Photoinduced Electron Transfer in Asymmetrical Perylene Diimide

Feb 27, 2017 - (2-6) Recently very small energy loss was observed in nonfullerene organic solar cells,(7-11) which would relate to the photoinduced ...
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Photo-Induced Electron Transfer in Asymmetrical Perylene Diimide: Understanding the Photophysical Processes of Light Absorbing Non-Fullerene Acceptors Biao Xu, Cong Wang, Weitao Ma, Linlin Liu, Zengqi Xie, and Yuguang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00263 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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Photo-Induced Electron Transfer in Asymmetrical Perylene Diimide: Understanding the Photophysical Processes of Light Absorbing Non-Fullerene Acceptors Biao Xu, Cong Wang, Weitao Ma, Linlin Liu*, Zengqi Xie*, Yuguang Ma

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China.

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ABSTRACT. In bulk-heterojunction based organic photovoltaics, strong light absorbing ability is one of the most important advantages of non-fullerene acceptors prior to fullerene derivatives. Herein, a series of electron donating units were asymmetrically connected at the imide positions of perylene diimide (PDI) that is a classic electron acceptor building block. The photo-induced electron transfer behaviors were compared in different substitutions by steady/time-resolved spectroscopy and theoretical simulation. The electron donating ability as well as the donor-acceptor distance has a significant impact on the photo-induced electron transition from electron donating units to the PDI core. Due to the process of charge transfer at excited states, a fast non-radiative deactivation is observed for the PDIs with relatively strong electron substituent. These results would help to understand the electron transfer processes from donor to acceptor in the case of the excited electron acceptor molecule.

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1. Introduction

Non-fullerene based organic solar cells have drawn much attention recently owning to the rapid developed power conversion efficiency over 12%.1 The advantages of non-fullerene electron acceptors includes tunable energy levels, various conjugation structures and especially the strong light absorbing ability that make it super compared with the fullerene based electron acceptors.2-6 Recently very small energy loss was observed in non-fullerene organic solar cells,7-11 which would related to the photo-induced electron transition process between the donors and acceptors; however the deep understanding of this process is challenging due to the complicated intermolecular interactions and multiple energy and electron processes in the bulk heterojunction systems. Small molecules and polymers with donor-acceptor structure have drawn much attention because of the easily occurred photo-induced intramolecular/intermolecular electron transfer,12-16 which competes with the radiation process from excited state. The electronic structures of the electron donating and accepting units, and the orientation and distance between them are the key components that affect the electron transfer processes.17-19 Understanding the processes of photo-induced electron transfer occurring between the electron donating units and the chromophore is a crucial step toward the identification of the relationship between structure and property for further development of novel electron accepter materials.20-21

Perylene diimide (PDI) derivatives are promising small molecules for use as non-fullerene acceptors in bulk-heterojunction organic solar cells,22-29 because of their high chemical/ thermal stability, strong molar absorption coefficient, and high electron mobility.30-33 3

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Furthermore, PDIs feature a relatively low reduction potential,34,35 which makes it also suitable as an electron acceptor in photoinduced charge transfer processes.18,19 Herein, we present the synthesis of a series of PDIs incorporated in functional molecular architectures, using electron donating units at imide position to mimic the electron donors in blending films. We varied the donor units, and the donor−acceptor distance to probe the influence of these factors on the photo-induced charge transfer process. Due to the different electronic properties of the donor units, the different fluorescence quantum yield and emission life time was observed. The rate constants of radiative transition and non-radiative transition of the PDIs were determined, which were used to assess the process of the photo-induced electron transition. Assistant by theoretical simulation, the investigation identify the possible intermediate species formed upon excitation.

2. RESULTS AND DISCUSSION

2.1 Synthesis A series of asymmetrically substituted PDIs containing ortho-cresol groups at bay areas and different electron donors at the imide nitrogen was synthesized with relatively high yields. Chemical structures of the asymmetric PDI derivatives were shown in Chart 1, which were named as PDI-POMe, PDI-PCz, PDI-TPA and PDI-PTPA for the molecules bearing different electron donating units. All these compounds were fully characterized by 1H NMR, 13C NMR and MALDI-TOF (see supporting information).

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O O

O O O

O

N

N

O

O

O

O

N

N

O

O

N

O

O O

O O

PDI-POMe

PDI-PCz

O O

O O

O

O

N

N

O

N

O

O

O

N

N

O

N

O

O O

O O

PDI-TPA

PDI-PTPA

Chart 1. Chemical structure of the asymmetric PDI derivatives.

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 400

450

500

550

600

650

700

Fluorescence (a. u.)

2.2 The Steady-State Absorption and Emission Spectra

Absorption (a. u.)

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0.0 750

Wavelength (nm)

Figure 1. UV-Vis absorption and emission spectra of PDI-POMe in toluene.

All the PDI derivatives are well soluble in common organic solvents such as dichloromethane, THF and toluene. Figure 1 shows the normalized absorption and emission spectra of PDI-POMe in toluene. The ground state absorption spectrum features a strong 0-0 vibronic transition at 574 nm and a shoulder at 533 nm corresponding to the 0-1 vibronic bands. The emission spectrum shows closely the mirror image of the absorption spectra. The 5

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introduction of different electron donors at the imide nitrogen of the PDI chromophore gives almost identical absorption and fluorescence spectral position35 (Table 1), which indicate that the radiative transition of all these molecules occurred between the frontier orbitals of the PDI core, regardless the variation of the electron donating substituents on imide position.

However, the fluorescence intensity exhibits significant change for different molecules in the same solvent. Fluorescence quantum yields (Ф) of PDIs were determined in toluene (Table1). Remarkably, the investigated PDI-POMe and PDI-PCz display high fluorescence quantum yields (94% and 92%), which show the minor effect of nonradiative processes. While the fluorescence of PDI-TPA and PDI-PTPA is strongly quenched in toluene leading to that the fluorescence quantum yields is quite low (0.6% and 14%). The fluorescence quantum yields for PDI-POMe, PDI-PCz and PDI-TPA confirm that charge transfer modification is viable through the various electron donor units of PDIs. As for PDI-POMe and PDI-PCz, there is neglected effect of different donor on enhancing the charge transfer in toluene. When a stronger donor is present in the system, as the case of PDI-TPA, the fluorescence is completely quenched. Hence, the fluorescence quantum yields decreased deeply with increasing electron donating ability, which means that the efficiency of electron transfer is tightly correlated with donor ability. We attribute this rapid fluorescence quenching to a photo-induced intramolecular charge transfer (ICT) from the electron donating units to the electron accepting PDI core. The stronger electron donating of the substituent provides a significantly increased driving force for the transition from the local excited (LE) state to the ICT state. Hence, the LE state is nearly totally converted to the ICT state leading to the 6

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efficient nonradiative deactivation in PDI-TPA, which will be discussed in detail in the following sections.

Then the systems of PDI-TPA and PDI-PTPA were designed to allow a direct comparison of the electron transfer via extending the distance between the donor and acceptor. A second phenyl spacer group was added in PDI-PTPA to increase the distance, which affords greater electronic isolation of the donor from electron accepting PDI core, and thus, the fluorescence quantum yields was increased significantly from near zero to 14% in toluene following the expected trend.

Table 1. Photophysical parameters of PDIs in toluene.

a)

ΦPL

Fluorescence decay times (ns)

Kf (108 s-1)

Knf (108 s-1)

598

0.94

6.0

1.55

0.11

576

600

0.92

5.6

1.64

0.15

PDI-TPA

575

600

0.006

0.27

0.22

36.8

PDI-PTPA

576

599

0.14

2.73a)

0.51

3.15

PDI

Absorption max. (nm)

Fluorescence max. (nm)

PDI-POMe

574

PDI-PCz

Double-exponential fitting results with τ1 of 1.67 ns (70%) and τ2 of 5.13 ns (30%).

2.3 The Time Resolved Fluorescence Spectra

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PL intensity (a. u.)

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PDI-POMe PDI-PCz PDI-TPA PDI-PTPA IRF

0.1

0.01

0

10

20

30

40

50

60

Time (ns)

Figure 2. Time resolved fluorescence spectra of PDIs in toluene, monitored at 610 nm. In order to figure out the influence of the imide substituent on charge transfer dynamics, a series of time resolved fluorescence experiments were performed (Figure 2). The fluorescence decay properties of the series of PDIs in toluene are summarized in Table 1. On the basis of the fluorescence quantum yield and fluorescence life time (τ) values, the rate constants of radiative transition (Kf) and non-radiative transition (Knf) of the PDIs were determined by equations 1-2, respectively.36

Φ = Kf /

( Kf +

K nf )

(1)

τ = 1 / ( K f + K nf )

(2)

The fluorescence traces recorded for PDI-POMe and PDI-PCz in toluene can be well fitted with a single-exponential decay with a time constant of ~6 ns, which is the characteristic for the fluorescence of the PDI derivatives. The rate constant of radiative transition (Kf) is over 10 times larger than the rate constants of non-radiative transition (1.64 × 108 s-1 vs 0.15 × 108 s-1), which reflects that radiative transition occupies a dominant position. The possibility is that the energy of LE state is inferior to ICT state in toluene, which induced restriction of the transition from the LE state to the ICT state. 8

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The quantitative analysis of the fluorescence lifetime of PDI-TPA was very small for the fact that the emission is too weak. The observed decrease of the fluorescence decay time and Φf indicates that the existence of an additional nonradiative decay channel from LE state. The rate constants of non-radiative transition is over 150 times faster than the rate constants of radiative transition (3.68 × 109 s-1 vs 0.02 × 109 s-1), showing the nonradiative deactivation have dominated the whole process, which leads to the transition from the LE state to the ICT state with a higher efficiency.

Adding another phenyl spacer group, PDI-PTPA exhibits an increased fluorescence lifetime about 2.7 ns and a relatively high radiative transition due to the greater electronic isolation between donor and PDI core. Here, two different deactivation channels were observed with corresponding life time of

τ1 = 1.67 ns (70%) and τ2 = 5.13 ns (30%). The

longer life time is accordance with the fluorescence life times of PDI-POMe and PDI-PCz indicating the radiation from LE state, while the short one must relate to the nonradiative deactivation that is relaxes from the ICT state. The ratio between the deactivation from the two channels reflects the competition of the radiation process and the charge transfer from TPA units to the PDI core. However, the nonradiative deactivation also remains in a dominant position, and a lower-speed charge transfer occurring from the electron rich to the electron poor PDI core when compared with that of PDI-TPA. These results show that the short donor-acceptor distance is in favor of electron transfer.

Solvatochromism is a characteristic of donor-acceptor molecules to change color due to a change in solvent polarity. But for all the PDIs discussed here show very small solvent effect 9

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even the polarity largely changed from toluene to DMF, which are due to the N node that isolates the donor unit and the acceptor PDI core. However, the fluorescence quantum yields and life times of these PDIs show various changes when the solvent was changed, as given in supporting information. The results indicate that the photo induced electron transfer processes were strongly related to the solvent polarity.

2.4 Theoretical calculations The density functional theory (DFT) calculation gave the frontier molecular orbital structures at B3pw91/def2TZVP level and the energy levels of the LUMO, HOMO and HOMO-1 in toluene are shown in Table 2 and Figure 3. All the compounds, the LUMOs are delocalized over the PDI core. For both the PDI-POMe and PDI-PCz, the HOMOs are delocalized over the PDI core, and the HOMO-1s are distributed on the donor substituents. However, for both the PDI-TPA and PDI-PTPA, the HOMOs are distributed on the donor units, and the HOMO-1s are delocalized over the PDI core. As shown in Figure 3, when the PDI core is excited, the transition of PDI-POMe and PDI-PCz are dominated by the radiative transition, as a result, they possess high rate constants of radiative transition, which directly leads to large fluorescence quantum yields. Whereas the HOMO-HOMO-1 gap of PDI-PCz has no distinct difference, the solvation effect has a strong impact on the emergence of an intramolecular charge transfer state relative to that of PDI-POMe (see supporting information). For PDI-TPA, because of the energy level of the HOMO located at the donor unit (TPA) is higher than that of the PDI core, when the PDI core is excited, a fast charge 10

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transfer from the donor units to the PDI core occurred easily. Hence, the electron on LUMO of PDI core getting back to the HOMO-1 is hindered, showing a quite low rate constant of radiative transition. While another phenyl spacer group is added in PDI-PTPA, the energy gap between HOMO and HOMO-1 is similar to that of PDI-TPA but the distance between the donor and the PDI core is increased, resulted in a relative lower charge transfer rate from the donor units to PDI core, causing increased rate constants of radiative transition. The rate constants of charge separation (Kcs) was calculated with the equation 3,37 in which the fluorescence lifetime of PDI-TPA or PDI-PTPA in toluene was taken as fluorescence lifetime of PDI-POMe in toluene was taken as

τf , and

the

τ0. The rate constants of charge

separation for PDI-TPA and PDI-PTPA were thus estimated to be 3.53×109 S-1 and 0.20×109 S-1, respectively. The results clearly indicate that the increased distance between the electron donor units and the PDI core obviously reduces the rate of the charge separation. We also noted that the Kcs values here are several orders lower than that reported in bulk-heterojunction solar cells,38 which might due to the nodes on nitrogen atoms that isolate the donor/acceptor units totally without any orbital overlap.

Kcs = 1

1 τ − τ f

(3) 0

Table 2. The calculated frontier molecular orbitals (HOMO, LUMO and HOMO-1) in the ground state of PDI-POMe, PDI-PCz, PDI-TPA and PDI-PTPA in toluene. PDI-POMe

PDI-PCz

PDI-TPA

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LUMO (eV) -3.36

-3.41

-3.37

-3.37

-5.68

-5.72

-5.40

-5.32

-6.73

-5.76

-5.68

-5.68

HOMO (eV)

HOMO-1 (eV)

Figure 3. Schematics of the photophysical processes of the PDIs. The energy levels of LUMO, HOMO and HOMO-1 for PDI-POMe, PDI-PCz, PDI-TPA and PDI-PTPA are based on the calculated results in Table 2.

3. Conclusion We have reported the synthesis of donor-acceptor systems taking advantage of substitution at the imide nitrogens of PDI, and discussed the influence of the photo-induced electron transfer process on the fluorescence properties. The various ability of electron-donating has a significant impact on the rate constants of radiative transition (Kf) and non-radiative transition

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(Knf). The DFT calculation for our interpretation provides an important basis. The different energy level alignment and distance between the donor units and the PDI core on the photo induced electron transfer processes contributes to understanding of the photophysical processes of light absorbing non-fullerene acceptors.

ASSOCIATED CONTENT

Supporting Information. Experiment conditions, synthesis, solvatochromism experiments, and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the financial supports from the National Natural Science Foundation of China (51373054, 51573055, 51473052), National Basic Research Program of China (973 Program) (2013CB834705, 2014CB643504), and Fundamental Research Funds for the Central Universities. 13

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