Excimer-Mediated Intermolecular Charge Transfer in Self-Assembled

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Excimer-Mediated Intermolecular Charge Transfer in Self-Assembled Donor-Acceptor Dyes on Metal Oxides Yongze Yu, Szu-Chia Chien, Jiaonan Sun, Elline C. Hettiaratchy, Roberto C Myers, Li-Chiang Lin, and Yiying Wu J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Journal of the American Chemical Society

Excimer-Mediated Intermolecular Charge Transfer in Self-Assembled Donor-Acceptor Dyes on Metal Oxides Yongze Yu†, Szu-Chia Chien‡, Jiaonan Sun†, Elline C. Hettiaratchy ‡, Roberto C. Myers‡, Li-Chiang Lin┴*, Yiying Wu†* † Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States ‡ Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, United States ┴ William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States Supporting Information Placeholder ABSTRACT: When conjugate molecules are self-assembled on

the surface of semiconductors, emergent properties resulting from the electronic coupling between the conjugate moieties are of importance in the interfacial electron transfer dynamics for photoelectrochemical and optoelectronics devices. In this work, we investigate the self-assembly of the triphenylamineoligothiophene-perylenemonoimide (PMI) molecules, denoted as BH4, on metal oxide surfaces via UV-Vis absorption, photoluminescence, transient near-infrared absorption spectroscopies, and molecular dynamic simulations, and report the excimer formation due to the π-π interaction of the PMI units between the neighboring dye molecules. To our best knowledge, this is the first experimental observation of intermolecular excimer formation when conjugate donor-acceptor molecules form a self-assembled monolayer (SAM). In addition, a longlived (4.3μs) intermolecular charge separation is observed, and a new excimer-mediated intermolecular charger transfer (EMICT) mechanism is proposed. This work demonstrates that through the design of dye molecules, the excited complexes or aggregates can provide a pathway to slow down the recombination rate in photoelectrodes that utilize donor-acceptor dyad molecules. Intermolecular electron coupling is of great interest in organic electronics, photovoltaics, and chromophore catalyst systems.1,2 Due to the interaction between neighboring molecules, a singlet excited state may undergo complicated electron transfer pathways such as forming a lower energetic excimer state or two triplet states from singlet fission process.3,4 Even though excimer states could trap exciton states and consequently diminish the desired charge transfer pathway5–7, recent studies have interestingly demonstrated that excimer intermediates may not dissipate the excitation energy but instead facilitate the singlet fission8–10 or the intramolecular charge transfer11,12. Therefore, whether the excimer state is beneficial or detrimental to optoelectronic devices is still one of the important topics that has drawn considerable attention. Most π-stacked systems can be generally classified into two categories. One is the solid-state materials such as molecular crystalline or thin-film aggregates13–15, and the other is the covalent dimer molecule8,11,16–20. It is reported that, in these systems, the subsequent processes after the excimer state formation are either the radiative relaxation of the trap states7,21, the intramolecular electron transfer,11,12 or the formation of triplet state8–10. While most studies have been focused on the symmetric covalent molecular dimmers or symmetric πstacked systems including PDI11,21–23, PMI19, pentacene10,14,24– 27. The excimer formation in the π-stacked donor-acceptor system has not yet been well-studied. The chromophores with

their extensive use in dye-sensitized solar cells (DSSCs)28,29 self-assembled on semiconductor surfaces are recently reported to be another promising systems for the excimer formation and multiexciton generation.24,30,31 Typically the dye aggregation is regarded as an undesired pathway that can be harmful to the DSSC performance32, although there are several examples showing that a fine packing of dye aggregates could possibly achieve efficient performance and superior stability.33 In this work, we have used photoluminescence (PL), near-infrared transient absorption (TA) and molecular dynamic (MD) simulations to investigate the intermolecular excimer formation and electron transfer kinetics when the triphenylamine-oligothiophene-perylenemonoimide (PMI) molecules, denoted as BH4, are self-assembled on metal oxide surfaces. Interestingly, unlike the widely reported intramolecular charge transfer, we have observed an intermolecular charge transfer mechanism and an ultra-slow recombination kinetics. Based on these results, we propose that the dye aggregates could facilitate the long-lived charge separations in the donor-acceptor system through an excimer-mediated intermolecular charge transfer process. Our results provide a new mechanism to slow down the recombination rate through dye aggregates and donor-acceptor dyad design in dye-sensitized systems.

Figure 1. a) Molecular structure of BH4. b) The shortest distance and angle between a pair of adjacent PMIs in BH4 molecules, computed from MD simulations. c) Side and d) top views of the PMI π-π stacking between neighboring BH4 molecules. For clear presentation of the stacking, only a selected portion of PMI is displayed.

The BH4 dye has recently been used in a stable water splitting dye-sensitized NiO photocathode and an aqueous p-type DSSC.34,35 The stability of these systems was attributed to the hydrophobic bulk of the hexyl groups of the BH4 dye (Figure 1a), which consists of triphenylamine (TPA), oligothiophenes,

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and perylenemonoimide (PMI).36 Our prior work showed that the SAM of BH4 can form a hydrophobic layer that prevents ionic/polar species such as protons or water from reaching the metal oxide surface and therefore decouples the surface local pH from aqueous electrolyte.37 MD simulations were also employed in the study to simulate the BH4-grafted NiO surface and demonstrated the role of BH4 in protecting the NiO surface. Interestingly, from further analyzing the obtained molecular trajectories, we identify the strong π-π interactions between PMI groups of BH4 molecules. Figure 1b displays the shortest distance and angle between a pair of adjacent PMI groups from two neighboring BH4 molecules. The average shortest distance and angle between two adjacent PMI groups were determined to be 3.28 ± 0.20 Å and 10.10 ± 4.43 degrees, respectively. These values resemble those determined for PMI dimer molecules19 reasonably. Figure 1c and 1d illustrate respectively the side and top views of the PMI stacking. Computational details can be found in the Supporting Information (SI).

Figure 2. a) Normalized photoluminescence (PL) spectra of 0.01mM BH4 in DMF solution and the BH4 sensitized Al2O3 film soaking with different time; Excitation: 450nm; b) Time-resolved photoluminescence of the overnight soaked BH4/Al2O3 film. Fitted with the function 𝑦𝑦 = 𝐴𝐴 × 𝑒𝑒 −𝑡𝑡/𝜏𝜏1 + 𝐵𝐵 × 𝑒𝑒 −𝑡𝑡/𝜏𝜏2 . A and B are 0.71 and 0.29, respectively (A plus B normalized to 1) and 𝜏𝜏1 and 𝜏𝜏2 are 0.43 ± 0.01 ns and 3.09 ± 0.04 ns, respectively. Single exponential is failed to fit the decay curve.

Covalently cofacial or slip-stacked perylene,16 PDI,17,18 and PMI derivatives19 have been reported to have the excimer formation in solution. A strong evidence of low-lying excimer states is the red-shift of the steady-state emission spectra. Therefore, PL techniques were used to probe the aggregation process. Figure 2a displays the self-assembly process of BH4 molecules on an insulating Al2O3 mesoporous film. Initially, the PL peak is at 585nm which agrees with the peak of dye monomers in solution. As the soaking time increases, interestingly, the monomer PL peak remains the same whereas the PL peak at 660 nm become more and more pronounced. This bathochromic-shifted PL peak can be attributed to the dimerized dye molecules on PMI units. We note that the formation of excimers of a series of cofacial and slip-stacked PMI dimers has been reported by Wasielewski et. al. with the evidence of red-shifted in emissions.19 They studied various PMI dimers with head-tohead and head-to-tail configurations and different spatial displacements, in which “head” represents the imide side of the PMI. The excimer emission energy reflects the energy gap between the vibrationally relaxed excimer and the ground state of the molecule in the same conformation. In general, a stronger electronic coupling between dimmers results in a lower emission energy. Compared with the emission energy in their study, the energy of electronic coupling in neighboring BH4 is similar to the energy of head-to-tail configuration in molecular dimers. In addition, we found that the PL intensity can be augmented after the sensitized film being annealed at elevated temperatures under air condition (Figure S1). This may

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be attributed to more dimers formed due to the enhanced thermal motion of the molecules on the oxide surface. The steadystate PL kinetics (Figure S2) and absorptions kinetic profiles (Figure S3 and S4) also shed light on how the molecules assemble on the surface of the oxide and when the dye molecules start to aggregate. Our results suggest that the aggregation exists at the beginning of the soaking process instead of until saturation.

Figure 3. Femtosecond Transient Absorption of Al2O3 / BH4 and Spectroelectrochemical measurement of BH4 solution. a) Spectra at different delay times; b) Decay Associated Spectra (DAS) by global analysis. Excitation: 500nm, time scale: 0-2.8ns. c) TA spectra at 2.8ns; Delta Absorbance of d) reduced BH4 and e) oxidized BH4. Notes: Triphenylamine-oligothiophene conjugated units are denoted as TPA and perylenemonoimide units are denoted as PMI. Besides the steady-state absorption and emission spectra, the formation of excimers can also be characterized by TA spectrum for the near-infrared band.16–19 The TA spectrum of BH4 sensitized Al2O3 film in NIR region can be seen in Figure 3 (NIR only region in Figure S5). Upon ultrafast pulse excitation at 500nm, locally excited PMI38 Frenkel exciton state has a broad feature at 1100nm. Then the formation of the excimer state from lower Frenkel state is observed by the appearance of TAS at 1550nm. The time-resolved photoluminescence (TRPL) of BH4/Al2O3 films displays the radiative pathway of excimer decays (Figure 2b). Double exponential decays have to be used to describe the kinetic curve with lifetimes of 0.43 ± 0.01 ns and 3.09 ± 0.04 ns. The slow component is assigned to the radiative relaxation of excimers and the fast component is due to a proposed pathway of the intermolecular charge separation (vide infra).

The visible region in Figures 3a shows the combined features of the intramolecular charge transfer (Figure S6) and the excimer formation. Our previous study38 has shown that BH4 molecules will form a charge separated state from a locally excited state with a formation lifetime of 0.5ps, followed by decaying to the ground state with a lifetime of 30ps in a DMF solution. However, when dye molecules aggregate on metal oxide surfaces and form dimers, the excimer formation will compete


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Journal of the American Chemical Society with the intramolecular charge separation process upon photoexcitation, which can be assigned to the new peak observed at 730nm at 1ps delay time. Generally, the excimer state is stabilized by mixing of two locally Frenkel states (PMIA*-PMIB and PMIA-PMIB*) and charge transfer states (PMIA+-PMIB− and PMIA−-PMIB+).21 Once the charge transfer state (PMI+-PMI−) is produced, the high-energy PMI+ becomes unstable because the HOMO of the BH4 molecule is located at TPA-oligothiophene units, whereas PMI− remains stable in the BH4 molecule because the LUMO is located at PMI units.38 Positive charge will quickly flow into TPA-oligothiophene units via intramolecular charge transfer. Therefore, a long-lived reduced BH4 featured at 600-630nm38 and oxidized BH4 featured at 700-750nm were observed at 2.8ns delay time (Figure 3d and 3e). Three decay times with an infinite residue are fitted through the global analysis on the whole spectrum ranging from 5201600nm as shown in Figure 3b. Because of both the intermolecular and intramolecular electron transfer process displayed in whole region, the fitted kinetics is more complicated than the kinetics fitted to the NIR-only region. The 0.6ps fast component is assigned to the mixing of the intramolecular charge separation and the excimer formation. The middle component with a lifetime of 30ps is assigned to the mixing of the recombination of intramolecular charge-separated state and the vibrational relaxation of the excimer state. And the slow component of 400ps is assigned to the formation of reduced and oxidized BH4 molecules, which is in excellent agreement with the TRPL fast component decay time.

Figure 4. Nanosecond Transient Absorption of Al2O3/BH4. a) Spectra at different delay times; b) Kinetic profiles at 715 and 630nm. Excitation: 500nm.

The recombination kinetics of long-lived charge separated states can be characterized using the nanosecond transient absorption technique. The kinetics profiles at 715nm and 630nm were chosen as representatives for oxidized BH4 and reduced BH4 (Figure 4). The identical decay kinetics confirms the bimolecular recombination pathway. A stretched exponential decay Kohlrausch−Williams−Watts (KWW) model38–41 is used to fit the decay lifetime (Details can be found in SI). The averaged lifetime τ KWW is calculated as 4.3μs with τ and β equal to 0.0192μs and 0.152, respectively. However, there is a mismatch at fast decay time. Therefore, an additional single exponential decay is fitted only on the 0-5ns data and the decay time is determined as 3.56ns. The fast decay component can be from the signal of the excimer state which is demonstrated in TRPL. Overall, the excimer-mediated charge separated surface exhibits an exceptional slow recombination rate.

Scheme 1. Energy diagrams of aggregated BH4 molecules on Al2O3 upon photoexcitation. a

a. Triphenylamine-oligothiophene conjugated units are denoted as TPA and perylenemonoimide units are denoted as PMI; trc, tct, texc, trlx, tem are lifetimes of the charge recombination, the charge transfer, the excimer formation, the vibrational relaxation, and the emission, respectively.

In conclusion, we have demonstrated the excimer formation of SAM on metal oxide surfaces due to the π-π interaction of the PMI moieties in BH4 molecules. The red-shift of photoluminescence and characteristic signature of NIR transient absorption spectrum cross validate the aggregation of dye molecules in the dye-sensitized system. This study provides a new perspective on the dye aggregates. Due to the conformation and push-pull character of the molecule, long-lived (4μs) charge separated state could form through the proposed excimer-mediated intermolecular charge transfer process between neighboring molecules (Scheme 1). Via the rational design of dye molecules, the excited complexes or aggregates can provide a pathway to generate the long-lived active charge separated state, which can be further utilized in the optoelectronics. Even though the triplet state does not exist in this case due to the unfavorable energetics of PMI (E(S1) < 2 E(T1)),19,25 it still paves the way to support the singlet fission optoelectronic devices on dye-sensitized systems.31,42–44 This work also sheds light on the importance of intermolecular non-covalent interactions and the resultant impact on interfacial electron transfer processes in the dye-sensitized system.

ASSOCIATED CONTENT Supporting Information

This material is available free of charge via the Internet at http://pubs.acs.org.” Experimental and computational methods, PL spectra under annealing condition, UV-vis absorption upon soaking, zoomedin TA spectra in NIR region, and Femtosecond TA of solution.

AUTHOR INFORMATION Corresponding Author

*E-mail: (Y. W.) [email protected]. Fax: +1-614-292-1685. Tel.: +1-614-247-7810. (L.-C. L.) [email protected]. Tel.: +1-614-688-2622

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

We acknowledge exceptional support from Dr. David J. Gosztola on TA experiments at the Argonne National Lab and the funding support from the U.S. Department of Energy (Award No. DE-FG02−07ER46427). The use of the Center for Nanoscale Materials (CNM) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under



Contract No. DE-AC02-06CH11357. The authors also gratefully thank the Ohio Supercomputer Center (OSC) for computational resources.

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Excimer-Mediated Intermolecular Charge Transfer in Self-Assembled

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