Article pubs.acs.org/JPCB
Donor−Acceptor Star-Shaped Conjugated Macroelectrolytes: Synthesis, Light-Harvesting Properties, and Self-Assembly-Induced Förster Resonance Energy Transfer Li Zhao,† Cheng-Fang Liu,† Wei-Dong Xu,† Yi Jiang,† Wen-Yong Lai,*,†,‡ and Wei Huang*,†,‡ †
Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ‡ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China S Supporting Information *
ABSTRACT: A novel series of donor−acceptor star-shaped conjugated macroelectrolytes (CMEs), denoted as 4FTs, including anionic carboxylic acid sodium groups (4FNaT), neutral diethanolamine groups (4FNOHT), and cationic ammonium groups (4FNBrT), were designed, synthesized, and explored as an excellent platform to investigate the impact of various polar pendent groups on self-assembly behaviors. The resulting CMEs with donor−acceptor star-shaped architectures exhibited distinct light-harvesting properties. The interactions between 4FTs and TrNBr, a star-shaped monodisperse CME grafted with cationic quaternary ammonium side chains, were investigated in H2O and CH3OH using steady-state, time-resolved fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Highly favored energy transfer has been proven by the excellent spectral overlap between TrNBr fluorescence and 4FTs absorptions which can be tuned by adjusting the pendent polar groups and solvents. It is suggested that selfassembled structures are formed between TrNBr and 4FNaT, while there is no obvious change for TrNBr/4FNOHT and TrNBr/4FNBrT in both H2O and CH3OH at low concentrations (1.5 × 10−6 M), not only electrostatic interactions but also intermolecular interactions (i.e., π−π stacking) contribute to the FRET process that causes notable luminescence quenching. Moreover, the same investigation of TrNBr and 4FTs in CH3OH (Figure 3 and Figure S9 (Supporting Information)) follows the same trends as the results in H2O. It is worthwhile to mention that the critical point of the concentration to achieve complete quenching in CH3OH is found to be around 1.1 × 10−6, 3 × 10−5, and 1 × 10−4 M for TrNBr/4FNaT, TrNBr/4FNOHT, and TrNBr/4FNBrT, respectively, following the trend of [TrNBr/4FNaT] < [TrNBr/4FNOHT] < [TrNBr/4FNBrT]. In this way, the molecular distances at a low concentration are determined by the electrostatic interactions. However, at a high concentration, in addition to intermolecular interactions, there exists aggregation of molecules due to the increase of concentrations. Therefore, we can draw the conclusion that
the electrostatic interactions are the main force in the process in dilute solutions with a concentration lower than 10−6 M. To get further understanding, the dependence of the optical spectral properties on increasing concentrations was investigated at length. Electronic energy transfer from donors (TrNBr) to acceptors (4FTs) was then studied in detail at a constant TrNBr concentration (5 × 10−7 M) by varying concentrations of 4FTs acceptors in a range of (0−9) × 10−7 M (Figure 4 and Figure S10 (Supporting Information). The absorption spectra of TrNBr (5 × 10−7 M) vary by increasing the concentrations of 4FTs in H2O (Supporting Information, Figure S10a,b) and CH3OH (Supporting Information, Figure S10c−e), resulting in an increase of the absorption intensity at the maximum wavelength (λabs max) and an appearance of a new absorption band at about 540 nm. Similarly, as shown in the Supporting Information (Figure S5a,c), gradually increasing the concentrations of TrNBr at a constant 4FNaT (Supporting Information, Figure S11a) or 4FNBrT (Supporting Information, Figure S11c) concentration causes an increase of the absorption at the wavelength maximum (λabs max) and a slight hypsochromic shift of λabs max. Comparing the absorption spectrum 6734
DOI: 10.1021/acs.jpcb.5b02851 J. Phys. Chem. B 2015, 119, 6730−6739
Article
The Journal of Physical Chemistry B of the TrNBr (5 × 10−7 M)/4FNaT (10−6 M) self-assembled system and the sum of the absorption spectra of isolated TrNBr (5 × 10−7 M) and 4FNaT (10−6 M) systems, it is possible to infer that the new absorption band at about 543 nm may arise from 4FNaT due to the ICT characteristic core structure (Supporting Information, Figure S12a, in H2O and Supporting Information, Figure S12c, in CH3OH). The absorption bands of the mixtures are slightly broader and red-shifted relative to the sum of the two components, suggesting the formation of TrNBr/4FNaT self-assembled structures. The absorption intensity of the TrNBr/4FNaT mixtures at λabs max is much smaller than the sum of these two isolated components, indicating strong intermolecular interactions and efficient FRET processes between TrNBr and 4FNaT induced by the self-assembly. In contrast, when we compare the absorption spectrum of the mixture of TrNBr/4FNBrT and TrNBr/ 4FNOHT with the sum of the absorption spectra of isolated 4FNBrT, 4FNOHT, and TrNBr solution, the absorption bands in terms of intensity and positions are almost identical (Supporting Information, Figure S12b,d,e). The results suggest that efficient FRET process happens in the TrNBr/4FNaT system even in very dilute solution, but not in TrNBr/4FNBrT and TrNBr/4FNOHT systems. This highlights the influence of the pendent polar ionic groups on FRET through the selfassembled system, as revealed by the difference in the spectra features. The effect of TrNBr on the spectral properties of 4FTs is more evident in the PL spectra and vice versa (Figure 4a,b in H2O and Figure 4c−e in CH3OH). Upon addition of 4FNaT, a decrease in intensity of TrNBr fluorescence is observed, together with the appearance of new emission bands in the range 640−770 nm assigned to 4FNaT in both H2O (Figure 4a) and CH3OH (Figure 4c). The PL spectra of TrNBr are obtained at the maximum absorption wavelength about at 360 nm, which is also within the absorption region of 4FNaT. Hence, the appearance of the PL band might be attributed to the direct excitation of the 4FNaT rather than any excitation energy transfer (FRET) from TrNBr to 4FNaT. In fact, both processes were occurring, as shown in Figure S11b (Supporting Information). The gradual addition of TrNBr results in an increase of the PL intensity in the range 640−770 nm, assigned to the 4FNaT, which implies the energy transfer from TrNBr to 4FNaT. Nevertheless, the PL intensity of TrNBr/4FNBrT at 691 nm assigned to 4FNBrT gets weak, and the PL intensity at λem = 402 nm assigned to TrNBr first increases and then decreases with a gradual increment of TrNBr concentration (Supporting Information, Figure S11d,e). These results reflect the aggregation of 4FNBrT and TrNBr. Furthermore, the pictures of different solutions under UV light give us an intuitive impression about the actual FRET process (Figure 5).
Either in H2O or in CH3OH, the solutions of TrNBr/4FNaT system are not able to emit blue light, suggesting the existence of FRET in this system due to the self-assembly induced by the interaction of oppositely charged ionic pendent groups. Table 2 shows the ΦF values of the two systems (TrNBr/ 4FNaT, TrNBr/4FNBrT) in H2O and the three systems Table 2. Fluorescence Quantum Yields of the Systems (λex = 361 nm) TrNBr/4FTs (4FNaT, 4FNBrT) (5 × 10−7 M) in H2O and the Systems (λex = 360 nm) TrNBr/4FTs (4FNaT, 4FNOHT, 4FNBrT) (5 ×10−7 M) in CH3OHa system
ΦF(370−608 nm)
ΦF(608−750 nm)
TrNBr/4FNaT (H2O) TrNBr/4FNBrT (H2O) TrNBr/4FNaT (CH3OH) TrNBr/4FNOHT (CH3OH) TrNBr/4FNBrT (CH3OH)
0.46 0.85 0.42 0.85 0.84
0.05 0.02 0.03 0.02 0.00
The fluorescence quantum yields (ΦF) were measured with the absolute methods by using an integrating sphere, and the excitation wavelength was the corresponding maximum absorption peak.
a
(TrNBr/4FNaT, TrNBr/4FNOHT, TrNBr/4FNBrT) in CH3OH at a total constant concentration (5 × 10−7 M). In the presence of the 4FNaT acceptors, the ΦF value of the TrNBr sharply decreases, while the ΦF values of the TrNBr/ 4FNaT systems are still much higher than those of 4FNaT alone (Table 1) in both H2O and CH3OH solutions. This phenomenon can be attributed to the self-assembled structures facilitated by the strong ionic attractive interactions of oppositely charged ionic groups that boost FRET. On the contrary, in the presence of the 4FNBrT, the ΦF values have no obvious changes both in H2O and CH3OH, manifesting no efficient FRET in this process. Moreover, we also investigated the variation of ΦF values of TrNBr/4FNOHT in CH3OH. The results show trends similar to that of TrNBr/4FNBrT. We believe that this may originate from the fact that the different charged ions induce different intermolecular interactions, although there may be some contribution from direct excitation of 4FNaT, 4FNOHT, and 4FNBrT at the wavelength about 360 nm. Based on these analyses, we can infer that oppositely charged ionic groups have strong electrostatic interactions to boost the self-assembly that induces obvious FRET even in very dilute solutions. Consequently, modulating the ionic pendent groups has provided an effective strategy to adjust the selfassembly behaviors in the systems. To confirm the self-assembly in these systems and provide further insight on the luminescence quenching according to analysis of the fluorescence behaviors, the Stern−Volmer plots of TrNBr/4FTs in H2O (Figure 6a) and CH3OH (Figure 6b), respectively, were established as shown in Figure 6. As shown in the plots, the I0/I ratio is given as a function of the TrNBr concentration, where I0 and I are the maximum PL intensities in the absence and presence of 4FTs. Φ0 I = 0 = 1 + kqτ0[A] = 1 + KSV[A] Φ I where kq is the quenching constant, KSV is the Stern−Volmer constant, and τ0 is the TrNBr lifetime in the absence of 4FTs. The plots of I0/I for TrNBr/4FTs are shown in Figure 6a in H2O and Figure 6b in CH3OH. Self-assembled systems of TrNBr/4FNaT (Figure 6, squares) reveal that they do not obey the linear Stern−Volmer relation expected for collisional
Figure 5. Schematic representation of TrNBr (5 × 10−7 M), 4FTs (4FNaT, 4FNOHT, 4FNBrT) (10−6 M), and the mixture of TrNBr (5 × 10−7 M)/4FTs (10−6 M) in H2O and CH3OH under UV light. 6735
DOI: 10.1021/acs.jpcb.5b02851 J. Phys. Chem. B 2015, 119, 6730−6739
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The Journal of Physical Chemistry B
Time-Resolved Fluorescence Measurements on the Self-Assembled Systems of TrNBr/4FNaT. The timeresolved fluorescence of TrNBr/4FNaT was plotted as shown in Figure 7. It was measured for the samples in H2O or CH3OH
Figure 6. Stern−Volmer plots of TrNBr in the presence of 4FTs. (a) 4FNaT (squares) and 4FNBrT (circles) in H2O. (b) 4FNaT (squares), 4FNBrT (circles), and 4FNOHT (triangles) in CH3OH. The Stern−Volmer plots of 4FNBrT (circles) and 4FNOHT (triangles) in CH3OH are shown in the inset.
quenching in all of the concentration range. Instead, the plot of I0/I of TrNBr/4FNaT is linear for low quencher concentration ((0−5) × 10−7 M) and then has a rapidly upward curvature, due to combined dynamic and static quenching, where the fluorophore can be quenched not only by collision but also by complex formation with the quencher. Besides, the plot of I0/I of the TrNBr/4FNBrT system (Figure 6, circles) follows the linear Stern−Volmer relation expected for collisional quenching. In addition, we also obtain the plot of I0/I of TrNBr/ 4FNOHT in CH3OH, which is similar to that of the TrNBr/ 4FNBrT system obeying the linear Stern−Volmer relation. KSV is determined from the slope of the linear region in this case, and kq is calculated from these values, using τ0 as the lifetime for TrNBr (1.06 ns in H2O, 0.85 ns in CH3OH) in the absence of quencher. Comparing the TrNBr/4FNaT system in the two solvents, it can be seen from the initial slope of the Stern− Volmer plots that the quenching efficiency is higher in H2O than in CH3OH (Figure 6 and Table 3). The results suggest
Figure 7. Fluorescence emission decays of (a) TrNBr (5 × 10−7 M), (b) 4FNaT (10−6 M), and TrNBr/4FNaT (5 × 10−7 M), collected at 402 and 688 nm in H2O (c) and collected at 397 and 698 nm in CH3OH (d).
following 1 MHz, 70 ps pulse excitation. The emission decays of TrNBr and 4FNaT were recorded by using two discrete exponentials both in H2O and in CH3OH with excellent fits (Figure 7a,b). The short lifetime of τ1(TrNBr) (0.88 ns in H2O, 0.69 ns in CH3OH) is similar to those reported for truxenecored molecules.39 The long lifetime of τ1(TrNBr) (2.03 ns in H2O, 2.20 ns in CH3OH) is linked with the decay of the relaxed structures. On the contrary, the long lifetime consisting of τ2(4FNaT) (4.37 ns in H2O, 3.11 ns in CH3OH) dominates the decay, which is generally believed to include contributions from possible intramolecular energy migration from the fluorene-based arms to the TPABT core. As shown in Figure 7a,b, it is noted that the different decay behaviors of TrNBr and 4FNaT are found to depend on the solvent polarity. Figure 7c,d shows the fluorescence decays of TrNBr/4FNaT self-assembly system (5 × 10−7 M), collected at the emission of TrNBr and 4FNaT. Comparing Figure 7c,d with Figure 7a,b, the decay times collected at 688 nm in H2O (1.25, 4.00 ns) and 698 nm in CH3OH (0.83, 3.45 ns) are associated with the emission decays from 4FNaT and the decay times collected at 402 nm in H2O (0.86 ns) and 397 nm in CH3OH (0.87 ns) may be correlated with the emission decays from TrNBr. However, the long decay time in TrNBr/4FNaT collected at 402 nm in H2O (3.36 ns) and at 397 nm in CH3OH (2.52 ns) does not occur in either TrNBr or 4FNaT, manifesting FRET from the donors to the acceptors in the self-assembled system to produce new chromophores. Although all these processes are possible in the present self-assembly systems, we believe that the increase of 4FNaT emission accompanied by the reduction of TrNBr implies a significant contribution from intermolecular energy transfer from TrNBr to adjacent 4FNaT molecules. Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM) Measurements on TrNBr/
Table 3. Quenching Constant (kq), Stern−Volmer Constant (KSV), Spectral Overlap (J),a and Förster Distance (R0)b of TrNBr/4FNaT in H2O and CH3OH system TrNBr/4FNaT (H2O) TrNBr/4FNaT (CH3OH) a
J (M−1 cm−1)
R0 (Å)
KSV (M−1)
kq (M−1 s−1)
23
38.08
5.80 × 10
6
6.59 × 1015
1.1 × 1023
27.23
3.14 × 106
4.55 × 1015
8.5 × 10
b 4 2 −4 1/6 J(λ) = ∫ ∞ 0 F(λ) ε(λ)λ dλ. R0 = 0.211(k n ΦDJ(λ)) .
that CMEs intend to ionize easier in H2O than in CH3OH and thus corroborate the idea again that TrNBr and 4FNaT experience self-assembly through electrostatic interactions (ion pairing). From the inset figure of Figure 6b, we can see that the quenching constant of TrNBr/4FNOHT is slightly higher than that of TrNBr/4FNBrT, suggesting collisional quenching has something to do with the intermolecular force. In order to further confirm the FRET process, the TrNBr/ 4FNaT complex spectral overlap (J), the donor fluorescence yield, and the lifetime were used to calculate the Förster distance (R0, distance at resonance energy transfer is 50%) of the TrNBr/4FNaT system as 38.08 Å in H2O and 27.23 Å in CH3OH, respectively. Results of these calculations are listed in Table 3. According to the calculation, the donor−acceptor distances of TrNBr/4FNaT system fall within the Förster distance, ensuring the existence of FRET. 6736
DOI: 10.1021/acs.jpcb.5b02851 J. Phys. Chem. B 2015, 119, 6730−6739
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The Journal of Physical Chemistry B 4FTs. In order to gain further insights into the self-assembly systems, dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to investigate the samples (Figure S13 (Supporting Information), Figure 8, and
larger than those of TrNBr or 4FNaT with the same concentrations, which might be attributed to the self-assembled aggregates of TrNBr and 4FNaT in both H2O and CH3OH. On the contrary, the mean diameters of TrNBr/4FNOHT and TrNBr/4FNBrT keep almost the same particle sizes as their discrete solutions (TrNBr, 4FNOHT, and 4FNBrT) in both H2O and CH3OH. All these phenomena have confirmed the fact that the self-assembly behaviors do exist in the TrNBr/ 4FNaT system due to the strong electrostatic interaction of oppositely charged ionic groups. Moreover, TEM images provide further clear evidence supporting this hypothesis in the mixed systems (Figures 8 and 9). Particles are easily identified and counted to follow similar trends. For example, TEM analysis conducted on TrNBr/4FNaT finds larger particles compared with that of TrNBr or 4FNaT alone in both H2O and CH3OH. However, in both H2O and CH3OH, the particle sizes in TrNBr/4FNOHT and TrNBr/4FNBrT are nearly the same as those of TrNBr, 4FNOHT, and 4FNBrT alone. Bearing these results in mind, we deduce that the selfassembled structures occur in the TrNBr/4FNaT system, further consolidating our previous statements.
■
CONCLUSIONS In summary, we have designed and synthesized a series of donor−acceptor star-shaped CMEs with various polar pendent groups, including anionic 4FNaT, neutral 4FNOHT, and cationic 4FNBrT, which exhibit distinct light-harvesting properties. This set of materials has been explored as acceptors to investigate the energy transfer process, where a star-shaped cationic CME, TrNBr, has been adopted as an energy donor. Among these, the cationic TrNBr as an energy donor and anionic 4FNaT as acceptors are observed to self-assemble in H2O and CH3OH even at very dilute concentrations (
