H), 6.92 (d, J = 8.9 Hz, 2H), 1.74 (s, 4H), 1.43–1.36 (m, 12H), 0.81–0.76 (m, 18H).

4-(2-Methylheptan-2-yl)-N-(4-(2-methylheptan-2-yl)phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) aniline (10). Compound 10 was synthesized according to the same procedure for preparing of compound 7 using 4-bromo-N,N-bis(4-(2-methylheptan-2-yl)phenyl)aniline 9 as reactant. The crude product was purified on silica column using petroleum ether as eluent afforded the pure compound 10 in 68% yield. ¹H NMR (600 MHz, CDCl₃) δ 7.64 (d, J = 8.5 Hz, 2H), 7.25 (d, J = 8.6 Hz, 4H), 7.01 (d, J = 8.6 Hz, 4H), 6.98 (d, J = 8.5 Hz, 2H), 1.71 (s, 4H), 1.42–1.35 (m, 12H), 1.33 (s, 12H), 0.78–0.71 (m, 18H).

5,5-Difluoro-10-(5-octylthiophen-2-yl)-3,7-di(thiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (BDP1). Compound 4 (200 mg, 0.37 mmol) was dissolved in toluene (20 mL), followed by addition of tributyl(thiophen-2-yl) stannane (330 mg, 0.88 mmol), Pd(PPh₃)₄ (11 mg, 0.01 mmol). The resulting mixture was refluxed at 110 °C under argon atmosphere for 24 h. Then the reaction mixture was brought to room temperature, washed with brine, and extracted by ethyl acetate. Organic layers were combined, dried over anhydrous Na₂SO₄, and evaporated to dryness. Purification was performed by column chromatography on silica gel using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent, from which the desired product BDP1 was obtained as purple solid (81 mg, yield 40%). ¹H NMR (600 MHz, CDCl₃) δ 8.19 (d, J = 3.6 Hz, 2H), 7.48 (d, J = 5.0 Hz, 2H), 7.29 (d, J = 3.5 Hz, 1H), 7.22–7.16 (m, 4H), 6.92 (d, J = 3.5 Hz, 1H), 6.84 (d, J = 4.3 Hz, 2H), 2.91 (t, J = 6.0 Hz, 2H), 1.78–1.73 (m, 2H), 1.46–1.25 (m, 10H), 0.92 (t, J = 7.0 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 151.68, 149.81, 136.22, 134.40, 132.36, 132.09, 131.20, 131.14, 131.09, 130.16, 129.03, 125.12, 120.47, 33.66, 31.54, 30.33, 30.01, 29.72, 28.88, 22.58, 14.09. MALDI-TOF-MS, m/z: calcd for C₂₉H₂₉BF₂N₂S₃ [M]⁺: 550.155, found: 550.360. HRMS, m/z: calcd for C₂₉H₂₉BF₂N₂S₃ [M]⁺: 550.1554, found: 550.1566.

3,7-Di([2,2′-bithiophen]-5-yl)-5,5-difluoro-10-(5-octylthio-phen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (BDP2). Compound BDP2 was synthesized according to the same procedure as for prepared BDP1 using [2,2′-bithiophen]-5-yltributylstannane as reactant. The crude product was purified on silica column using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent to give the pure compound BDP2 as purple solid in 52% yield. ¹H NMR (600 MHz, CDCl₃) δ 8.13 (d, J = 4.0 Hz, 2H), 7.31–7.26 (m, 7H), 7.17 (d, J = 4.3 Hz, 2H), 7.06–7.04 (m, 2H), 6.90 (d, J = 3.2 Hz, 1H), 6.83 (d, J = 4.3 Hz, 2H), 2.90 (t = 6.5, 2H), 1.78–1.72 (m, 2H), 1.45–1.24 (m, 10H), 0.98 (t, J = 7.4 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 151.52, 148.89, 140.99, 136.89, 136.72, 136.66, 133.02, 132.44, 132.18, 131.97, 129.91, 128.12, 125.82, 125.53, 125.10, 124.70, 120.41, 33.67, 31.55, 30.34, 30.02, 28.90, 28.29, 22.59, 14.10. MALDI-TOF-MS, m/z: calcd for C₃₇H₃₃BF₂N₂S₅ [M − F]⁺: 695.131, found: 695.312. HRMS, m/z: calcd for C₃₇H₃₃BF₂N₂S₅ [M]⁺: 714.1308, found: 714.1321.

5,5-Difluoro-3,7-bis(9-octyl-9H-carbazol-3-yl)-10-(5-octyl thiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f] [1,3,2]diazaborinin-4-ium-5-uide (BDP3). To the solution of compound 4 (200 mg, 0.37 mmol) and 9-octyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (357 mg, 0.82 mmol) in toluene (20 mL) was added Pd(PPh₃)₄ (11 mg, 0.01 mmol) and sodium carbonate solution (10 mL, 2 M in water), the mixture was stirred at 90 °C under argon atmosphere for 24 h. Then the reaction mixture was brought to room temperature, washed with brine, and extracted by ethyl acetate. Organic layers were combined, dried over anhydrous Na₂SO₄, and evaporated to dryness. The residue was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent, afforded the desired product BDP3 as purple solid (150 mg, yield 41%). ¹H NMR (600 MHz, CDCl₃) δ 8.70 (s, 2H), 8.14 (d, J = 7.5 Hz, 2H), 8.07 (d, J = 8.6 Hz, 2H), 7.92 (d, J = 8.3 Hz, 1H), 7.46–7.42 (m, 2H), 7.40–7.35 (m, 4H), 7.30 (d, J = 4.2 Hz, 2H), 7.21 (d, J = 7.7 Hz, 2H), 6.94 (d, J = 3.2 Hz, 1H), 6.79 (d, J = 4.2 Hz, 2H), 4.24 (t, J = 7.3 Hz, 4H), 2.94 (t, J = 7.6 Hz, 2H), 1.88–1.82 (m, 4H), 1.80–1.76 (m, 2H), 1.36–1.19 (m, 30H), 1.01 (t, J = 7.4 Hz, 3H), 0.88–0.83 (m, 6H). ¹³C NMR (151 MHz, CDCl₃) δ 159.10, 142.61, 141.09, 140.91, 132.09, 130.07, 127.69, 125.78, 125.60, 123.67, 123.24, 122.84, 122.06, 120.69, 120.59, 119.22, 119.18, 108.80, 108.74, 108.43, 108.10, 83.59, 43.24, 43.12, 31.80, 29.39, 29.36, 29.18, 29.02, 28.95, 27.33, 22.61, 14.09. MALDI-TOF-MS, m/z: calcd for C₆₁H₇₁BF₂N₄S [M]⁺: 940.546, found: 940.631. HRMS, m/z: calcd for C₆₁H₇₁BF₂N₄S [M]⁺: 940.5461, found: 940.5457.

3,7-Bis(9,9-dimethyl-9H-fluoren-2-yl)-5,5-difluoro-10-(5-octylthiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diaza borinin-4-ium-5-uide (BDP4). According the similar synthetic procedure as for preparing BDP3, using 2-(9,9-dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane as reactant. The crude product was purified on silica column using petroleum ether/ethyl acetate (10 : 1, v/v) as eluent to give the pure compound BDP4 as purple solid in 58% yield. ¹H NMR (600 MHz, CDCl₃) δ 8.02 (s, 2H), 7.89 (d, J = 9.2 Hz, 2H), 7.75–7.71 (m, 4H), 7.41 (d, J = 8.1 Hz, 2H), 7.39 (d, J = 3.5 Hz, 1H), 7.34–7.29 (m, 6H), 6.96 (d, J = 3.4 Hz, 1H), 6.75 (d, J = 4.2 Hz, 2H), 2.98–2.91 (m, 2H), 1.81–1.75 (m, 2H), 1.48 (s, 12H), 1.39–1.23 (m, 10H), 1.01 (t, J = 7.3 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 158.62, 154.41, 153.32, 151.87, 140.51, 138.70, 136.05, 133.89, 132.45, 131.65, 130.57, 128.72, 127.67, 127.00, 125.20, 124.02, 122.60, 120.79, 120.42, 119.65, 83.75, 46.95, 33.73, 30.06, 29.77, 29.69, 29.35, 27.08, 24.93, 22.32, 13.85. MALDI-TOF-MS, m/z: calcd for C₅₁H₄₉BF₂N₂S [M]⁺: 770.368, found: 770.479. HRMS, m/z: calcd for C₅₁H₄₉BF₂N₂S [M]⁺: 770.3678, found: 770.3691.

3,7-Bis(4-(bis(4-(2-methylheptan-2-yl)phenyl)amino)phenyl)-5,5-difluoro-10-(5-octylthiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (BDP5). According the similar synthetic procedure as for preparing BDP3, 4-(2-methyl heptan-2-yl)-N-(4-(2-methylheptan-2-yl)phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline was used as reactant. The crude product was purified on silica column using petroleum ether/ethyl acetate (8 : 1, v/v) as eluent to give the pure compound BDP5 as purple solid in 45% yield. ¹H NMR (600 MHz, CDCl₃) δ 7.78 (d, J = 8.8 Hz, 4H), 7.62 (d, J = 8.4 Hz, 4H), 7.44 (d, J = 8.7 Hz, 1H), 7.24 (d, J = 8.6 Hz, 8H), 7.18 (d, J = 4.3 Hz, 2H), 7.07 (d, J = 8.6 Hz, 8H), 6.89 (d, J = 3.5 Hz, 1H), 6.65 (d, J = 4.3 Hz, 2H), 2.92–2.89 (m, 2H), 1.83 (d, J = 8.6 Hz, 2H), 1.71 (d, J = 11.8 Hz, 8H), 1.37–1.25 (m, 34H), 0.98 (t, J = 7.4 Hz, 3H), 0.78–0.73 (m, 36H). ¹³C NMR (151 MHz, CDCl₃) δ 151.04, 149.14, 145.67, 145.10, 144.48, 144.07, 135.70, 130.53, 127.13, 127.08, 127.02, 126.18, 125.15, 124.96, 124.56, 124.46, 124.36, 123.54, 120.90, 119.97, 57.21, 57.18, 38.28, 38.21, 32.45, 31.79, 31.72, 31.44, 29.96, 29.69, 29.37, 29.25, 27.19, 24.87, 22.34, 14.14. MALDI-TOF-MS, m/z: calcd for C₈₉H₁₁₅BF₂N₄S [M]⁺: 1320.890, found: 1321.053. HRMS, m/z: calcd for C₈₉H₁₁₅BF₂N₄S [M]⁺: 1320.8904, found: 1320.8911.

Conclusions

In summary, we have successfully designed and synthesized a series of novel structurally constrained meso-thiophene BODIPY dyes (BDP1–5) with a variety of electron-donating substituents at 3,5-positions. These dyes exhibit excellent panchromatic absorption with high extinction coefficients both in solution and in film, especially, BDP1-2 dramatically promote the ε values in film state, which benefit them harvesting the whole range of solar light. Additionally, all these light-harvesters show good solubility and perfect film-forming ability, which are attractive factors for solution-processed organic film devices. Luminescence of these dyes located at near-infrared and the red regions with relative low fluorescent quantum yields. The HOMO/LUMO energy levels investigated by CV were about 5.4 eV and 3.8 eV, which are fascinating for BHJ donor materials. The photovoltaic device based on BDP2 and PC₆₁BM exhibit a potential power conversion efficiency of 2.12%, a moderate open-circuit voltage of 0.73 V and a relative high short-circuit photocurrent density of 7.64 mA cm⁻².

Acknowledgements

We greatly acknowledge the financial support from the Nature Science Foundation of Guangdong Province (2014A030313630), the National Nature Science Foundation of China (51476036, 21342003), Project for High-level Personnel of University in Guangdong Province ([2013]246), the Production-Study-Research Combinational Project of Guangdong Province and Ministry of Education (2012B091100296), the University Science and Technology Innovation Key Project of Guangdong Province (cxzd1148) and the University and Scientific Research Institution Key Project of Dongguan City (2012108101005).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15389a

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