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Conversion of Large Bandgap TriphenylamineBenzothiadiazole to Low Bandgap, Wide-Band Capturing Donor-Acceptor Systems by Tetracyanobutadiene and/or Dicyanoquinodimethane Insertion for Ultrafast Charge Separation Yogajivan Rout, Youngwoo Jang, Habtom B. Gobeze, Rajneesh Misra, and Francis D'Souza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06632 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Conversion of Large Bandgap Triphenylamine-Benzothiadiazole to Low Bandgap, Wide-band Capturing Donor-Acceptor Systems by

Tetracyanobutadiene

and/or

Dicyanoquinodimethane

Insertion for Ultrafast Charge Separation Yogajivan Rout,a Youngwoo Jang,b Habtom B. Gobeze,b Rajneesh Misra,a,* and Francis D’Souzab,* a

Department of Chemistry, Indian Institute of Technology, Indore 453552, India.

b

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX

76203-5017, USA

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

ABSTRACT. Usage of multi-modular donor-acceptor systems capable of revealing tunable ground and excited state properties is gaining momentous interest for applications in light energy harvesting and optoelectronics. Here, we demonstrate conversion of a large bandgap donoracceptor-donor (D-A-D) type system, (triphenylamine-benzothiadizole-triphenylamine, TPABTD-TPA) into low bandgap, unsymmetrical, D-A′-A-D and D-A′-A-A″-D type donor-acceptor systems by the insertion of tetracyanobutadiene (A′) or dicyanoquinodimethane (A″) by [2+2] cycloaddition–retroelectrocyclization reactions. Due to the existence of strong charge transfer in the ground and excited states, these low band gap unsymmetrical donor-acceptor chromophores exhibit strong electronic absorption covering the visible and near IR regions. Electrochemical, spectroelectrochemical and computational studies are performed to evaluate their redox potentials, spectral characterization of oxidized/reduced species, and to realize their electronic structures. Finally, occurrence of ultrafast charge separation in these conjugates has been established from femtosecond transient absorption covering the visible-near IR regions in polar and nonpolar solvents; properties relevant towards their optoelectronic applications.


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INTRODUCTION Studies on symmetrical and unsymmetrical molecular systems covalently linked by electron donor and acceptor entities have attracted considerable interest in organic electronic applications including light energy harvesting.1-22 The use of appropriately positioned, easy to oxidize donor and easy to reduce acceptor molecules promote the possibility of D-to-A charge transfer extending their absorption well into the visible and near-IR regions that can be exploited in organic photovoltaic (OPV) devices.1-22 Additionally, the possibility of both hole and electron injection from appropriate electrode materials promote their ambipolar organic field effect transistors (OFET) properties.23-30 Extending the simple D-A dyad concept to build multi-modular D-A systems (having different types of donors and/or acceptors varying in their electron donor and acceptor strengths) would promote multiple intramolecular charge transfer events within the systems.31-46

An advantage of such a design is in the possibility of extending absorption

characteristics covering the visible and near-IR regions along with ultrafast excited state charge separation events. However, although highly desirable, building such complex multi-modular systems is often challenging due to synthetic difficulties. In the multi-modular D-A systems, the 2,1,3-benzothiadiazole (BTD) unit has gained significant attention to be a good electron acceptor due to its several advantages such as favorable reduction potential, a prominent bathochromic shift of the charge-transfer absorption band, and strong electron affinity (EA).43-46 The literature reveals that the BTD derivatives have widely been used as π-conjugated organic materials for two-photon absorption, photo-induced intramolecular charge transfer (ICT), organic light emitting diodes (OLED) and solar cells. 47-54 In the present study, we have successfully built multi-modular donor-acceptor conjugates capable of absorbing the visible and near-IR light while undergoing ultrafast photoinduced electron transfer upon



photoexcitation. Starting from two triphenylamine (TPA) connected to (BTD) via acetylene linkers, BTD2, a D-A-D type donor-acceptor-donor type system, we have been successful in synthesizing, for the first time, unsymmetrical D-A′-A-D and D-A′-A-A′′-D type multi-modular donor-acceptor systems. Here, A′ represents tetracyanobutadiene (TCBD) while A″ represents dicyanoquinodimethane (DCNQ) electron acceptors (Figure 1). As demonstrated here, the TPATCBD-BTD-TPA (BTD3), TPA-DCNQ-BTD-TPA (BTD4), and TPA-TCBD-BTD-DCNQ-TPA (BTD5) exhibit strong intramolecular charge transfer interactions extending the absorption characteristics well into the near-IR region. Excited state interactions leading to charge separation has also been possible to achieve in these systems wherein the measured kinetic parameters reveal occurrence of ultrafast photochemical events.

Figure 1. Structure of the investigated multi-modular systems bearing closely associated to promote charge transfer, TPA, BTD, TCNE and DCNQ electron donor and acceptor entities. Synthesis of the multi-modular donor-acceptor conjugates is depicted in Scheme 1. The control compound, BTD1 was synthesized using Sonogashira cross-coupling reaction of 4,7-


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dibromo-2,1,3-benzothiadiazole (1) with phenylacetylene (2) at 70 °C for 12 h.55-56 BTD2 was synthesized by the Sonogashira cross-coupling reaction of 4,7-dibromo-2,1,3-benzothiadiazole (1) with 2.1 equiv of 4-ethynyl-N,N-diphenylaniline at 70 °C for 12 h.55-56 The unsymmetrical BTD3 and BTD4 with D-A′-A-π-D configuration were synthesized by the [2 + 2] cycloaddition– retroelectrocyclization reaction of BTD2 with one equivalent of TCNE or 7,7,8,8tetracyanoquinodimethane (TCNQ) in dichloromethane (DCM) at 40 °C and in dichloroethane at 80 °C in 89% and 88% yields, respectively.

Scheme 1. Synthesis of BTD1-BTD5.



RESULTS AND DISCUSSION For the synthesis of BTD5 having both TCNE and DCNQ acceptors, BTD4 was subjected to [2 + 2] cycloaddition–retroelectrocyclization reaction with TCNE in dichloroethane (DCE), which resulted BTD5 with D-A′-A-A′′-D configuration at 80 °C in 90% yield. The purity of the newly synthesized compounds was arrived by thin-layer chromatography while the structural integrity was solved by 1H and 13C NMR and HR-MS techniques (see SI for spectral data).

Figure 2. (a) Normalized absorption spectra of BTD1 (dark) and BTD2 (red) in benzonitrile. (b) Normalized fluorescence spectra of BTD2 in different solvents and the picture inset shows the color of BTD2 solution in solvents of increasing polarity, toluene, THF, DCB, PhCN, and DMF at ambient light (top) and UV irradiation (bottom) conditions.


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Figure 2a shows the absorption spectra of BTD1 and BTD2 in benzonitrile while those in toluene are shown in Figure S1 in ESI. The control BTD1 having no electron donor groups revealed a peak 415 nm originating from the BTD -system. For BTD2, having TPA electron donor and BTD electron acceptor, two peaks at 347 and 486 nm were observed. In nonpolar toluene, these two peaks were located at 338 and 479 nm. The fluorescence of BTD1 was located at 498 nm while that for BTD2 it was at 653 nm and was quenched over 95% in benzonitrile (see Figure S2a). In contrast, in toluene, the emission peaks of BTD1 and BTD2 were located at 484 and 567 nm with the relative quenching of only 45% (Figure S2b). Significant quenching in polar solvent support charge transfer type interactions between TPA and BTD entities in BTD2.57 Earlier, in case of covalently linked TPA-BTD systems, the origin of the second peak was attributed to a charge transfer transition.16 This also seems to be the case for BTD2 as shown in Figure 2b wherein the visible band reveals systematic red-shift with increasing the polarity (also, see picture of solution color in the absence and presence of UV illumination of BTD2 showing color shift to red) Owing to the presence of two TPA entities, a quadrupolar charge transfer state in the case of BTD2 could be envisioned. Fluorescence lifetimes measured using time-correlated single photon counting (TCSPC) technique and nanoLED excitation revealed monoexponential decays for both BTD1 and BTD2. The measured lifetimes were found to be 5.07 and 1.77 ns in benzonitrile and 4.19 and 3.43 ns in toluene, respectively for BTD1 and BTD2 (see Figure S3 for decay plots). Significant lifetime quenching in the case of BTD2 in polar benzonitrile as a result of strong charge transfer was witnessed.54,58 For BTD3–BTD5 having one or two powerful electron acceptors, an additional strong charge transfer band spanning the near-infrared region, both in polar and nonpolar solvents, was observed, as shown in Figures 3 and S4. Efforts were also made to locate any charge transfer



emission by exciting the samples at wavelengths corresponding to both the high energy locally excited (LE) and low energy charge transfer (CT) peaks. Such efforts revealed no detectable peaks (vide infra) suggesting the CT emission is too weak to detect in these systems.

Figure 3.

Absorption spectrum of BTD3 (blue), BTD4 (magenta) and BTD5 (green) in

benzonitrile. Assessment for the strong electron deficient nature of TCBD and DCNQ in the hybrids was arrived by performing electrochemical studies in benzonitrile containing 0.1 M (TBA)ClO 4 (Figure 4). The first reduction of BTD1 was located at -1.49 V vs. Fc/Fc+ suggesting it to be a moderate level electron acceptor. BTD1 also revealed an irreversible oxidation at Epa = 0.54 V. In the case of BTD2 having two TPA entities, the reduction was found to be at almost the same potential while two quasi-reversible oxidations located at Epa = 0.48 and 0.58 V were observed. By analogy, the first oxidation peak was assigned to TPA entity while the second oxidation peak was for the BTD entity, establishing the donor role of TPA and acceptor role of BTD in BTD2. In BTD3 having an additional electron acceptor TCBD between the TPA and BTD entities, two reversible reductions at -0.61 and -1.04 V corresponding to first and second reduction of TCBD and a third reduction at -1.92 corresponding to BTD were observed. At the anodic side, two quasi8 Environment ACS Paragon Plus

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reversible peaks at 0.62 and 0.80 V, both anodically shifted from that of BTD2, due to close association of TCBD, were observed. In the case of BTD4 having DCNQ between TPA and BTD entities, two reversible reductions at -0.59 and -0.72 V corresponding to first and second reduction of DCNQ and a third reduction at -1.98 corresponding to BTD entity were observed. In the anodic side, two peaks at Epa = 0.57 and 0.72 V were observed. In the case of BTD5 having both TCBD and DCNQ electron acceptors between the TPA and BTD entities, four reductions at -0.52, -0.66, -0.85 and -1.32 V involving both electron acceptor (first and second reductions) were observed. At more cathodic potentials, the expected BTD reduction was also observed.

Figure 4. Cyclic voltammograms of the indicated compounds in benzonitrile containing 0.1 M (TBA)ClO4 supporting electrolyte. Having two electron acceptor entities made the first reduction process easier by 70 mV compared to BTD4. In the anodic side, two peaks at Epa = 0.57 and 0.80 V were observed. In all these compounds the irreversible to quasi-reversible electrode processes made the site of oxidation 9 Environment ACS Paragon Plus


difficult to assign. However, by comparison, the first oxidation was assigned to the TPA entity that was far from the electron acceptor entity in the case of BTD3 and BTD4 (see the structures in Figure 1). Importantly, bandgaps of 1.23 V for BTD3, 1.16 V for BTD4, and 1.07 V for BTD5 was possible to achieve in the present series of donor-acceptor systems.

Figure 5. Optimized structure, frontier HOMO and LUMO of indicated compounds performed at the B3LYP/6-31G* level. Computational studies performed at the B3LYP/6-31G* level59 to help seek geometry and spread of frontier orbitals. The presence of TCBD and DCNQ electron acceptors distorted the molecular structures to a considerable extent (Figure 5). In the case of BTD1 and BTD2, the HOMO was spread all over the molecule while majority of the LUMO was on the BTD entity. 10 Environment ACS Paragon Plus

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Presence of strong electron acceptors changed this situation completely. That is, in the case of BTD3 and BTD4, the HOMO was spread only on the BTD-TPA segment that was far from the strong electron acceptors while the LUMO was mainly on the electron acceptor entities. For BTD5 having two electron acceptors, the HOMO was on the TPA entity while the LUMO was on the DCNQ-BTD-TCBD segment, following their redox potential trends. Spectroelectrochemical studies were performed to characterize one-electron oxidized and one-electron reduced products of the investigated compounds. During first oxidation and first reduction of BTD1 and BTD2, an overall small increase in absorbance covering the region of 500– 900 nm, accompanied by a decrease in the main peak intensity, was observed. In contrast, BTD3– BTD5 revealed significant changes during both first oxidation and first reduction, as shown in Figure S5. Noticeable changes include disappearance of the low-energy charge transfer band during oxidation and reduction, and appearance of a new peak in the near-IR region during reduction in the case of BTD4 and BTD5. Energy level diagrams were established to visualize the excited state events by utilizing the spectral and electrochemical data, and by free-energy calculations,60-61 as shown in Figure S6 in SI for BTD2, and in Figure 6 for BTD3 and BTD4 tetrads. For BTD2, excitation of the D-A-D system would promote symmetry breaking charge separation.62-64 There are two channels to excite the tetrads, that is, direct excitation related to high-energy locally excited (LE) state, and excitation related to the low-energy charge transfer (CT) band. In the tetrads of the type D-A1-A-D (where D = TPA, A1=TCBD or DCNQ, and A = BTD), excitation of BTD LE state would populate its singlet excited state, D-A1-1A*-D. From this state, formation of excited state charge transfer, where the partial positive charges on the D entity and partial negative charges on A1-A entity, resulting in D+-(A1-A)--D or D-(A1-A)--D+ CT states is a possibility, although no charge



transfer emission was observed.

Alternatively, D-A1-1A*-D could directly undergo charge

separation to yield D-(A1-A)--D+ or D+-(A1-A)--D states in the case of BTD3 and BTD4. For the pentad, BTD5 such a process could yield at least two types of charge separation products, D-A1(A-A2) --D.+, and D-(A1-A-A2).--D+ where A1 and A2 represent TCBD or DCNQ, respectively. However, based on the location of frontier orbitals it would be safer to say that a charge separated state involving all three acceptors (TCBD, BTD and DCNQ), that is, a D-(A1-A-A2).--D+ type product is likely to form. In order to verify occurrence of such photochemical processes, femtosecond transient absorption (fs-TA) spectral studies were performed as discussed below.

Figure 6. Energy level diagram of BTD3 and BTD4 in benzonitrile reflecting energetic pathways of CS and CR after excitation of LE, and the CT states. The energy of the polarized CT states, and the CS were derived by the CT absorption maxima, and from the respective redox potentials, that is, first oxidation and first reduction of respective donor-acceptor systems.


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The fs-TA were recorded in both polar benzonitrile and toluene by exciting the samples both corresponding to the LE and CT peaks. Figure 7 illustrates fs-TA spectra at the indicated delay times of BTD1 probe and BTD2 in benzonitrile while such data in toluene are shown in Figure S7. The instantaneously formed 1BTD* in the case of BTD1 revealed peaks at 750 and 1300 nm range whose decay was slow consistent with the earlier discussed lifetime data.

Figure 7. Fs-TA spectra at the indicated delay times of (a) BTD1 (425 nm excitation) and (b) BTD2 (475 nm excitation) in benzonitrile. The right hand panel shows the time profiles of the peak to estimate the decay kinetics. In the case of BTD2 where charge transfer interactions involving TPA and BTD were observed, the transient spectral signals were different from that observed for the BTD probe in BTD1 (see Figures 7b and S7b). The instantaneously formed BTD had broad spectral features with a peak maxima at 640 nm (see spectrum at 1.2 ps). Decay of this signal was accompanied by initial new peaks at 570, 620 and 883 nm (see spectrum at 3.2 ps). With time, the 570 and 620 nm



peaks merged into a broad peak centered at 607 nm (see spectrum at 348 ps). In toluene, within the first 1 ps, transient peaks with a negative peak at 500 nm range corresponding to ground state bleaching (GSB), and positive peaks at 630, 842 and 990 nm corresponding to excited state absorption (ESA) were observed. With increasing the delay time, recovery of the GSB and decay of the ESA peaks revealed new peaks at 536 and 1010 nm. With prolonged delay times, these near-IR peak diminished in intensity while the 536 nm peak had significant peak intensity. To gather kinetic information, multi-wavelength global analysis was performed. The time constants for charge separation (CS) and charge recombination (CR) from this analysis are given in Table 1. The time constants for CR of 1.55 ns in benzonitrile and 1.76 ns in toluene were consistent with similar BTD derived donor-acceptor conjugates in the literature.14 Incorporation of one or more strong electron acceptors into the BTD2 framework accelerated the excited state charge separation and recombination processes to a great extent both in benzonitrile and toluene, and at both LE and CT excitation wavelengths, as shown in Figures 8, S8 – S10. Figures 8a-c show the fs-TA spectra at the indicated delay times respectively for BTD3, BTD4 and BTD5 in benzonitrile at the excitation wavelength of 475 nm. To establish the occurrence of excited state charge separation in these strongly interacting systems, the earlier discussed spectroelectrochemical results shown in Figure S5 and frontier orbital locations shown in Figure 5 were found to be invaluable. In the case of BTD3 in benzonitrile, immediately after excitation, a strong near-IR peak at 1260 nm was observed, there was some growth in the 800 nm region near the window of our near-IR detector (see spectrum at 1.02 ps). The decay of this peak was associated with a blue-shift and was accompanied by a negative peak in the 560 nm range corresponding to GSB and a positive peak at 715 nm corresponding to oxidize/reduced product within the first 10 ps was observed. This peak was also short-lived whose decay was associated


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with a new peak in the 620 nm range. Using multi-wavelength global analysis, the time constants for CS and CR were determined to be 1.67 and 10.69 ps revealing ultrafast charge separation and charge recombination processes. Changing the solvent to nonpolar toluene also revealed such transient spectral trends, however, in this case a negative peak at 560 nm corresponding to GSB was better developed (see Figure S8). The time constants for CS and CR were determined to be 4.35 and 41.19 ps suggesting relatively slow CS and CR compared to that observed in benzonitrile. Changing the excitation wavelength to 575 nm corresponding to the CT band also revealed efficient formation of electron transfer products. There was also a well-defined negative peak in the 900 nm range. Comparison with the absorption band clearly ruled out that this negative signal is a GSB but a stimulated emission (SE) signal, likely corresponding to the inverse transition of the absorption peak. The time constants for CS and CR were found to be 4.35 and 41.19 ps, being slightly higher than that observed when the sample was excited at high energy peak at 475 nm. The fs-TA spectral features observed for BTD4 and BTD5 at the excitation wavelength corresponding to both LE and CT states also revealed efficient charge separation in both benzonitrile and toluene (see Figures S8 – S10). The transient spectra was composed with peaks related to charge separated products, negative peaks in the visible range corresponding to GSB and inverse transition in the near-IR region. The time constants obtained from multi-wavelength analysis is tabulated in Table 1. An analysis of kinetic data from Table 1 reveal the following. Introduction of one or two powerful electron acceptors into BTD2 promotes both CS and CR, more so in polar benzonitrile than in toluene. Worthy of mention is that the excitation of BTD3–BTD5 either at the LE and CT peak maxima has little or no effect on the overall photoprocesses. This suggests that the LE state undergoes rapid CS process likely involving an intermediate charge transfer state that is too fast



to detect. As predicted earlier, in the case of BTD5 a charge separated state involving all three acceptors, D-(A1-A-A2).--D+ is likely to be the product.

Figure 8. Fs-TA spectra at the indicated delay times of (a) BTD3, (b) BTD4 and (c) BTD5 in benzonitrile, the samples were excited at 475 nm. The right had panel shows the kinetic profiles of the visible peaks to evaluate the time constants for CS and CR.


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Table 1. Time constants for charge separation and charge recombination as a function of solvent polarity and excitation wavelength for the investigated series of compounds. Compound

BTD2

Solvent

PhCN

ex, nm

475

Toluene BTD3

PhCN

475

Toluene BTD4

CS, ps

CR, ps

4.91

1550

12.11

1763

1.67

10.69

4.35

41.19

PhCN

575

2.53

10.98

PhCN

475

0.48

7.24

3.29

20.25

0.98

11.87

2.92

18.36

0.64

6.38

4.78

20.77

0.31

7.16

3.72

16.97

Toluene PhCN

625

Toluene BTD5

Time constants

PhCN

475

Toluene PhCN Toluene

675

CONCLUSIONS In summary, the TPA-BTD-TPA donor-acceptor system (BTD2) revealed charge transfer absorption and emission that was highly dependent on the solvent polarity covering the visible-region. Insertion of powerful electron acceptors, TCBD and DCNQ between the TPABTD entities of BTD2 resulted in a new series of low bandgap systems whose absorption extended well into the near-IR region, a result that was never been observed in BTD derived donor-acceptor systems. In contrast to BTD2 where strong CT emission was possible to achieve, there was little or no charge transfer emission BTD3–BTD5. The established energy level diagram suggested



thermodynamic accessibility of excited state charge transfer and separation in these conjugates. Finally, in this series of donor-acceptor systems, complementary transient absorption studies demonstrated the occurrence of ultrafast photoinduced charge separation processes, which, together with the ground and excited state spectral features, render these systems promising materials for applications in the field of molecular optoelectronics. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS publications website at DOI.. Synthetic details, experimental section, additional spectral, spectroelectrochemical, energy level diagram and femtosecond transient spectral data. AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] *E-mail: [email protected] ORCID Francis D’Souza: 0000-0003-3815-8949 Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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Support from the DST, (DST/TMD/SERI/D05 (C)), INSA (SP/YSP/139/ 2017/2293), Govt. of India, New Delhi, and US-National Science Foundation (1401188 to FD) is gratefully acknowledged. REFERENCES 1) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733–758. 2) E. Bundgaard E.; Krebs, F. C. Low Band Gap Polymers for Organic Photovoltaics. Sol. Energy Mater. Sol. Cells, 2007, 91, 954–985. 3) Beaujuge, P. M.; Pi, W.; Tsao, H. N.; Ellinger, S.; Müllen, K.; Reynolds, J. R. Tailoring Structure−Property

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