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Tuning intramolecular charge transfer in conjugated polyphenylenes through benzoquinone functionalization and post-polymerization modificationsMasahiroTomita1
KoheiNishitani1
IsaoYamaguchi1✉Emailiyamaguchi@riko.shimane-u.ac.jp
1Department of Materials ChemistryShimane University1060, 690- 8504Nishikawatsu, MatsueJapan
Masahiro Tomita • Kohei Nishitani • Isao Yamaguchi*
Department of Materials Chemistry, Shimane University, 1060 Nishikawatsu, Matsue 690–8504, Japan
Email: iyamaguchi@riko.shimane-u.ac.jp
Abstract
Charge-transfer (CT) type π-conjugated polymers (CPs) comprising donor and acceptor aromatic units offer tunable optical and electrochemical properties, which are important for electronic and photonic applications. In this study, we synthesized CT-type polyphenylenes, P(Flu-BQ) and P(Ph-BQ), containing 9,9-dihexylfluorene or 1,4-dihexyloxybenzene as donor units and benzoquinone (BQ) as an acceptor. Reduction of BQ to hydroquinone (HQ) allowed systematic investigation of CT effects by comparison of optical and electrochemical properties before and after reduction. Polymers composed of BQ and HQ units, P(BQ-HQ), were also prepared via sulfuric acid-mediated polymerization, and their composition ratios were controlled by varying acid concentration. Subsequent reactions of HQ units yielded acetylated polymers, P(BQ-AcQ), and TCNQ-substituted polymers, P(TCNQ-AcQ), to modulate CT characteristics. UV–vis absorption, photoluminescence, fluorescence lifetime, and cyclic voltammetry studies revealed that CT along the polymer backbone strongly influences emission behavior and redox properties. Specifically, reduction of BQ suppressed intramolecular CT (ICT), leading to enhanced fluorescence in P(Flu-BQ) and P(Ph-BQ), while incorporation of TCNQ units enhanced CT in P(TCNQ-AcQ), as confirmed by formation of a 1:1 CT complex with 2,6-dimethyltetrathiafulvalene (DM-TTF). These results provide a clear relationship between CT and the optical/electrochemical properties of CPs, highlighting strategies to tune polymer functionality via donor–acceptor design.
Keywords -
πconjugated polymer
donor–acceptor system
charge-transfer interaction
benzoquinone
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Introduction
Charge-transfer (CT) type π-conjugated polymers (CPs) composed of donor-type and acceptor-type aromatic rings typically exhibit lower band gaps and more efficient charge carrier transport compared to conventional CPs [1, 2]. Moreover, the optical and electrochemical properties of CT-type CPs can be tuned by introducing substituents on the donor and acceptor aromatic rings, making them suitable for applications such as organic solar cell materials [3 − 7], organic thin-film transistors [8 − 12], and nonlinear optical materials [13,14]. To elucidate the influence of CT on the properties of typical CT-type CPs, it is important to synthesize reference CPs lacking either donor or acceptor units and to compare the properties of both systems. In this study, to facilitate straightforward evaluation of the effect of CT on CP properties, we synthesized CT-type π-conjugated polyphenylenes, P(Flu-BQ) and P(Ph-BQ), using 9,9-dioctylfluorene-2,7-diyl (Flu) or 1,4-dihexyloxybenzene-2,5-diyl (Ph) as donor units, and 1,4-benzoquinone (BQ) as an acceptor unit. In these polymers, reduction of the BQ units to hydroquinone (HQ) units eliminates the acceptor character and suppresses CT within the polymer. Therefore, by comparing the properties of the polymers before and after reduction of the BQ units, the effect of CT on their properties can be clarified. To date, reports of CPs containing BQ units in the main chain are limited [15–17], and, to the best of our knowledge, no examples of CT-type CPs with BQ units in the main chain have been reported.
It is known that BQ can polymerize upon reaction with sulfuric acid, yielding polymers containing HQ units reduced by the acid (P(BQ-HQ)) [18]. P(BQ-HQ) functions as a CT-type CP in which HQ units act as donors and BQ units as acceptors. However, there have been no studies focusing on controlling the composition ratio of P(BQ-HQ) or evaluating its properties with respect to CT. In this study, we investigated whether the composition ratio of P(BQ-HQ) could be controlled by varying the sulfuric acid concentration during BQ polymerization. Furthermore, we performed polymer reactions utilizing the reactive hydroxyl and carbonyl groups of P(BQ-HQ). Specifically, HQ units were acetylated to yield P(BQ-AcQ), and BQ units were converted to 7,7,8,8-tetracyanoquinodimethane (TCNQ) units to obtain P(TCNQ-AcQ). Because AcQ units are more π-electron deficient than HQ, P(BQ-AcQ) is expected to exhibit lower CT character than P(BQ-HQ). Conversely, TCNQ units are more electron-deficient than BQ, and thus CT is anticipated to be enhanced in P(TCNQ-AcQ) relative to P(BQ-AcQ).
In this paper, we report the relationship between CT and optical properties based on UV–vis absorption, photoluminescence (PL), and fluorescence lifetime measurements of P(Flu-BQ) and P(Ph-BQ) before and after BQ reduction. We also present the results of composition control for P(BQ-HQ) and the CT behavior before and after polymer reactions on HQ units. In addition to optical properties, the electrochemical properties of the synthesized polymers were evaluated using cyclic voltammetry (CV). These optical and electrochemical data are expected to provide a foundation for understanding the relationship between CT and the properties of CPs.
Experimental section
Reagents and measurements
Reagents were obtained and used without further purification. Solvents were dried, distilled, and stored under a nitrogen atmosphere. All reactions were performed under nitrogen using standard Schlenk techniques.
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IR spectra were recorded using a JASCO FT/IR-660 PLUS spectrophotometer using the KBr pellet method. 1H NMR spectra were collected on a JEOL ECX-500 spectrometer. GPC analyses were carried out with a TOSO HLC-8320 system equipped with polystyrene gel columns and an RI detector, with DMF containing 0.06 M LiBr as the eluent. UV-vis and PL spectra were measured using JASCO V-560 spectrometer and JASCO FP-6200 spectrometers, respectively. Fluorescence lifetime measurements were determined on a HORIBA FluoroCube Model1000U with a 340 nm diode laser (Horiba NanoLED) for excitation. An aqueous suspension of TM-40 colloidal silica (40wt%) was used for prompt measurements. Cyclic voltammetry was performed using a Hokuto Denko HSV-110 electrochemical analyzer. Platinum plates (1 cm × 1 cm and 1 cm × 2 cm) served as the working and counter electrodes, respectively, and a silver wire was used as the reference electrode. Tetraethylammonium tetrafluoroborate was employed as the supporting electrolyte, and the scan rate was 50 mV•s− 1.Synthesis of P(Flu-BQ)
M1 (0.48 g, 1.0 mmol) and M3 (0.27 g, 1.0 mmol) were dissolved in 10 mL of dried THF. To this solution, Pd(PPh3)4 (0.12 g, 0.10 mmol) and 2 M K2CO3(aq) (2.5 mL), which had been bubbled with nitrogen prior to use, were added. The reaction mixture was then refluxed for 72 h. After completion, the solvent was removed under vacuum. The resulting solid was washed with 200 mL of water, and the water-insoluble fraction was collected by filtration, dissolved in 2 mL of dichloromethane, and reprecipitated from 200 mL of methanol. The precipitate was collected by filtration and dried under vacuum to afford P(Flu-BQ) as a vermilion powder (0.096 g, 18%). 1H NMR (500 MHz, CDCl3): d 7.00-7.76 (8H), 1.96 (4H), 1.05 (20H), 0.74 (10H). IR (KBr, cm− 1): 3059 (w), 2925 (s), 2853 (s), 1652 (m), 1586 (m), 1462 (m), 1173 (s), 891 (w), 819 (m), 743 (w).
Synthesis of P(Ph-BQ)
M2 (0.37 g, 1.0 mmol) and M3 (0.27 g, 1.0 mmol) were dissolved in 10 mL of dried THF. To this solution, Pd(PPh3)4 (0.12 g, 0.10 mmol) and 2 M K2CO3(aq) (2.5 mL), which had been bubbled with nitrogen prior to use, were added. The reaction solution was then refluxed for 48 h. After completion, the solvent was removed under vacuum. The resulting solid was washed sequentially with 200 mL of water and 20 mL methanol, collected by filtration, and dried under vacuum to afford P(Ph-BQ) as a dark brown powder (0.036 g, 9%). 1H NMR (500 MHz, CDCl3): d 6.89–7.05 (4H), 3.89 (4H), 1.77 (4H), 1.30 (12H), 0.86 (6H). IR (KBr, cm− 1): 2931 (s), 2859 (m), 1655 (m), 1595 (m), 1468 (m), 1378 (m), 1210 (s), 1028 (m), 804 (w).
Synthesis of P(BQ-HQ)-a
p-Benzoquinone (9.0 g, 84 mmol) was dissolved in 150 mL of 0.061 M sulfuric acid. The reaction mixture was refluxed for 5 h. After completion, the precipitate was collected by filtration, washed several times with 150 mL of water at 75 ˚C. The water-insoluble fraction was collected by filtration and dried under vacuum to afford P(BQ-HQ)-a as a black powder (2.5 g, 28%). IR (KBr, cm− 1): 3450 (ཌྷ), 1794 (w), 1619 (m), 1503 (s), 1344 (m), 1200 (s), 817 (m).
P(BQ-HQ)-b and P(BQ-HQ)-c were synthesized following the same procedure as that used for the preparation of P(BQ-HQ)-a.
Synthesis of P(BQ-AcQ)-a
P(BQ-HQ)-a (2.8 g, 13 mmol) and sulfuric acid (0.14 g, 1.4 mmol) were added to 10 mL of acetic anhydrite. The reaction mixture was ultrasonically stirred for 10 min, then poured in 100 mL of water. The precipitate was collected by filtration, washed with 300 mL of water, and dried under vacuum to afford P(BQ-AcQ)-a as a black solid (2.1 g, 64%). 1H NMR (500 MHz, DMSO-d6): d 6.60–7.73 (4H), 2.33 (1.2H). IR (KBr, cm− 1): 1790 (w), 1621 (m), 1500 (s), 1346 (w), 1199 (s), 816 (m).
P(BQ-AcQ)-b and P(BQ-AcQ)-c were synthesized following the same procedure as that used for the preparation of P(BQ-AcQ)-a. IR data of P(BQ-AcQ)-b (KBr, cm− 1): 1759 (m), 1622 (m), 1496 (s), 1369 (m), 1199 (s), 820 (m). IR data of P(BQ-AcQ)-c (KBr, cm− 1): 1764 (s), 1623 (w), 1479 (m), 1370 (m), 1187 (s), 1015(w), 825 (w).
Synthesis of P(TCNQ-AcQ)
P(BQ-AcQ)-a (56 mg, 0.16 mmol) and malononitrile (0.33 g, 5.0 mmol) was dissolved in 51 mL of dry THF and stirred at 0 ˚C for 15 min. A solution of TiCl4 (1.1 mL, 10 mmol) in 1.6 mL of dry pyridine solution was added dropwise to the mixture. The reaction mixture was refluxed for 36 h. The precipitate was separated by centrifugation, and the solution was concentrated to 2 mL under reduced pressure before being poured into 200 mL of water. The resulting precipitate was collected by filtration and dried under vacuum to afford P(TCNQ-AcQ) as a black solid (88 mg, 10%). 1H NMR (500 MHz, CDCl3): d 6.89–7.05 (4H), 2.25 (1.2H). IR (KBr, cm− 1): 2202 (w), 1624 (m), 1494 (w), 1207 (s), 1157 (s), 821 (m), 501 (m).
DFT calculations
All theoretical calculations were carried out using Firefly version 8.2.0. Full geometry optimizations were performed at the DFT level using the hybrid exchange-correlation functional B3LYP and the 6-31G+(d) basis set. The Cartesian coordinates obtained from the DFT calculations are provided in the Supporting Information.
Results and discussion
Synthesis
P(Flu-BQ) and P(Ph-BQ) were synthesized via Suzuki–Miyaura coupling of 9,9-dioctylfluorene-2,7-diboronic acid (M1) or 2,5-bis(hexloxy)benzene-1,4-diboronic acid (M2) with 2,5-dibromobenzoquinone (M3), affording the polymers in 18% and 9% yields, respectively (Scheme 1). It has been reported that benzoquinone can form complexes with palladium, which may account for the low yields [19, 20]. Additionally, the purification steps to remove palladium from the products also contributed to yield loss.
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Polymers composed of BQ and HQ units, P(BQ-HQ)-a, P(BQ-HQ)-b, and P(BQ-HQ)-c, were synthesized by oxidative polymerization of BQ in aqueous sulfuric acid at concentrations of 11 mol%, 15 mol%, and 20 mol%, respectively (Scheme 2a). These polymers were subsequently reacted with acetic anhydride to acetylate the HQ units, yielding P(BQ-AcQ)-a, P(BQ-AcQ)-b, and P(BQ-AcQ)-c (Scheme 2). Furthermore, P(BQ-AcQ)-a was reacted with malononitrile to convert the BQ unit into TCNQ unit, affording P(TCNQ-AcQ) (Scheme 2). The results of these reactions are summarized in Table 1.P(Flu-BQ) and P(Ph-BQ) were soluble in chloroform, dichloromethane, N,N-dimethylformamide (DMF), and tetrahydrofuran (THF) due to the presence of alkyl substituents. P(BQ-HQ) and P(BQ-AcQ) were soluble in DMF and dimethyl sulfoxide (DMSO), partially soluble in acetone and methanol, and insoluble in water. P(TCNQ-AcQ) was soluble in DMSO but insoluble in acetone and ethanol.
Scheme 1 Synthesis of P(Flu-BQ) and P(Ph-BQ)
Polymer | H2SO4 (mol%)a | Yield (%) | BQ : AcQb | Mnc | Mwc |
|---|---|---|---|---|---|
P(Flu-BQ) | - | 18 | - | 6700 | 7500 |
P(Ph-BQ) | - | 9 | - | 6500 | 7700 |
P(BQ-HQ)-a | 11 | 22 | - | -d | -d |
P(BQ-HQ)-b | 15 | 31 | - | -d | -d |
P(BQ-HQ)-c | 20 | 29 | - | -d | -d |
P(BQ-AcQ)-a | - | 64 | 79 : 21 | 6800 | 7800 |
P(BQ-AcQ)-b | - | 40 | 56 : 44 | 7000 | 8100 |
P(BQ-AcQ)-c | - | 74 | 30 : 70 | 12000 | 14500 |
P(TCNQ-AcQ)-a | - | 10 | - | -d | -d |
a Amount of H2SO4 used for the synthesis of P(BQ-HQ). b Calculated from the IR absorbance. c Determined by GPC (eluent = DMF containing 0.06M LiBr). d Not measured.
IR and NMR spectra
In the IR spectra of M1 and M2, absorptions attributable to B–O stretching vibrations were observed at 1340 cm− 1 and 1326 cm− 1, respectively, while M3 exhibited an absorption at 992 cm− 1 corresponding to the C–Br stretching vibration. In the IR spectra of P(Flu-BQ) and P(Ph-BQ), these absorptions disappeared, confirming the progression of the polymerization reaction (Figure S1). Additionally, the C = O stretching vibrations of P(Flu-BQ) and P(Ph-BQ) were observed at 1653 cm− 1 and 1654 cm− 1, respectively.
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Figure 1 shows the IR spectra of P(BQ-HQ)-a, P(BQ-AcQ)-a, -b, -c, and P(TCNQ-AcQ). In the IR spectrum of P(BQ-HQ)-a, a broad absorption attributable to O–H stretching was observed at 3360 cm− 1, and the C = O stretching vibration appeared at 1620 cm− 1. For P(BQ-AcQ)-a, -b, and -c, the C = O stretching vibrations of the BQ and AcQ units were observed at 1760 cm− 1 and 1650 cm− 1, respectively. The composition of BQ and AcQ units in P(BQ-AcQ)-a, -b, and -c was calculated from the absorbance ratios of these peaks. As shown in Table 1, the content of AcQ units increased with increasing sulfuric acid concentration used during the polymerization of BQ. This result supports that the HQ unit in P(BQ-HQ) was generated by reduction of BQ unit with aqueous sulfuric acid. In the IR spectrum of P(TCNQ-AcQ), the intensity of the C = O stretching vibration of the BQ unit decreased, and a new absorption corresponding to the C ≡ N stretching vibration of the TCNQ unit appeared at 2202 cm− 1.A
Figure 2 shows the ¹H NMR spectra of P(Flu-BQ) and P(Ph-BQ), with peak assignments indicated. For P(Flu-BQ), aromatic and alkyl proton signals were observed at δ 7.0–7.8 and δ 0.7–1.9, respectively. The integration values of these peaks were consistent with the polymer structure. For P(Ph-BQ), aromatic and alkyl proton signals appeared at δ 6.9–7.0 and δ 0.9–3.9, respectively, and the integration values also agreed with the polymer structure. Figure 3 presents the ¹H NMR spectra of P(BQ-HQ)-c and P(BQ-AcQ)-c. Signals attributable to benzene ring protons and acetyl protons were observed at δ 6.5–8.2 and around δ 2.2, respectively. The integration ratio of these peaks was 2:1.2, which is roughly consistent with the composition ratio of BQ and AcQ units calculated from IR absorbances. Similarly, for P(BQ-AcQ)-b and P(BQ-AcQ)-c, the integration values of benzene and acetyl protons were also consistent with the composition ratios of BQ and AcQ units determined from IR spectroscopy.Figure 1. IR spectra of (a) P(BQ-HQ)-a, (b) P(BQ-AcQ)-a, (c) P(BQ-AcQ)-b, (d) P(BQ-AcQ)-c, and (e) P(TCNQ-AcQ)
Fig. 2
1H NMR spectra of P(Flu-BQ) and P(Ph-BQ) in CDCl3
Figure 3. 1H NMR spectra of (a) P(BQ-AcQ)-c and (b) P(TCNQ-AcQ) in DMSO-d6
UV-vis spectra
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Table 2 summarizes the optical properties of the polymers synthesized in this study. Figures 4a and 4b show the UV–vis spectra of P(Flu-BQ), P(Ph-BQ), and M1 in DMSO. The absorption maxima (λmax) of P(Flu-BQ) and P(Ph-BQ) were observed at 342 nm and 316 nm, respectively. These values are red-shifted compared to that of M1 (λmax = 315 nm), indicating that the π-conjugation along the polymer backbone is extended in both P(Flu-BQ) and P(Ph-BQ). Moreover, the λmax values suggest that the π-conjugation is more extended in P(Flu-BQ) than in P(Ph-BQ). The difference in the degree of π-conjugation between the two polymers is attributed to the torsion of the polymer backbone. To confirm this, DFT calculations were performed on the structural units Flu-BQ and Ph-BQ, revealing that the dihedral angle of the blue-highlighted portion in Flu-BQ is 37.7° and that in Ph-BQ is 40.3° (Fig. 5). These results suggest that the backbone of P(Flu-BQ) is less twisted than that of P(Ph-BQ), contributing to the greater π-conjugation in P(Flu-BQ).The absorptions at 445 nm for P(Flu-BQ) and 462 nm for P(Ph-BQ) are attributed to intramolecular charge transfer (ICT) bands from the π-electron-rich 9,9-dihexylfluorene or 1,4-dihexyloxybenzene units to the electron-deficient BQ unit. This assignment was further confirmed by the disappearance of these bands upon addition of NaBH4 to solutions of P(Flu-BQ) and P(Ph-BQ) (Figs. 4a and 4b). The result indicates that reduction of the BQ units to HQ units by NaBH4 decreased their electron-withdrawing ability, thereby suppressing CT.
Figure 4c shows the UV–vis spectra of BQ, P(BQ-AcQ)-a, P(BQ-AcQ)-b, P(BQ-AcQ)-c, and P(TCNQ-AcQ) in DMSO. The absorption onset (λonset) of P(BQ-AcQ)-a, -b, and -c was observed at 670 nm, which is red-shifted compared to that of BQ (λonset = 330 nm), suggesting that π-conjugation along the polymer backbone is extended in these polymers. No absorption attributable to ICT from the electron-donating AcQ units to the electron-accepting BQ units was observed in P(BQ-AcQ). In contrast, the UV–vis spectrum of P(TCNQ-AcQ) in DMSO showed an absorption at 530 nm, corresponding to ICT from AcQ unit to TCNQ unit. The reported electron affinities of TCNQ and BQ are 2.82 eV [21] and 1.89 eV [22], respectively. Thus, ICT occurs in P(TCNQ-AcQ) but not in P(BQ-AcQ) because the electron-accepting ability of TCNQ is higher than that of BQ. Upon addition of 2,3-dimethyl-tetrathiafulvalene (DM-TTF) to a DMSO solution of P(TCNQ-AcQ), the absorption at 530 nm disappeared, and a new absorption appeared at 505 nm. This result indicates that the ICT in P(TCNQ-AcQ) was suppressed, and a charge-transfer complex was formed between the TCNQ units and DM-TTF, which is more electron-donating than AcQ.
Polymer | Absorption/ nma | Emission/ nma | t/ nsa | |||
|---|---|---|---|---|---|---|
Without additive | With additive | Without additive | With additive | Without additive | With additive | |
P(Flu-BQ) | 342, 445 | 350c | 410 | 414c | 0.77 | 0.77c |
P(Ph-BQ) | 316, 462 | 341c | 411 | 414c | 1.21 | 1.39c |
P(BQ-AcQ)-a | 304, 670b | -d | - | - | - | - |
P(BQ-AcQ)-b | 306, 670b | -d | - | - | - | - |
P(BQ-AcQ)-c | 306, 670b | -d | - | - | - | - |
P(TCNQ-AcQ) | 530, 700b | 505e, 740b | 379 | 380e | 2.44 | 2.25e |
a In DMSO. b Onset wavelength. c With NaBH4. d Not measured. e With DM-TTF.
Figure 4. (a) UV-vis spectra in DMSO of M1 (black curve), P(Flu-BQ) (red solid curve), and P(Flu-BQ) upon addition of NaBH4 (red hashed curve). (b) UV-vis spectra in DMSO of P(Ph-BQ) (blue solid curve) and P(Ph-BQ) upon addition of NaBH4 (blue hashed curve). (c) UV-vis spectra in DMSO of BQ (black curve), P(BQ-AcQ)-a (purple curve), P(BQ-AcQ)-b (green curve), P(BQ-AcQ)-c (yellow curve), P(TCNQ-AcQ) (red solid curve), and P(TCNQ-AcQ) upon addition of DM-TTF (red hashed curve)
Fig. 5
Structures of Flu-BQ and Ph-BQ
PL spectra
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Figures 6a and 6b show the photoluminescence (PL) spectra of P(Flu-BQ) and P(Ph-BQ) in dichloromethane before and after the addition of NaBH4. Upon the addition of NaBH4, the PL intensities of both polymers increased markedly, accompanied by a slightly red shift. In π-conjugated polymers, it has been reported that CT along the polymer backbone leads to a decrease in emission intensity [23]. Therefore, the observed increase in PL intensity upon addition of NaBH4 is attributed to the suppression of ICT.P(BQ-AcQ) exhibited no detectable fluorescence, whereas P(TCNQ-AcQ) showed distinct emission (Fig. 6c). This contrast can be rationalized by the different nature of the CT excited states in the two systems. In P(BQ-AcQ), the strong electron-accepting ability of the BQ unit stabilizes the CT state at a deep energy level, promoting efficient nonradiative deactivation via internal conversion and intersystem crossing, which completely quenches fluorescence. In contrast, P(TCNQ-AcQ) forms a more delocalized and partially charge-separated excited state, in which radiative recombination competes effectively with nonradiative decay. The extended π-conjugation and moderate D–A coupling along the P(TCNQ-AcQ) backbone thus facilitate observable fluorescence emission. The PL intensity of P(TCNQ-AcQ) gradually decreased with increasing amounts of 2,6-dimethyltetrathiafulvalene (DM-TTF) (Fig. 6b). This observation suggestes that the addition of DM-TTF perturbs the ICT of P(TCNQ-AcQ), leading instead to the formation of an intermolecular CT complex between the TCNQ units of P(TCNQ-AcQ) and DM-TTF. Figure 6c shows the Stern–Volmer plot for the PL quenching of P(TCNQ-AcQ) by DM-TTF. Using the equation
where
is the PL intensity in the absence of the quencher and
is the PL intensity in the presence of the quencher, the Stern–Volmer constant was determined to be
M−1. The formation of the complex between P(TCNQ-AcQ) and DM-TTF was further analyzed using the Benesi–Hildebrand method according to the equation
where
and
are the PL intensities of P(TCNQ-AcQ) in the absence and presence of DM-TTF, respectively,
is the PL intensity at saturation,
is the concentration of DM-TTF, and
is the association constant. Figure 6d shows the 1:1 Benesi–Hildebrand plot of
versus
. The observed linearity of the plot indicates that P(TCNQ-AcQ) and DM-TTF form a 1:1 CT complex.
PL lifetimes
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Figure 7a shows the fluorescence lifetime profiles of P(TCNQ-AcQ) before and after the addition of DM-TTF. The fluorescence lifetimes of P(TCNQ-AcQ) were 2.44 ns and 2.25 ns before and after DM-TTF addition, respectively. The decrease in fluorescence lifetime upon DM-TTF addition is attributed to the suppression of ICT and the formation of a charge-transfer (CT) complex between the TCNQ units and DM-TTF. It has been reported that the formation of CT complexes in emissive polymers leads to a reduction in fluorescence lifetime [24, 25].Fluorescence lifetime measurements were also performed for P(Flu-BQ) and P(Ph-BQ). In P(Flu-BQ) solution, the fluorescence lifetime was 0.77 ns and remained essentially unchanged upon addition of NaBH4 (Fig. 7b). In contrast, for P(Ph-BQ), the fluorescence lifetime increased from 1.21 ns to 1.39 ns after NaBH4 treatment (Fig. 7c). These results can be explained in terms of the role of ICT states. In the P(Flu-BQ) system, the partial ICT from the Flu donor to the BQ acceptor generates a non-emissive state, which does not significantly contribute to the observed fluorescence; therefore, NaBH4-mediated suppresses ICT has little effect on the fluorescence lifetime. Conversely, in P(Ph-BQ), the ICT state competes with radiative decay by providing a non-radiative relaxation pathway. Reduction of BQ units by NaBH4 suppresses ICT, decreasing the non-radiative decay rate and leading to an increase in the fluorescence lifetime. The difference in ICT behavior between the two systems likely arises from variations in donor structure and electron–acceptor interactions. The rigid, extended π-conjugation of Flu favors partial charge transfer that predominantly populates a non-emissive state, whereas the smaller, more flexible Ph donor allows ICT states that are energetically accessible and partially coupled to the radiative decay channel.
Figure 7.
Cyclic voltammograms
The electrochemical properties of the synthesized polymers were evaluated by cyclic voltammetry (CV). Figure 8a shows the cyclic voltammograms of P(BQ-AcQ)-a, P(BQ-AcQ)-b, and P(BQ-AcQ)-c. The cathodic peak potentials (Epc) for reduction were observed at lower potentials for polymers with higher BQ unit content. This result indicates that the BQ unit functions as an electron acceptor.
Figures 8b and 8c show the cyclic voltammograms of cast films of P(Flu-BQ) and P(Ph-BQ). The BQ units in both P(Flu-BQ) and P(Ph-BQ) undergo a two-step electrochemical reduction, from the BQ radical anion to the BQ dianion. For P(Flu-BQ), the reduction potentials for the formation of the BQ radical anion (Epc(1)) and the BQ dianion (Epc(2)) were − 1.39 V and − 1.53 V (vs. Ag⁺/Ag), respectively. For P(Ph-BQ), the corresponding potentials were Epc(1) = -1.38 V and Epc(2) = -1.85 V (vs. Ag⁺/Ag). The lower Epc(2) of P(Flu-BQ) compared to P(Ph-BQ) is attributed to the more extended π-conjugation in P(Flu-BQ).
Fig. 8
CV curves of (a) electrolyte solutions of P(BQ-AcQ)-a (purple), P(BQ-AcQ)-b (green), and P(BQ-AcQ)-c (yellow), (b) P(Flu-BQ) cast film, and (c) P(Ph-BQ) cast film
Conclusion
We synthesized and characterized a series of CT-type π-conjugated polymers to investigate the influence of donor–acceptor interactions on polymer properties. P(Flu-BQ) and P(Ph-BQ) were prepared via Suzuki–Miyaura coupling, and BQ reduction to HQ enabled evaluation of CT suppression effects. P(BQ-HQ) polymers were obtained through sulfuric acid-mediated polymerization, and their composition ratios were controlled by acid concentration. Subsequent functionalization afforded P(BQ-AcQ) and P(TCNQ-AcQ) with tunable CT characteristics. UV–vis spectroscopy revealed extended π-conjugation and ICT absorption bands, which disappeared upon BQ reduction, indicating CT suppression. Photoluminescence studies showed enhanced emission intensity and modified lifetimes after reduction, while TCNQ incorporation promoted CT and allowed formation of a charge-transfer complex with DM-TTF, confirmed by PL quenching and Benesi–Hildebrand analysis. Cyclic voltammetry demonstrated that BQ unit acts as an electron acceptor, with reduction potentials dependent on polymer composition and π-conjugation. Overall, the study establishes a direct correlation between CT strength and optical/electrochemical properties, providing guidance for designing functional CPs with controlled electronic interactions.
Supplementary Information The online version contains supplementary material available at
Author contributions Masahiro Tomita: Formal Analysis, Investigation, Writing–original draft. Kohei Nishitani: Formal Analysis, Investigation. Isao Yamaguchi: Conceptualization, Supervision, Project administration, Writing–review & editing.
Funding No funding was received for this research.
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Data availability
Data sharing does not apply to this article as no datasets were generated or analyzed during the current study.
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DeclarationsConflict of interest
The authors declare that they have no competing interests.
Electronic Supplementary Material
Below is the link to the electronic supplementary material
References
1.
Shahjad PA (2024) Impact of composition ratio of donor and acceptor moieties in conjugated polymer: optical and electrochemical properties. Mol Syst Des Eng 9:754–764
2.
Reynolds JR, Thompson BC, Skotheim TA (2019) Conjugated polymers: perspective, theory, and new materials. CRC, New York
A
3.Yang C, Cheng B, Xu J, Yu J, Cao S (2024) Donor-acceptor-based conjugated polymers for photocatalytic energy conversion. EnergyChem 6:100116
A
4.Mdluli SB, Ramoroka ME, Yussuf ST, Modibane KD, John-Denk VS, Iwuoha EI (2022) π-Conjugated polymers and their application in organic and hybrid organic-silicon solar cells. Polymers 14:716 5. Holliday S, Li Y, Luscombe CK (2017) Recent advances in high performance donor-acceptor polymers for organic photovoltaics. Prog Polym Sci 70:34 – 51 6. Holliday S, Li Y, Luscombe CK (2017) Recent advances in high performance donor-acceptor polymers for organic photovoltaics. 70:34 – 51 7. Wu JS, Cheng SW, Cheng YJ, Hsu CS (2015) Donor–acceptor conjugated polymers based on multifused ladder-type arenes for organic solar cells. Chem Soc Rev 44:1113 – 1154
A
8.Cao X, Han Y Control of donor – acceptor conjugated polymer crystallization for optimized film structures in organic transistors. Polym Sci Technol 1:413 – 435 9., Ren S, Wang Z, Chen J, Wang S, Yi Z (2025) (2024) Organic transistors based on highly crystalline donor–acceptor π-conjugated polymer of pentathiophene and diketopyrrolopyrrole. Molecules 29:457
10.
Li YF, Guo YL, Liu YQ (2023) Recent progress in donor-acceptor type conjugated polymers for organic field-effect transistors. Chin J Poym Sci 41:652–670
11.
Ren S, Wang Z, Zhang W, Ding Y, Yi Z (2023) Donor-acceptor-based organic polymer semiconductor materials to achieve high hole mobility in organic field-effect transistors. Polymers 15:3713
A
12.Doumbia A, Tong J, Wilson RJ, Turner ML (2021) Investigation of the performance of donor–acceptor conjugated polymers in electrolyte-gated organic field-effect transistors. Adv Electron Mater 7:2100071
13.
Liu Z, Sun J, Yan C, Xie Z, Zhang G, Shao X, Zhang D, Zhou S Diketopyrrolopyrrole based donor–acceptor π-conjugated copolymers with near-infrared absorption for 532 and 1064 nm nonlinear optical materials. J Mater Chem C 8:12993 – 13000 14., Ellinger S, Graham KR, Shi P, Farley RT, Steckler TT, Brookins RN, Taranekar P, Mei J, Padilha LA, Ensley TR, Hu H, Webster S, Hagan DJ, Stryland EWV, Schanze KS, Reynolds JR (2020) (2011) Donor–acceptor–donor-based π-conjugated oligomers for nonlinear optics and near-IR emission. Chem Mater 23:3805–3817
15.
Sugiura R, Imai H, Oaki Y (2024) Morphology and size control of an amorphous conjugated polymer network containing quinone and pyrrole moieties via precipitation polymerization. Nanoscale Adv 6:1084
A
16.Yano S, Sato K, Suzuki J, Imai H, Oaki Y (2019) Amorphous 2D materials containing a conjugated polymer network. Commun Chem 2:97
17.
Yamamoto T, Kimura T, Shirashi K (1999) Preparation of p-conjugated polymers composed of hydroquinone, p-benzoquinone, and p-diacetoxyphenylene units. Optical and redox properties of the polymers. Macromolecules 32:8886–8896
18.
Sabaa MW, Madkour TM, Yassin AA (1988) Polym Degrad Stabil 22:195–203
19.
Grennberg H, Gogoll A, Báckvall JE (1993) Acid-induced transformation of palladium(0)-benzoquinone complexes to palladium(II) and hydroquinone. Organometallics 12:1790–1793
20.
Minematsu K, Takahashi S, Hagihara N (1975) Bonding interaction of p-quinones with palladium-phospiune complexes. J Organomet Chem 91:389–398
21.
Farragher AL, Page FM (1966) Experimental determination of electron affinities. Part 9.—Benzoquinone, chloranil and related compounds. Trans Faraday Soc 62:3072–3080
22.
Zhu GZ, Wang LS (2015) Communication: vibrationally resolved photoelectron spectroscopy of the tetracyanoquinodimethane (TCNQ) anion and accurate determination of the electron affinity of TCNQ. J Chem Phys 143:221102
23.
Hameed F, Mohanan M, Ibrahim N, Ochonma C, Rodrzíguez-Lípez J, Gavvalapalli N (2023) Controlling π–conjugated polymer – acceptor interactions by designing polymers with a mixture of π–face strapped and nonstrapped monomers. 56:3421–3429
24.
Thazhathethil S, Muramatsu T, Tamaoki N, Weder C, Sagara Y (2022) Mechanophores excited state charge-transfer complexes enable fluorescence color changes in a supramolecular cyclophane mechanophore. Angew Chem Int Ed 61:e202209225
25.
Rout Y, Montanari C, Pasciucco E, Misra R, Carlotti B (2021) Tuning the fluorescence and the intramolecular charge transfer of phenothiazine dipolar and quadrupolar derivatives by oxygen functionalization. J Am Chem Soc 143:9933–9943