Solar cells sensitized by donor-linked concerted companion dyes

Abstract

Recently, concerted companion (CC) dyes have been developed by covalently linking the acceptors of organic and porphyrin dye units. Herein, a new class of CC dyes XW85 and XW86 have been designed by linking the donors of porphyrin and organic dye units with C₆H₁₂ and C₁₂H₂₄ chains, respectively. The DSSCs of XW85 based on the I₃⁻/I⁻ electrolyte show significant J_SC (17.20 mA cm⁻²) and PCE (8.96%), and XW86 exhibits higher J_SC (18.55 mA cm⁻²) and PCE (9.76%), which are also higher than those of the corresponding cosensitization systems. However, the PCEs for XW85 and XW86 are lower than that of the acceptor-linked reference dye XW76 despite the obviously larger dye adsorption amounts. Desorption studies reveal that the CC dyes may be either double-anchored or single-anchored, with the double/single anchoring ratios lying in a sequence of XW85 (1.31) < XW86 (1.88) < XW76 (6.34), consistent with that of increasing PCE. These observations indicate that the non-adsorbed sub-dye unit cannot effectively contribute to electron injection, and thus relatively large mono-anchoring proportions for XW85 and XW86 result in their relatively low J_SC, and the difference between XW85 and XW86 indicates that a longer linking chain is beneficial for partially alleviating the unfavorable single anchoring, resulting in superior performance. The results indicate that the photovoltaic behavior for the CC dyes may be further enhanced by avoiding the unfavorable alignment of the two carboxyl groups in opposite directions and thus simultaneously anchoring the two carboxyl groups, which may be realized through more rational molecular design.

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Introduction

The conversion and utilization of solar energy have attracted extensive attention because of the demand for energy consumption, carbon emission reduction, and environmental protection.¹ As a representative technique in the field of photoelectric conversion, dye-sensitized solar cells (DSSCs) have been considerably developed since the first report by Grätzel and O’Regan in 1991.^2–5 As an essential component of DSSCs, light absorbing sensitizers harvest sunlight and transfer the photogenerated electrons into the semiconductor (TiO₂). Various types of sensitizers, such as ruthenium-based complexes,^6,7 metal-free organic dyes,^8–14 and porphyrin dyes,^15–21 have been designed with the purpose of improving the photovoltaic performance.^22–25

Although high power conversion efficiencies (PCEs) have been achieved for the DSSCs based on porphyrin dyes due to their excellent light-harvesting properties,^{3,4,15,26–28} one of the main drawbacks for porphyrin sensitizers is the weak absorption in the green color region between the Soret and Q bands, which limits sunlight harvesting and the PCE.^29–32 To address this problem, a cosensitization strategy has been developed and extensively employed by using a porphyrin dye and an organic dye with complementary absorption to achieve panchromatic absorption and PCE improvement.^33–36 Despite the success in this respect, the optimization process of the cosensitization approach is somewhat complicated and it is challenging to control the distribution of the two dyes on the semiconductor interface.^37,38 Recently, we have developed a class of concerted companion (CC) dyes by covalently linking the organic dye and porphyrin dye units through flexible chains at the acceptor moieties.^15,39–41 For example, XW76 has been constructed by linking the acceptors of two sub-dye units corresponding to XW10 and XL3 (Fig. 1a).^26,40 Notably, these dyes exhibit panchromatic absorption in the visible region, resulting in high efficiencies and long-term photostability because of the double anchoring effect.

Fig. 1 Molecular structures of dyes (a) XW10,²⁶XL3, XW76,⁴⁰XW85–XW86 and (b) DB-B, DB-D.⁴³

It has been reported that the capability of suppressing charge recombination may be improved by linking two branches of organic dye units at the donor parts, and thus the photovoltaic performance may be enhanced (Fig. 1b).^42–46 Inspired by these results, the sub-dye units identical to those used for XW76 have been employed to construct two novel CC dyes XW85 and XW86 by linking the phenothiazine donors with flexible chains of C₆H₁₂ and C₁₂H₂₄, respectively (Fig. 1a).

As a result, XW86 exhibits higher incident photon-to-current conversion efficiency (IPCE) plateau, larger J_SC, and higher PCEs (9.76% for I₃⁻/I⁻ and 8.78% for [Co(bpy)₃]^3+/2+) compared to those of XW85. Notably, the PCE of XW86 is also higher than those obtained by the cosensitizations of XW10 and XL3. However, the PCE obtained for XW86 is lower than that of the reference acceptor-linked CC dye XW76. The desorption behavior measurements have revealed that considerable amounts of dyes are singly anchored in the devices of XW85 and XW86, which lowers the PCE of the cells. Compared with XW85, XW86 with the longer linking chain affords an enhanced double/single anchoring ratio (1.88 for XW86vs. 1.31 for XW85), resulting in superior performance for XW86. The results indicate that the photovoltaic performance for CC dyes may be further improved by enhancing the double-anchoring proportion through elaborate molecular design in the future.

Results and discussion

Syntheses of the CC dyes

The synthetic procedures for the donor-linked CC dyes XW85 and XW86 are outlined in Scheme 1. Initially, the Suzuki coupling of diphenothiazine tetrabromides PTZ-a and PTZ-b⁴⁷ with 4-(hexyloxy)phenylboronic acid yielded the donor-attached dibromides 1a and 1b, respectively. Subsequent Pd-catalyzed Sonogashira coupling reactions with porphyrin alkynyl intermediate ZnP-A⁴⁸ produced 2a and 2b, respectively. Thereafter, Pd-catalyzed Miyaura coupling reactions of 2a and 2b with bis(pinacolato)diboron followed by Suzuki coupling with the organic dye acceptor unit BT-BTD-A⁴⁹ provided esters 3a and 3b, respectively, and then hydrolysis of the ester groups afforded XW85 and XW86. The structures of all the synthesized compounds have been characterized by ¹H NMR, ¹³C NMR, mass spectra, IR spectra and melting point (see the Experimental section and the ESI†).

Scheme 1 Synthetic routes for XW85–XW86. Reaction conditions: (i) 4-(hexyloxy)phenylboronic acid, Pd(PPh₃)₄, K₂CO₃, THF, H₂O; (ii) AsPh₃, Pd₂(dba)₃, THF, Et₃N; (iii) bis(pinacolato)diboron, Pd(PPh₃)₂Cl₂, KOAc, 1,4-dioxane; (iv) Pd(PPh₃)₄, K₂CO₃, THF, H₂O; (v) LiOH·H₂O, THF, H₂O.

Optical properties

The absorption spectra of the dyes in THF and anchored on thin TiO₂ films are presented in Fig. 2, with the corresponding data summarized in Table S1 (ESI†). Due to the structural similarity with the reported CC dye XW76, the donor-linked CC dyes XW85 and XW86 exhibit panchromatic absorption in the visible region similar to that of XW76.⁴⁰ The similar absorption characteristics of the three CC dyes indicate that the linking chain positions and lengths hardly perturb their light-harvesting capability. Upon anchoring onto the TiO₂ films, all the dyes show broadened bands (Fig. 2b), which can facilitate light-harvesting in the DSSCs.

Fig. 2 Absorption spectra of the sensitizers (a) in THF, and (b) adsorbed on TiO₂ films (2 μm).

To explore the possibility of energy transfer between the sub-dye units of the CC dyes, the emission spectra of the dyes have been measured. The porphyrin dye XW10 and the organic dye XL3 exhibit emission at 675 nm and 629 nm, respectively (Fig. S1a, ESI†), and the CC dyes XW85, XW86, and XW76 demonstrate a strong emission peak at ca. 675 nm corresponding to the porphyrin unit (Fig. S1a and b, ESI†), with the peak at ca. 629 nm contributed by the organic dye unit considerably weakened, irrespective of the excitation wavelength (Fig. S1c, ESI†). These results are indicative of effective energy transfer from the organic dye unit to the porphyrin unit, which may partially compensate for the photocurrent loss for the singly anchored CC dyes via the porphyrin unit (vide infra).

Electrochemical properties

The electrochemical behavior of the CC dyes was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Fig. S2, ESI†) to estimate the thermodynamic feasibility of the charge transfer processes. The energy level diagrams for the sensitizers, electrolyte redox potential, together with the conduction band edge of TiO₂ are shown in Fig. 3. The oxidation potentials (E_ox) of XW85 and XW86, corresponding to their highest occupied molecular orbital (HOMO) levels, were found to be 0.83 and 0.80 V (vs. NHE), respectively, which are lower than the I₃⁻/I⁻ and [Co(bpy)₃]^3+/2+ redox potentials (∼0.4 V and 0.56 V vs. NHE, respectively) indicative of enough driving force for the reduction of the dye cations.⁵⁰ The optical band gaps (E_0–0) of XW85 and XW86 were both estimated to be 1.91 eV from the intersection of the normalized absorption and fluorescence spectra in THF. From the difference between the E_0–0 and E_ox values, the excited state potentials were calculated to be −1.08 and −1.11 V (vs. NHE), considerably higher than that of the conduction band of TiO₂ (−0.5 V vs. NHE) suggesting that both of the sensitizers provide sufficient driving force for electron injection.

Fig. 3 Energy level diagrams of the sensitizers with respect to the conduction band of TiO₂ and the redox potentials of I₃⁻/I⁻ and [Co(bpy)₃]^3+/2+.

Photovoltaic performance

To evaluate the photovoltaic behavior of the donor-bridged CC dyes, the corresponding DSSCs have been fabricated using the I₃⁻/I⁻ and [Co(bpy)₃]^3+/2+ electrolytes (see the ESI† for details). The photocurrent density–voltage (J–V) curves (Fig. 4a and c) and IPCE spectra (Fig. 4b and d) were measured under simulated sunlight (AM 1.5) and the corresponding photovoltaic parameters are collected in Table 1. For the DSSCs based on the I₃⁻/I⁻ electrolyte, XW85 exhibits V_OC, J_SC, and PCE of 767 mV, 17.20 mA cm⁻², and 8.96%, respectively. Compared with XW85, XW86 exhibits a slightly decreased V_OC (755 mV) but an elevated J_SC (18.55 mA cm⁻²), and the resulting PCE (9.76%) is higher than that of XW85. Compared to the corresponding component dyes XL3 (PCE of 8.69%)⁴⁰ and XW10 (PCE of 8.6%),²⁶XW85 and XW86 show moderately improved photovoltaic performance. Furthermore, the efficiency achieved for XW86 is higher than that of 9.28% obtained for the device cosensitized with XW10 and XL3, which exhibits a considerably lower J_SC as a result of the considerably lowered adsorption amount of the porphyrin dye accompanied with the coadsorption of the organic dye (Table 1 and Table S2, S3, ESI†). Compared with the iodine-based DSSCs, the devices based on the cobalt electrolyte exhibit improved V_OC attributable to the higher potential of the cobalt redox mediator.⁵⁰ Despite the improved V_OC, the J_SC values are considerably lowered because of the mass transport limitation of the bulky cobalt complex.⁵¹ As a result, XW85 and XW86 exhibit lowered efficiencies of 7.80% and 8.78%, respectively.

Fig. 4 J–V characteristic curves of the DSSCs using (a) iodine-based and (c) cobalt-based redox mediators, and IPCE spectra of the DSSCs using (b) iodine-based and (d) cobalt-based redox mediators.

Table 1 Photovoltaic parameters of DSSCs under AM1.5 illumination (power, 100 mW cm⁻²)^a

Dyes	Electrolyte	V OC [mV]	J SC [mA cm^−2]	J ^IPCE [mA cm^−2]^c	FF [%]	PCE [%]	Dye loading [× 10^−8 mol cm^−2]
a The data were collected from three parallel cells (average values and errors). b The TiO2 electrode was dipped in a mixed dye solution (XW10/XL3, 1/2) in a mixture of chloroform and ethanol (1/1, v/v) for 10 h (device M3 in the ESI). c The J^IPCE values were obtained by integrating the IPCE spectra, and the deviations from the corresponding JSC values obtained from the J–V tests are listed in the parentheses.
XW85	I3^−/I^−	767 ± 2	17.20 ± 0.27	15.91 (8.1%)	67.92 ± 0.52	8.96 ± 0.08	12.6
XW86	I3^−/I^−	755 ± 2	18.55 ± 0.32	16.83 (10.2%)	69.66 ± 0.59	9.76 ± 0.16	11.7
XW10 + XL3^b	I3^−/I^−	794 ± 4	16.60 ± 0.26	14.78 (12.3%)	70.40 ± 0.23	9.28 ± 0.15	0.42 (XW10)/23.5(XL3)
XL3 ^40	I3^−/I^−	810 ± 3	16.31 ± 0.16	14.84 (9.9%)	65.81 ± 0.24	8.69 ± 0.07	25.0
XW10 ^26	I3^−/I^−	711 ± 5	17.90 ± 0.04	16.08 (11.3%)	68.4 ± 0.5	8.6 ± 0.1	9.4
XW76 ^40	I3^−/I^−	765 ± 5	19.94 ± 0.18	18.28 (9.1%)	70.70 ± 0.66	10.78 ± 0.11	8.2
XW85	[Co(bpy)3]^3+/2+	923 ± 2	11.79 ± 0.14	11.72 (0.6%)	71.65 ± 0.55	7.80 ± 0.12	7.4
XW86	[Co(bpy)3]^3+/2+	911 ± 2	13.33 ± 0.29	13.02 (2.4%)	72.29 ± 0.80	8.78 ± 0.11	6.9
XW10 + XL3^b	[Co(bpy)3]^3+/2+	951 ± 1	11.27 ± 0.15	11.11 (1.4%)	71.59 ± 0.69	7.68 ± 0.17	0.2 (XW10)/17.2(XL3)
XL3	[Co(bpy)3]^3+/2+	975 ± 1	10.79 ± 0.11	10.58 (2.0%)	72.10 ± 1.15	7.59 ± 0.15	17.6
XW10	[Co(bpy)3]^3+/2+	891 ± 2	12.09 ± 0.21	11.91 (1.5%)	72.78 ± 0.49	7.84 ± 0.16	5.1
XW76	[Co(bpy)3]^3+/2+	919 ± 2	14.63 ± 0.22	14.55 (0.5%)	72.12 ± 1.00	9.69 ± 0.13	4.2

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As illustrated in Fig. 4b and d, the IPCE spectra of XW85 and XW86 exhibit a panchromatic response over the visible region, similar to that of XW76. By integrating the IPCE data, J^IPCE values could be obtained, and they were found to be lower than the corresponding J_SC values obtained from the J–V tests by 0.5%–12.3% (Table 1 and Fig. S4, ESI†). These results may be rationalized by the fact that the full irradiation applied in the J–V test generates more heat and electrons, resulting in higher charge transport and collecting efficiencies, and thus higher J_SC values were obtained in the J–V tests.³⁰ Compared with the IPCE of XW85, XW86 presents an elevated plateau, consistent with its higher J_SC, indicating that the longer linking chain in XW86 is favorable for improving the J_SC. Compared with XW85 and XW86, the traditional cosensitization approach affords inferior IPCE values around the Q-band region of the porphyrin dyes within 600–750 nm, which may be related to the very small amount of adsorbed porphyrin dye XW10 (0.42 × 10⁻⁸ mol cm⁻² for I₃⁻/I⁻ and 0.2 × 10⁻⁸ mol cm⁻² for Co^3+/2+) relative to the organic dye XL3 (23.5 × 10⁻⁸ mol cm⁻² for I₃⁻/I⁻ and 17.2 × 10⁻⁸ mol cm⁻² for Co^3+/2+) (Table 1). As a result, relatively small J_SC values were obtained for the cosensitized devices. These results indicate that the donor-linked CC dyes XW85 and XW86 are indeed effective in enhancing the spectral response and the J_SC of the DSSCs because of their panchromatic absorption. However, the IPCE plateau and the J_SC for XW85 and XW86 are still obviously lower than those of the acceptor-linked CC dye XW76, despite their similar absorption spectra. As a result, XW85 and XW86 exhibit lowered PCE values relative to XW76 (Table 1).

Considering that internal factors like electron injection efficiency (η_inj) and dye regeneration efficiency (η_reg) may induce the difference in the IPCE values between the two types of CC dyes,^52,53 time-resolved fluorescence spectra of the dyes adsorbed on Al₂O₃ and TiO₂ films were checked to evaluate the electron injection dynamics. Compared to those of the dyes on Al₂O₃, the emission for all the dyes on the TiO₂ films are strongly quenched, indicating that the electrons in the excited singlet states of all the adsorbed dyes can effectively inject into TiO₂, showing high η_inj values exceeding 85% (Fig. S5 and Table S4, ESI†). The transient absorption measurements were further used to study the regeneration kinetics of the dyes with inert electrolyte and active redox electrolytes. The decay signals for the dye-sensitized TiO₂ films with inert electrolyte originate from the recombination between the dye cations and the injected electrons, with the recombination lifetimes (τ_rec) lying within 79–99 μs (Fig. S6 and Table S5, ESI†). Upon contact with the redox electrolytes, the electron transfer from the I⁻ or Co²⁺ species to the dye cations accelerates the decay processes, showing regeneration lifetimes of less than 10 μs and η_reg exceeding 90%. The relatively high η_inj and η_reg values for all the dyes ensure smooth electron injection and dye regeneration processes. Although η_inj and η_reg roughly lie in the sequence of XW85 < XW86 < XW76, identical with that of gradually improved IPCE and J_SC, the relatively small differences for the η_inj and η_reg values of these dyes cannot fully account for the considerable IPCE and J_SC differences. Another possible factor affecting IPCE and J_SC is the adsorption amounts of the dyes. In contrast to the fact that XW85 and XW86 exhibit lowered J_SC values relative to XW76, the adsorption amounts of XW85 and XW86 are obviously higher than that of XW76 (Table 1). This disagreement indicates that the anchoring states for the CC dyes linked at different positions may be quite different. Hence, we continued to check their anchoring behavior.

Desorption behavior

The anchoring behavior was explored by checking the dye desorption processes. Thus, the TiO₂ films adsorbed with the dyes were soaked in 0.01 M NaOH solution (THF/H₂O), and the plots of the remaining dye percentages versus the soaking time are shown in Fig. 5. As a result, XL3 and XW10 are completely desorbed within 14 minutes, indicating that the desorption processes are rather fast for the mono-anchored dyes. In contrast, the desorption behavior of the CC dyes is considerably different. A fast desorption process in the initial 14 minutes affords 55%, 63% and 84% of the remaining adsorption amounts for XW85, XW86 and XW76, respectively. Subsequently, a much slower desorption process was observed. When the TiO₂ films were soaked for a total of 1 h, the remaining adsorption amounts were found to be 46%, 55%, and 82%, respectively. The two desorption processes imply that the CC dyes may adsorb on the TiO₂ nanoparticles in two distinct modes, i.e., single-anchoring or double-anchoring. Thus, the single-anchoring mode corresponds to the fast desorption in the first 14 minutes, while the double-anchoring relates to the subsequent slow desorption process. Judging from the desorption ratios, the acceptor-bridged CC dye XW76 exhibits a higher ratio of double anchoring on TiO₂, compared with those of XW85 and XW86. Thus, the slow desorption process for each CC dye was linearly fitted and extrapolated to evaluate the single-anchored dye amount (Fig. S7 and Table S6, ESI†). As a result, the double/single anchoring ratios were obtained to be 1.31, 1.88 and 6.34 for XW85, XW86 and XW76, respectively. These data indicate that extending the linking chain between the donor units is favorable for enhancing the double-anchoring ratio, and the double/single anchoring ratios for the donor-linked CC dyes (XW85 and XW86) are considerably lower than that for the acceptor-linked one (XW76). These desorption results are consistent with the theoretical calculations (vide infra).

Fig. 5 Plots of the decrease in dye adsorption amounts versus the soaking time in 0.01 M NaOH solution (THF/H₂O, 1/1).

To further monitor the anchoring states of the dyes, the ATR-FTIR spectra of the dyes have been collected. All the dyes show a characteristic C=O stretching band of the carboxyl group at ∼1690 cm⁻¹ (Fig. S8, ESI†). Upon anchoring onto the TiO₂ films, this peak disappears for XL3, XW10, and XW76, while the peak at ∼1390 cm⁻¹ attributable to symmetric stretching vibrations (ν_sym) of COO⁻ is considerably intensified, indicating the nearly complete deprotonation and adsorption of the dyes on the TiO₂ surface. In contrast, the corresponding spectra for the adsorbed dyes of XW85 and XW86 still exhibit a weak peak at 1689 cm⁻¹, indicating the presence of considerable amounts of non-adsorbed carboxyl groups, which is consistent with the existence of mono-anchored dyes as revealed by the desorption study (vide supra). Similar results have also been reported for other multi-anchoring dyes.^54,55 In addition, the desorption processes for the dyes were also monitored by ATR-FTIR. Unfortunately, no obvious peak could be observed at ∼1690 cm⁻¹ corresponding to free carboxyl groups after the initial fast desorption stage (2 min) for XW85 and XW86, although the single-anchored dye has not been completely desorbed at this stage. This result may be attributed to the formation of carboxylate salts under strongly basic conditions. Further prolonged desorption time (14 and 60 min) afforded similar IR characters.

Accompanying the trend of decreasing double/single anchoring ratios, decreasing J_SC values were obtained for the CC dyes XW76, XW86, and XW85, despite the increasing adsorption amounts (Table 1). These observations indicate that each singly anchored CC dye molecule may contain a sub-dye unit hanging away from TiO₂, and thus it occupies a smaller area on the TiO₂ film. As a result, more dye molecules may be adsorbed, leading to an increase in the adsorption amounts, which may partially compensate for the J_SC loss induced by the single anchoring effect. In addition, the effective energy transfer from the organic dye unit to the porphyrin dye unit may also partially compensate for the J_SC loss. As a consequence, the lowered J_SC values obtained for XW85 and XW86 (17.20 and 18.55 mA cm⁻²) are still comparable to that of XW76 (19.94 mA cm⁻²) despite the unfavorable single anchoring effect.

Theoretical calculations

To further understand the anchoring behavior of the CC dyes, theoretical calculations were performed (Fig. S9, ESI†).^56,57 The binding energies (E_be) were calculated by subtracting the energy of the total system from the sum of the respective energies of the dye and TiO₂: E_be = (E_dye + E_TiO2) − E_dye/TiO2. The calculated E_be values for the double-anchored dyes lie in the order of XW85 (23.8 kcal mol⁻¹) < XW86 (26.0 kcal mol⁻¹) < XW76 (38.4 kcal mol⁻¹) (Table S7, ESI†), consistent with the much stronger adsorption of XW76 than XW85 and XW86 revealed by the desorption study. Furthermore, the E_be values for the two single-anchoring modes of XW85 and XW86 were calculated to be much smaller, lying within the range of 1.7–11.0 kcal mol⁻¹, implying that the adsorption of the mono-anchored dyes may be weaker and easier to desorb from the film, which is fully consistent with the desorption studies (vide supra). As mentioned above, the porphyrin dye XW10 adsorbs much less efficiently than the organic dye XL3 in the cosensitized systems. However, theoretical calculations reveal that there is no obvious difference between their E_be values, indicating that the preferable adsorption of XL3 in the cosensitization systems may be related to its smaller molecular size. In contrast, the single-anchored CC dyes XW85 and XW86 exhibit quite different adsorption behavior. The adsorption via the porphyrin unit may be preferable, as evidenced by the higher E_be values (8.9 and 11.0 kcal mol⁻¹) relative to those of adsorption via the organic unit (1.7 and 6.1 kcal mol⁻¹) (Table S7, ESI†). As a result, the energy transfer from the organic dye unit to the porphyrin unit may indeed partially compensate for the J_SC loss induced by the single anchoring of XW85 and XW86 (vide supra).

Electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) data for the DSSCs were measured in the dark to explore the effect of electrode interface properties on the V_OC. Theoretically, for a certain redox couple, the V_OC is mainly dominated by the semiconductor conduction band energy level (E_CB) and the free electron density,⁵¹ and the E_CB for TiO₂ is affected by the density of states (DOS),⁵⁸ which is proportional to the capacitance (C_μ) level. As a result, the DSSCs based on the I₃⁻/I⁻ electrolyte exhibit slightly different DOS values for all the dyes at a fixed bias voltage (Fig. 6a), indicative of a relatively weak effect of the conduction band shift on the V_OC. On the other hand, the free electron density is mainly governed by the charge recombination resistance (R_rec) values, which increase in the order of XW10 (60.0 Ω cm²) < XW86 (81.8 Ω cm²) < XW76 (171.6 Ω cm²) < XW85 (199.9 Ω cm²) < XW10 + XL3 (698.6 Ω cm²) < XL3 (784.1 Ω cm²) at the fixed bias voltage of −0.75 V (Fig. 6b), in a trend the same as that of increasing V_OC values. On this basis, the electron lifetimes (τ) were calculated from the equation of τ = C_μ × R_rec and plotted against DOS (Fig. 6c). At the same DOS, the order of increasing τ values is roughly consistent with those of increasing R_rec and V_OC values. These results indicate that the densities of the free electrons in the TiO₂ films dominate the V_OC of the devices. Similar EIS properties were also observed for the cobalt-based DSSCs (Fig. S10 and Table S8, ESI†).

Fig. 6 (a) Plots of DOS versus the bias voltages, (b) complex-plane plots at applied voltage of −0.75 V, and (c) τ versus DOS of the DSSCs using I₃⁻/I⁻ electrolyte measured in the dark.

The similar C_μ values obtained for XW85 and XW86 indicate that the linking chain lengths only slightly affect the conduction band positions. On the other hand, ethylene glycol units can usually trap Li⁺, shift the CB upward and thus enhance the V_OC.⁵⁹ However, XW85 exhibits a C_μ close to that of XW76. The slightly higher V_OC obtained for XW85 may be related to the fact that the adsorption amount for XW85 is obviously higher than that of XW76 and thus a more compact dye layer is formed for XW85, which suppresses the charge recombination process as thus a higher R_rec is observed for XW85. Compared with XW86, XW85 possesses a shorter linkage and it shows higher R_rec and V_OC values (Table 1), indicating that the presence of a more non-anchored sub-dye unit may block the penetration of oxidized species in the electrolyte to approach TiO₂, and thus suppress charge recombination. Based on the EIS data, the electron collection efficiencies (η_col) were also calculated, which lie in the ranges of 85.8–98.1% and 83.8–88.4% for I₃⁻/I⁻ (V_bias = −0.75 V) and Co^3+/2+ (V_bias = −0.85 V), respectively (Table S8, ESI†). The lower η_col values obtained for the cobalt electrolyte are consistent with the lower J_SC and IPCE platforms observed for the corresponding DSSCs (Fig. 4b and d).

Durability

The PCE of the DSSCs soaked in long-term visible light was monitored to evaluate the photostability, which is a crucial factor for practical applications of DSSCs.⁶⁰ The cells sensitized by XW85 and XW86 with I₃⁻/I⁻ redox shuttle provided PCEs of 8.05% and 8.86% after 500 h of visible light soaking, which are 89.8% and 90.8% of the initial efficiencies, respectively (Fig. 7), lying between those of 88.8% and 93.1% observed for XW10 and XW76, respectively. The sequence agrees well with that of the double/single anchoring ratios, i.e., the singly anchored dye XW10 exhibits the worst photostability. Whereas, XW85, XW86 and XW76 exhibit increasing photostability with the double/single anchoring ratios gradually improving from 1.31 to 6.34. These results indicate that enhancing the double anchoring proportion will improve the photostability, which is favorable for real applications.

Fig. 7 Plots of the changes in the PCEs vs. the illumination time for the solar cells sensitized by XW85, XW86, XL3, XW10 and XW76, respectively.

Conclusions

In summary, two novel CC dyes XW85 and XW86 have been constructed by linking the donors of two sub-dye units corresponding to XL3 and XW10 with flexible chains of C₆H₁₂ and C₁₂H₂₄, respectively. Compared to XW85, XW86 exhibits higher J_SC values of 18.55 and 13.33 mA cm⁻² and PCEs of 9.76% and 8.78% for I₃⁻/I⁻ and [Co(bpy)₃]^3+/2+ electrolytes, respectively, which are also higher than those of the cosensitized DSSCs based on XL3 and XW10. However, the PCE of XW86 does not outperform that of the acceptor-linked reference CC dye XW76. The desorption study revealed that the CC dyes may adsorb either in the double anchoring mode or the single anchoring mode, and the double/single anchoring ratios lie in the increasing sequence of XW85 (1.31) < XW86 (1.88) < XW76 (6.34), consistent with the sequence of increasing PCE. These results indicate that the lower photovoltaic performance of XW85 and XW86 relative to XW76 may be related to the fact that the non-adsorbed sub-dye unit in the mono-anchored dye molecules may not effectively contribute to electron injection, and thus large portions of mono-anchoring observed for XW85 and XW86 result in their relatively poor J_SC and PCE. Compared with XW85, the larger double/single anchoring ratio observed for XW86 indicates that elongation of the linking chain is favorable for double anchoring, leading to an enhanced J_SC of XW86, despite its smaller adsorption amount than that of XW85.

The adverse single-anchoring problem of the CC dyes revealed in this work indicates that the photovoltaic behavior may be further enhanced by addressing this problem through more rational molecular design in the future. For example, both the donors and the acceptors of the respective sub-dye units may be linked with flexible chains to rule out the possibility of opposite alignment of the two carboxyl groups between sub-dye units and thus simultaneously anchor the carboxyl groups, resulting in enhanced J_SC and PCE.

Experimental

Synthesis of the CC dyes XW85–XW86

Compounds PTZ-a, PTZ-b, BT-BTD-A, and ZnP-A were synthesized according to the literature.^47–49 The synthetic routes for dyes XW85 and XW86 are shown in Scheme 1 with the details described as follows:

1a . To the mixture of PTZ-a (440 mg, 0.55 mmol) and (4-(hexyloxy)phenyl)boronic acid (307 mg, 1.38 mmol) in THF (20 mL) were added 2 M K₂CO₃ aqueous solution (1.40 mL, 2.80 mmol) and Pd(PPh₃)₄ (32 mg, 0.03 mmol) under a nitrogen atmosphere. The mixture was refluxed for 18 h, and then a saturated aqueous solution of ammonium chloride was added at room temperature. The mixture was extracted with dichloromethane (DCM) (50 mL × 3) and the organic phase was dried over Na₂SO₄, filtered, and then concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography with DCM/petroleum ether (1/6, v/v) as the eluent to give a yellow solid of 1a (100 mg, 19%). ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.41 (d, J = 8.5 Hz, 4H), 7.30 (d, J = 8.7 Hz, 2H), 7.27 (s, 2H), 7.23–7.19 (m, 4H), 6.95–6.90 (m, 4H), 6.83 (d, J = 8.4 Hz, 2H), 6.66 (d, J = 8.4 Hz, 2H), 3.98 (t, J = 6.6 Hz, 4H), 3.80 (s, 4H), 1.84–1.74 (m, 8H), 1.51–1.42 (m, 8H), 1.38–1.32 (m, 8H), 0.95–0.87 (m, 6H). ¹³C NMR (151 MHz, CDCl₃): δ 158.54, 144.32, 143.40, 135.57, 132.06, 129.84, 129.65, 127.44, 126.99, 125.57, 125.43, 124.59, 116.45, 115.68, 114.76, 114.35, 68.05, 47.09, 31.57, 29.23, 26.44, 26.25, 25.71, 22.59, 14.03. HRMS (ESI, m/z): [M + H]⁺ calcd for C₅₄H₅₉Br₂N₂O₂S₂, 989.2385; found, 989.2382. FT-IR (ATR, cm⁻¹): 2931 (m), 2855 (m), 1604 (m), 1580 (w), 1518 (w), 1491 (m), 1458 (s), 1393 (s), 1375 (m), 1234 (s), 1187 (m), 1107 (m), 1023 (m), 876 (m), 798 (s), 749 (m), 679 (w), 606 (m). mp 119–121 °C.

1b . The synthetic procedure resembles that for synthesizing compound 1a except using compound PTZ-b (2.88 g, 3.27 mmol) instead of PTZ-a. The obtained crude product was purified by silica gel column chromatography with DCM/petroleum ether (1/6, v/v) as the eluent to give a yellow viscous liquid of 1b (600 mg, 17%). ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.42 (d, J = 8.5 Hz, 4H), 7.35–7.28 (m, 4H), 7.25–7.18 (m, 4H), 6.95–6.91 (m, 4H), 6.86 (d, J = 8.4 Hz, 2H), 6.69 (d, J = 8.5 Hz, 2H), 3.97 (t, J = 6.6 Hz, 4H), 3.80 (s, 4H), 1.84–1.73 (m, 8H), 1.50–1.38 (m, 8H), 1.37–1.31 (m, 8H), 1.29–1.18 (m, 12H), 0.94–0.88 (m, 6H). ¹³C NMR (151 MHz, CDCl₃): δ 158.55, 144.32, 143.43, 135.49, 132.11, 129.83, 129.57, 127.43, 126.77, 125.58, 125.37, 124.35, 116.39, 115.61, 114.78, 114.27, 68.07, 47.47, 31.62, 29.48, 29.41, 29.27, 29.17, 26.82, 26.67, 25.75, 22.63, 14.10, 14.08. HRMS (ESI, m/z): [M + H]⁺ calcd for C₆₀H₇₁Br₂N₂O₂S₂, 1073.3324, found, 1073.3317. FT-IR (ATR, cm⁻¹): 2931 (m), 2855 (m), 1605 (m), 1580 (w), 1518 (w), 1491 (m), 1458 (s), 1393 (s), 1375 (m), 1235 (s), 1186 (m), 1107 (m), 1023 (m), 876 (m), 798 (s), 749 (m), 678 (w), 606 (m).

2a . To the mixture of Pd₂(dba)₃ (6 mg, 0.01 mmol), AsPh₃ (34 mg, 0.11 mmol), compound 1a (110 mg, 0.11 mmol), and ZnP-A (80 mg, 0.06 mmol) in THF (12 mL) was added triethylamine (3 mL) under a nitrogen atmosphere. The solution was refluxed for 18 h. After cooling down to room temperature, the reaction mixture was concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography with DCM/petroleum ether (2/3, v/v) as the eluent to give a dark green solid of 2a (90 mg, 69%). ¹H NMR (400 MHz, CDCl₃, ppm): δ 9.63 (d, J = 4.6 Hz, 2H), 9.59 (d, J = 4.5 Hz, 2H), 8.88 (d, J = 4.6 Hz, 2H), 8.84 (d, J = 4.5 Hz, 2H), 8.18–8.14 (m, 2H), 8.01–7.97 (m, 2H), 7.77–7.68 (m, 4H), 7.48–7.44 (m, 2H), 7.38–7.29 (m, 3H), 7.25–7.20 (m, 5H), 7.02 (d, J = 8.4 Hz, 4H), 6.99–6.89 (m, 4H), 6.82 (d, J = 8.5 Hz, 1H), 6.69 (d, J = 9.3 Hz, 1H), 6.43 (d, J = 8.7 Hz, 2H), 4.01–3.94 (m, 7H), 3.90–3.78 (m, 10H), 3.37 (t, J = 6.6 Hz, 2H), 1.93–1.75 (m, 6H), 1.39–1.32 (m, 6H), 1.30–1.23 (m, 16H), 1.19–1.10 (m, 10H), 1.09–0.83 (m, 52H), 0.70–0.59 (m, 8H), 0.58–0.39 (m, 24H). ¹³C NMR (151 MHz, CDCl₃): δ 166.67, 159.93, 158.49, 158.11, 151.65, 151.34, 150.60, 150.48, 144.34, 143.40, 139.60, 135.59, 133.70, 132.15, 132.06, 131.95, 131.63, 131.06, 130.53, 130.17, 129.87, 129.77, 129.69, 129.64, 128.64, 128.44, 127.43, 127.26, 125.66, 125.50, 125.45, 125.41, 124.68, 124.44, 123.98, 123.27, 122.46, 121.91, 121.54, 120.94, 116.45, 115.73, 115.63, 115.31, 115.27, 114.78, 114.75, 114.35, 105.10, 68.58, 68.06, 67.97, 67.92, 64.80, 47.24, 47.15, 31.84, 31.61, 31.53, 31.50, 30.21, 30.15, 30.13, 29.95, 29.90, 29.87, 29.87, 29.83, 29.80, 29.49, 29.40, 29.29, 29.25, 29.14, 29.09, 28.98, 28.76, 28.65, 26.54, 26.45, 26.35, 26.28, 26.20, 25.73, 25.57, 25.27, 23.27, 22.64, 22.62, 22.56, 16.11, 14.09, 14.06, 14.03. HRMS (ESI, m/z): [M + H]⁺ calcd for C₁₄₆H₁₈₀BrN₆O₈S₂Zn, 2352.1779, found, 2352.1724. FT-IR (ATR, cm⁻¹): 2920 (s), 2850 (s), 2187 (m), 1720 (m), 1602 (m), 1586 (m), 1518 (w), 1492 (m), 1457 (s), 1392 (m), 1237 (s), 1206 (m), 1099 (s), 995 (m), 876 (w), 795 (m), 714 (m). mp 78–80 °C.

2b . The synthetic procedure resembles that for synthesizing compound 2a except using compound 1b (200 mg, 0.19 mmol) instead of 1a. The obtained crude product was purified by silica gel column chromatography with DCM/petroleum ether (2/3, v/v) as the eluent to give a dark green solid of 2b (90 mg, 59%). ¹H NMR (400 MHz, CDCl₃, ppm): δ 9.62 (d, J = 4.6 Hz, 4H), 8.89 (d, J = 4.6 Hz, 2H), 8.86 (d, J = 4.6 Hz, 2H), 8.17–8.08 (m, 2H), 7.95 (t, J = 6.2 Hz, 2H), 7.78–7.67 (m, 4H), 7.47 (d, J = 8.7 Hz, 2H), 7.40–7.31 (m, 4H), 7.25–7.16 (m, 4H), 7.02 (d, J = 8.5 Hz, 4H), 6.98 (d, J = 8.4 Hz, 1H), 6.94 (dd, J = 8.6, 2.7 Hz, 3H), 6.81 (dd, J = 8.8, 5.5 Hz, 3H), 6.66 (d, J = 8.5 Hz, 1H), 4.01–3.91 (m, 7H), 3.87 (t, J = 6.5 Hz, 10H), 3.78 (t, J = 7.1 Hz, 2H), 1.96–1.86 (m, 2H), 1.82–1.67 (m, 6H), 1.39–1.21 (m, 22H), 1.21–1.10 (m, 8H), 1.09–0.94 (m, 32H), 0.95–0.83 (m, 12H), 0.83–0.74 (m, 20H), 0.68–0.58 (m, 8H), 0.56–0.36 (m, 24H). ¹³C NMR (151 MHz, CDCl₃): δ 166.36, 159.95, 158.36, 158.25, 151.73, 151.41, 150.63, 150.51, 145.01, 144.31, 143.40, 143.18, 135.47, 135.34, 132.15, 132.05, 131.92, 131.76, 130.74, 130.68, 130.00, 129.96, 129.88, 129.81, 129.55, 129.31, 127.32, 127.19, 126.78, 125.48, 125.34, 125.25, 124.46, 124.37, 124.27, 124.24, 120.83, 116.37, 115.58, 115.54, 115.50, 114.77, 114.68, 114.23, 114.20, 105.16, 68.65, 68.06, 68.00, 47.71, 47.46, 31.86, 31.62, 31.60, 29.56, 29.49, 29.40, 29.29, 29.27, 29.21, 29.15, 29.09, 28.77, 28.67, 26.95, 26.85, 26.82, 26.68, 26.64, 25.73, 25.69, 25.33, 22.66, 22.64, 22.61, 14.12, 14.07. HRMS (ESI, m/z): [M + H]⁺ calcd for C₁₅₂H₁₉₂BrN₆O₈S₂Zn, 2436.2718, found, 2436.2712. FT-IR (ATR, cm⁻¹): 2920 (s), 2849 (s), 2187 (m), 1719 (m), 1602 (m), 1586 (m), 1518 (w), 1495 (w), 1456 (s), 1391 (m), 1240 (s), 1206 (m), 1096 (s), 996 (m), 879 (w), 791 (m), 712 (m). mp 66–68 °C.

3a . To the mixture of compound 2a (71 mg, 0.03 mmol), bis(pinacolato)diboron (9 mg, 0.04 mmol), and KOAc (15 mg, 0.15 mmol) in 1,4-dioxane (10 mL) was added Pd(PPh₃)₂Cl₂ (5 mg, 0.01 mmol) under a nitrogen atmosphere. The mixture was refluxed for 18 h, and then a saturated aqueous solution of ammonium chloride was added to the reaction mixture at room temperature. The mixture was extracted with DCM (50 mL × 3) and the organic phase was dried over Na₂SO₄, filtered, and then concentrated under reduced pressure. The obtained residue was purified by flash silica gel column chromatography with DCM/petroleum ether (2/1, v/v) as the eluent. The obtained green residue was used for the next reaction without further characterization. To the mixture of the obtained residue and BT-BTD-A (24 mg, 0.03 mmol) in THF (10 mL) were added 2 M K₂CO₃ aqueous solution (0.10 mL, 0.20 mmol) and Pd(PPh₃)₄ (3 mg, 0.003 mmol) under a nitrogen atmosphere. The reaction was refluxed for 18 h, and then a saturated aqueous solution of ammonium chloride was added to the reaction mixture at room temperature. The mixture was extracted with DCM (50 mL × 3) and the organic phase was dried over Na₂SO₄, filtered, and then concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography with DCM/petroleum ether (3/2, v/v) as the eluent to give a brown solid of 3a (25 mg, 29% (two steps)). ¹H NMR (400 MHz, CDCl₃, ppm): δ 9.57 (d, J = 4.5 Hz, 2H), 9.51 (d, J = 4.6 Hz, 2H), 8.85 (d, J = 4.6 Hz, 2H), 8.80 (d, J = 4.5 Hz, 2H), 8.14 (d, J = 8.3 Hz, 2H), 8.00 (d, J = 8.3 Hz, 2H), 7.97 (d, J = 8.3 Hz, 2H), 7.74 (d, J = 8.5 Hz, 2H), 7.70 (d, J = 6.4 Hz, 2H), 7.55–7.50 (m, 3H), 7.48–7.43 (m, 2H), 7.40–7.36 (m, 4H), 7.34–7.28 (m, 4H), 7.23 (d, J = 2.1 Hz, 1H), 7.04 (d, J = 8.6 Hz, 5H), 6.97–6.91 (m, 5H), 6.87 (dd, J = 8.5, 6.9 Hz, 2H), 6.61 (d, J = 8.6 Hz, 2H), 3.99–3.94 (m, 12H), 3.90 (t, J = 6.6 Hz, 8H), 3.58 (t, J = 6.6 Hz, 2H), 2.64 (t, J = 7.9 Hz, 4H), 1.95–1.75 (m, 8H), 1.63 (s, 8H), 1.38–1.22 (m, 28H), 1.19–1.11 (m, 8H), 1.09–0.98 (m, 20H), 0.96–0.86 (m, 24H), 0.84–0.75 (m, 20H), 0.72–0.65 (m, 8H), 0.61–0.48 (m, 20H). ¹³C NMR (151 MHz, CDCl₃): δ 166.68, 166.55, 159.87, 158.49, 151.63, 151.28, 150.47, 150.39, 149.20, 144.92, 143.48, 143.20, 139.33, 139.30, 138.64, 136.04, 135.61, 135.46, 132.28, 132.17, 131.88, 131.57, 131.12, 130.96, 130.87, 130.55, 130.25, 130.13, 130.03, 130.00, 129.96, 129.87, 129.84, 129.79, 129.65, 129.53, 129.48, 129.37, 129.01, 128.79, 128.70, 128.16, 127.72, 127.59, 127.43, 127.25, 125.58, 125.51, 125.43, 125.11, 124.68, 124.64, 124.59, 124.52, 123.87, 120.79, 118.52, 118.31, 115.70, 115.45, 115.33, 115.22, 115.09, 114.79, 114.74, 105.13, 68.65, 68.04, 68.03, 47.23, 31.78, 31.68, 31.62, 31.56, 31.48, 30.89, 30.27, 30.19, 29.82, 29.76, 29.74, 29.66, 29.52, 29.48, 29.44, 29.42, 29.32, 29.28, 29.21, 29.18, 29.16, 29.13, 29.08, 29.06, 29.01, 28.99, 28.97, 28.95, 28.92, 28.72, 28.63, 27.16, 27.14, 26.23, 26.16, 26.07, 25.70, 25.51, 25.27, 22.65, 22.60, 22.58, 22.53, 14.14, 14.10, 14.08, 14.04, 13.99. MS (MALDI-TOF, m/z): [M]⁺ calcd for C₁₈₂H₂₁₆N₈O₁₀S₅Zn, 2897.5; found, 2897.2. FT-IR (ATR, cm⁻¹): 2919 (s), 2849 (s), 2188 (m), 1720 (s), 1601 (s), 1586 (s), 1498 (w), 1455 (s), 1392 (m), 1270 (s), 1240 (s), 1205 (m), 1097 (s), 996 (m), 878 (w), 792 (m), 766 (m), 713 (m). mp 85–87 °C.

3b . The synthetic procedure resembles that for synthesizing compound 3a except using compound 2b (70 mg, 0.03 mmol) instead of 2a. The obtained crude product was purified by silica gel column chromatography with DCM/petroleum ether (3/2, v/v) as the eluent to give a brown solid of 3b (25 mg, 29% (two steps)). ¹H NMR (400 MHz, CDCl₃, ppm): δ 9.62–9.57 (m, 4H), 8.87 (d, J = 4.6 Hz, 2H), 8.84 (d, J = 4.5 Hz, 2H), 8.15 (d, J = 7.6 Hz, 2H), 7.97 (d, J = 8.2 Hz, 2H), 7.91 (s, 1H), 7.88 (d, J = 4.9 Hz, 2H), 7.76 (s, 1H), 7.73 (d, J = 8.3 Hz, 2H), 7.69 (d, J = 2.4 Hz, 2H), 7.62 (s, 2H), 7.46 (d, J = 8.7 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 8.7 Hz, 2H), 7.38–7.31 (m, 2H), 7.31 (d, J = 2.1 Hz, 1H), 7.30–7.27 (m, 3H), 7.02 (d, J = 8.5 Hz, 4H), 6.93 (d, J = 8.5 Hz, 4H), 6.90–6.86 (m, 2H), 6.84 (d, J = 8.8 Hz, 2H), 3.98 (d, J = 2.0 Hz, 3H), 3.96 (d, J = 6.6 Hz, 2H), 3.91 (s, 5H), 3.90–3.83 (m, 12H), 2.73 (t, J = 7.7 Hz, 2H), 2.67 (t, J = 7.9 Hz, 2H), 1.93–1.82 (m, 4H), 1.81–1.61 (m, 8H), 1.45–1.39 (m, 4H), 1.38–1.31 (m, 20H), 1.30–1.23 (m, 16H), 1.19–1.11 (m, 8H), 1.08–0.96 (m, 22H), 0.94–0.85 (m, 22H), 0.84–0.77 (m, 20H), 0.68–0.60 (m, 8H), 0.59–0.47 (m, 16H), 0.46–0.38 (m, 8H). ¹³C NMR (151 MHz, CDCl₃): δ 166.67, 166.42, 159.91, 159.87, 158.47, 158.35, 152.18, 151.66, 151.36, 150.56, 150.44, 149.45, 144.95, 144.49, 143.39, 143.14, 139.85, 139.51, 138.92, 136.40, 135.47, 135.30, 132.19, 132.15, 131.99, 131.66, 131.06, 130.94, 130.81, 130.78, 130.62, 130.41, 130.26, 129.94, 129.81, 129.64, 129.52, 129.38, 129.27, 129.00, 128.81, 128.45, 128.12, 128.01, 127.84, 127.71, 127.40, 127.31, 126.03, 125.46, 125.42, 125.40, 125.34, 124.63, 124.54, 124.51, 124.36, 124.32, 120.80, 118.77, 118.11, 115.53, 115.45, 115.39, 115.01, 114.97, 114.75, 114.70, 105.16, 101.26, 98.79, 68.62, 68.05, 68.02, 67.98, 47.66, 47.46, 32.03, 31.80, 31.66, 31.62, 31.58, 31.55, 31.47, 31.40, 31.01, 30.97, 30.92, 30.42, 30.36, 30.31, 30.28, 30.22, 30.15, 30.10, 30.03, 29.99, 29.90, 29.89, 29.85, 29.76, 29.74, 29.68, 29.50, 29.43, 29.36, 29.33, 29.31, 29.29, 29.24, 29.21, 29.17, 29.15, 29.07, 29.00, 28.99, 28.70, 28.60, 27.64, 27.18, 27.12, 27.09, 26.90, 26.78, 26.76, 26.68, 26.65, 26.62, 26.56, 26.52, 26.47, 25.71, 25.67, 25.25, 22.64, 22.60, 22.57, 14.12, 14.10, 14.06, 14.01. MS (MALDI-TOF, m/z): [M]⁺ calcd for C₁₈₈H₂₂₈N₈O₁₀S₅Zn, 2981.5; found, 2981.4. FT-IR (ATR, cm⁻¹): 2919 (s), 2849 (s), 2188 (m), 1720 (s), 1600 (s), 1586 (s), 1498 (w), 1454 (s), 1392 (m), 1269 (s), 1240 (s), 1205 (m), 1096 (s), 996 (m), 878 (w), 792 (m), 765 (m), 713 (m). mp 76–78 °C.

XW85 . Compound 3a (25 mg, 0.01 mmol) and LiOH·H₂O (56 mg, 1.3 mmol) were mixed in a solvent of THF/H₂O (10 mL, 4/1, v/v). The mixture was refluxed for 24 h and then cooled down to room temperature. The reaction mixture was acidified with 2 M HCl to pH = 1–3 and extracted with DCM (50 mL × 3). The organic phase was dried over Na₂SO₄, filtered, and then concentrated under reduced pressure. The crude product was purified by silica gel column chromatography with DCM/methanol (30/1, v/v) as the eluent to give a brown solid of XW85 (22 mg, 89%). ¹H NMR (400 MHz, DMSO-d₆: CDCl₃ = 2 : 1(v: v), ppm): δ 9.56 – 9.49 (m, 4H), 8.72 (d, J = 4.6 Hz, 2H), 8.69 (d, J = 4.5 Hz, 2H), 8.17 (s, 2H), 8.15 (s, 2H), 8.05 (d, J = 8.4 Hz, 2H), 8.01–7.97 (m, 3H), 7.92 (s, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.79 (d, J = 7.7 Hz, 1H), 7.76 (d, J = 8.8 Hz, 1H), 7.71–7.66 (m, 2H), 7.66 (d, J = 1.8 Hz, 1H), 7.57 (d, J = 8.2 Hz, 2H), 7.44 (d, J = 8.4 Hz, 2H), 7.37 (d, J = 8.5 Hz, 2H), 7.33 (d, J = 8.1 Hz, 1H), 7.30–7.27 (m, 2H), 7.24 (d, J = 8.0 Hz, 1H), 7.21 (d, J = 1.8 Hz, 1H), 7.03 (d, J = 8.5 Hz, 4H), 6.94 (d, J = 8.5 Hz, 2H), 6.90–6.85 (m, 3H), 6.81 (d, J = 8.5 Hz, 2H), 3.91 (t, J = 6.5 Hz, 4H), 3.83 (t, J = 6.6 Hz, 12H), 2.80–2.75 (m, 2H), 2.71 (t, J = 1.9 Hz, 2H), 1.74–1.64 (m, 8H), 1.44–1.39 (m, 8H), 1.33–1.27 (m, 18H), 1.23–1.18 (m, 12H), 1.16–1.09 (m, 8H), 1.07–1.00 (m, 16H), 0.97–0.91 (m, 14H), 0.90–0.82 (m, 20H), 0.79–0.72 (m, 18H), 0.59–0.50 (m, 14H), 0.40–0.31 (m, 8H). MS (MALDI-TOF, m/z): [M]⁺ calcd for C₁₈₀H₂₁₂N₈O₁₀S₅Zn, 2869.4; found, 2869.3. FT-IR (ATR, cm⁻¹): 2920 (s), 2848 (s), 2187 (m), 1689 (s), 1585 (s), 1500 (w), 1454 (s), 1390 (m), 1242 (s), 1205 (m), 1095 (s), 997 (m), 879 (w), 791 (m), 768 (m), 710 (m). mp 121–123 °C.

XW86 . The synthetic procedure resembles that for synthesizing compound XW85 except using compound 3b (25 mg, 0.01 mmol) instead of 3a. The crude product was purified by silica gel column chromatography with DCM/methanol (30/1, v/v) as the eluent to give a brown solid of XW86 (21 mg, 85%). ¹H NMR (400 MHz, DMSO-d₆: CDCl₃ = 2 : 1 (v: v), ppm): δ 9.56–9.49 (m, 4H), 8.72 (d, J = 4.7 Hz, 2H), 8.69 (d, J = 4.5 Hz, 2H), 8.16 (s, 2H), 8.15 (s, 2H), 8.05 (d, J = 8.4 Hz, 2H), 8.02–7.96 (m, 3H), 7.91 (s, 1H), 7.85 (d, J = 8.3 Hz, 1H), 7.79 (d, J = 7.5 Hz, 1H), 7.75 (d, J = 8.6 Hz, 1H), 7.71–7.66 (m, 2H), 7.66 (d, J = 1.7 Hz, 1H), 7.58 (d, J = 8.1 Hz, 2H), 7.43 (d, J = 8.3 Hz, 2H), 7.37 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.8 Hz, 1H), 7.30–7.26 (m, 2H), 7.24 (d, J = 8.4 Hz, 1H), 7.21 (s, 1H), 7.03 (d, J = 8.5 Hz, 4H), 6.94 (d, J = 8.3 Hz, 2H), 6.88 (d, J = 7.7 Hz, 3H), 6.81 (d, J = 8.4 Hz, 2H), 3.91 (t, J = 6.5 Hz, 4H), 3.84 (t, J = 6.5 Hz, 12H), 2.80–2.75 (m, 2H), 2.68–2.62 (m, 2H), 1.77–1.60 (m, 12H), 1.45–1.38 (m, 8H), 1.35–1.29 (m, 18H), 1.27–1.23 (m, 24H), 1.16–1.10 (m, 8H), 1.08–1.00 (m, 12H), 0.96–0.82 (m, 36H), 0.79–0.72 (m, 16H), 0.59–0.49 (m, 14H), 0.41–0.29 (m, 8H). MS (MALDI-TOF, m/z): [M]⁺ calcd for C₁₈₆H₂₂₄N₈O₁₀S₅Zn, 2953.5; found, 2953.4. FT-IR (ATR, cm⁻¹): 2920 (s), 2848 (s), 2185 (m), 1689 (s), 1600 (s), 1585 (s), 1497 (w), 1456 (s), 1392 (m), 1240 (s), 1203 (m), 1095 (s), 995 (m), 878 (w), 791 (m), 768 (m), 714 (m). mp 108–110 °C.

Further details about characterization figures, fabrication of DSSCs, and photovoltaic behavior measurements can be found in the ESI.†

Author contributions

Jiaxin Luo: formal analysis, investigation, methodology, visualization, and writing – original draft. Yuqing Wang and Glib Baryshnikov: formal analysis. Shaojin Shi, Yuankun Wu, Taochun Ma, and Leyao Wang: investigation. Xinyan Wu, Chengjie Li and Yongshu Xie: conceptualization, formal analysis, investigation, methodology, visualization, writing – review & editing, funding acquisition, and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22131005, 21971063, 22201074, and 22075077), the Program of Shanghai Academic Research Leader (No. 20XD1401400), the Natural Science Foundation of Shanghai (No. 20ZR1414100, 22ZR1416100), and the Fundamental Research Funds for the Central Universities (No. 222201717003).

Notes and references

1. N. Yan, C. Zhao, S. You, Y. Zhang and W. Li, Chin. Chem. Lett., 2020, 31, 643–653.
2. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737–740.
3. A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629–634.
4. S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6, 242–247.
5. Y. Ren, D. Zhang, J. Suo, Y. Cao, F. T. Eickemeyer, N. Vlachopoulos, S. M. Zakeeruddin, A. Hagfeldt and M. Grätzel, Nature, 2023, 613, 60–65.
6. Q. Yu, Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang and P. Wang, ACS Nano, 2010, 4, 6032–6038.
7. C. E. Housecroft and E. C. Constable, Chem. Sci., 2022, 13, 1225–1262.
8. H. Jiang, Y. Ren, W. Zhang, Y. Wu, E. C. Socie, B. I. Carlsen, J.-E. Moser, H. Tian, S. M. Zakeeruddin, W.-H. Zhu and M. Grätzel, Angew. Chem., Int. Ed., 2020, 59, 9324–9329.
9. C. Liao, K. Zeng, H. Wu, Q. Zeng, H. Tang, L. Wang, H. Meier, Y. Xie and D. Cao, Cell Rep. Phys. Sci., 2021, 2, 100326.
10. D. Zhang, M. Stojanovic, Y. Ren, Y. Cao, F. T. Eickemeyer, E. Socie, N. Vlachopoulos, J.-E. Moser, S. M. Zakeeruddin, A. Hagfeldt and M. Grätzel, Nat. Commun., 2021, 12, 1777.
11. L. Zhang, X. Yang, W. Wang, G. G. Gurzadyan, J. Li, X. Li, J. An, Z. Yu, H. Wang, B. Cai, A. Hagfeldt and L. Sun, ACS Energy Lett., 2019, 4, 943–951.
12. Y. K. Eom, I. T. Choi, S. H. Kang, J. Lee, J. Kim, M. J. Ju and H. K. Kim, Adv. Energy Mater., 2015, 5, 1500300.
13. Z. Chai, J. Wang, Y. Xie, P. Lin, H. Li, K. Chang, T. Xu, A. Mei, Q. Peng, M. Wang, H. Han, Q. Li and Z. Li, ACS Appl. Mater. Interfaces, 2019, 11, 27648–27657.
14. Z. Chai, M. Wu, M. Fang, S. Wan, T. Xu, R. Tang, Y. Xie, A. Mei, H. Han, Q. Li and Z. Li, Adv. Energy Mater., 2015, 5, 1500846.
15. K. Zeng, Y. Chen, W.-H. Zhu, H. Tian and Y. Xie, J. Am. Chem. Soc., 2020, 142, 5154–5161.
16. K. Zeng, Y. Lu, W. Tang, S. Zhao, Q. Liu, W. Zhu, H. Tian and Y. Xie, Chem. Sci., 2019, 10, 2186–2192.
17. S. H. Kang, M. J. Jeong, Y. K. Eom, I. T. Choi, S. M. Kwon, Y. Yoo, J. Kim, J. Kwon, J. H. Park and H. K. Kim, Adv. Energy Mater., 2017, 7, 1602117.
18. Y. Kurumisawa, T. Higashino, S. Nimura, Y. Tsuji, H. Iiyama and H. Imahori, J. Am. Chem. Soc., 2019, 141, 9910–9919.
19. T. Higashino, Y. Kurumisawa, A. B. Alemayehu, R. F. Einrem, D. Sahu, D. Packwood, K. Kato, A. Yamakata, A. Ghosh and H. Imahori, ACS Appl. Energy Mater., 2020, 3, 12460–12467.
20. Y. Hu, A. Alsaleh, O. Trinh, F. D'Souza and H. Wang, J. Mater. Chem. A, 2021, 9, 27692–27700.
21. Y. Hu, W. A. Webre, M. B. Thomas, A. Moss, S. N. Hancock, J. Schaffner, F. D'Souza and H. Wang, J. Mater. Chem. A, 2019, 7, 10712–10722.
22. H. Song, Q. Liu and Y. Xie, Chem. Commun., 2018, 54, 1811–1824.
23. K. Zeng, Z. Tong, L. Ma, W.-H. Zhu, W. Wu and Y. Xie, Energy Environ. Sci., 2020, 13, 1617–1657.
24. T. Higashino and H. Imahori, ACS Energy Lett., 2022, 7, 1926–1938.
25. M. Aftabuzzaman, S. Sarker, C. Lu and H. K. Kim, J. Mater. Chem. A, 2021, 9, 24830–24848.
26. Y. Xie, Y. Tang, W. Wu, Y. Wang, J. Liu, X. Li, H. Tian and W.-H. Zhu, J. Am. Chem. Soc., 2015, 137, 14055–14058.
27. J.-M. Ji, H. Zhou, Y. K. Eom, C. H. Kim and H. K. Kim, Adv. Energy Mater., 2020, 10, 2000124.
28. A. Mahmood, J.-Y. Hu, B. Xiao, A. Tang, X. Wang and E. Zhou, J. Mater. Chem. A, 2018, 6, 16769–16797.
29. C.-C. Chen, J.-S. Chen, V. S. Nguyen, T.-C. Wei and C.-Y. Yeh, Angew. Chem., Int. Ed., 2021, 60, 4886–4893.
30. H. Song, J. Zhang, J. Jin, H. Wang and Y. Xie, J. Mater. Chem. C, 2018, 6, 3927–3936.
31. Y. Zhang, K. Ren, L. Wang, L. Wang and Z. Fan, Chin. Chem. Lett., 2022, 33, 33–60.
32. Y. Tang, X. Liu, Y. Wang, Q. Liu, X. Li, C. Li, X. Shen and Y. Xie, Chin. Chem. Lett., 2020, 31, 1927–1930.
33. C.-L. Wang, J.-Y. Hu, C.-H. Wu, H.-H. Kuo, Y.-C. Chang, Z.-J. Lan, H.-P. Wu, E. Wei-Guang Diau and C.-Y. Lin, Energy Environ. Sci., 2014, 7, 1392–1396.
34. S. Chang, H. Wang, Y. Hua, Q. Li, X. Xiao, W.-K. Wong, W. Y. Wong, X. Zhu and T. Chen, J. Mater. Chem. A, 2013, 1, 11553–11558.
35. T. Hua, K. Zhang, Z.-S. Huang, L. Wang, H. Tang, H. Meier and D. Cao, J. Mater. Chem. C, 2019, 7, 10379–10388.
36. J. M. Cole, G. Pepe, O. K. Al Bahri and C. B. Cooper, Chem. Rev., 2019, 119, 7279–7327.
37. Y. Lu, Q. Liu, J. Luo, B. Wang, T. Feng, X. Zhou, X. Liu and Y. Xie, ChemSusChem, 2019, 12, 2802–2809.
38. J. Zou, Y. Tang, G. Baryshnikov, Z. Yang, R. Mao, W. Feng, J. Guan, C. Li and Y. Xie, J. Mater. Chem. A, 2022, 10, 1320–1328.
39. Y. Chen, Y. Tang, J. Zou, K. Zeng, G. Baryshnikov, C. Li and Y. Xie, ACS Appl. Mater. Interfaces, 2021, 13, 49828–49839.
40. J. Luo, Z. Xie, J. Zou, X. Wu, X. Gong, C. Li and Y. Xie, Chin. Chem. Lett., 2022, 33, 4313–4316.
41. J. Zou, Y. Wang, G. Baryshnikov, J. Luo, X. Wang, H. Ågren, C. Li and Y. Xie, ACS Appl. Mater. Interfaces, 2022, 14, 33274–33284.
42. S. Jiang, S. Fan, X. Lu, G. Zhou and Z.-S. Wang, J. Mater. Chem. A, 2014, 2, 17153–17164.
43. Z.-S. Huang, C. Cai, X.-F. Zang, Z. Iqbal, H. Zeng, D.-B. Kuang, L. Wang, H. Meier and D. Cao, J. Mater. Chem. A, 2015, 3, 1333–1344.
44. C.-Y. Lo, D. Kumar, S.-H. Chou, C.-H. Chen, C.-H. Tsai, S.-H. Liu, P.-T. Chou and K.-T. Wong, ACS Appl. Mater. Interfaces, 2016, 8, 27832–27842.
45. H. Meier, Z.-S. Huang and D. Cao, J. Mater. Chem. C, 2017, 5, 9828–9837.
46. Z. Wang, Q. Chen, Y. Zou, J. Chen, Y. Luo, Y. Liu, S. Ding, P. Cai, J. Yuan and M. Liang, Dyes Pigm., 2021, 187, 109134.
47. Y. H. Lee, R. K. Chitumalla, B. Y. Jang, J. Jang, S. Thogiti and J. H. Kim, Dyes Pigm., 2016, 133, 161–172.
48. K. Zeng, W. Tang, C. Li, Y. Chen, S. Zhao, Q. Liu and Y. Xie, J. Mater. Chem. A, 2019, 7, 20854–20860.
49. J. An, X. Yang, B. Cai, L. Zhang, K. Yang, Z. Yu, X. Wang, A. Hagfeldt and L. Sun, ACS Appl. Mater. Interfaces, 2020, 12, 46397–46405.
50. S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo and A. Hagfeldt, J. Am. Chem. Soc., 2010, 132, 16714–16724.
51. S. C. Pradhan, A. Hagfeldt and S. Soman, J. Mater. Chem. A, 2018, 6, 22204–22214.
52. M. Pazoki, U. B. Cappel, E. M. J. Johansson, A. Hagfeldt and G. Boschloo, Energy Environ. Sci., 2017, 10, 672–709.
53. Y. Wang, L. Yang, M. Xu, M. Zhang, Y. Cai, R. Li and P. Wang, J. Phys. Chem. C, 2014, 118, 16441–16446.
54. R. Ambre, K.-B. Chen, C.-F. Yao, L. Luo, E. W.-G. Diau and C.-H. Hung, J. Phys. Chem. C, 2012, 116, 11907–11916.
55. W. Zhang, X. Dai, X. Liao, X. Zang, H. Zhang, X. Yin, C. Yu, C. Ke and Y. Hong, Sol. Energy, 2020, 212, 220–230.
56. J. J. P. Stewart, J. Mol. Model., 2007, 13, 1173–1213.
57. M. Korth, M. Pitoňák, J. Řezáč and P. Hobza, J. Chem. Theory Comput., 2010, 6, 344–352.
58. J. Bisquert, F. Fabregat-Santiago, I. Mora-Seró, G. Garcia-Belmonte and S. Giménez, J. Phys. Chem. C, 2009, 113, 17278–17290.
59. S. Vijaya, G. Landi, H.-C. Neitzert and S. Anandan, Phys. Chem. Chem. Phys., 2020, 22, 18183–18191.
60. M. Kokkonen, P. Talebi, J. Zhou, S. Asgari, S. A. Soomro, F. Elsehrawy, J. Halme, S. Ahmad, A. Hagfeldt and S. G. Hashmi, J. Mater. Chem. A, 2021, 9, 10527–10545.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc05167b

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