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Letter

Direct Observation of an Efficient Triplet Exciton Diffusion Process in a Platinum-Containing Conjugated Polymer Lili Du, Wenjuan Xiong, Shun-Cheung Cheng, Haiting Shi, Wai Kin Chan, and David Lee Phillips J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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The Journal of Physical Chemistry Letters

Direct Observation of an Efficient Triplet Exciton Diffusion

Process

in

a

Platinum-Containing

Conjugated Polymer Lili Du,‡ Wenjuan Xiong,‡ Shun-Cheung Cheng,a Haiting Shi, Wai Kin Chan,* and David Lee Phillips* Institute of Molecular Functional Materials# and Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China ‡

These co-authors contributed equally to this work

a

Present address: Department of Biology and Chemistry, City University of Hong Kong, Tat

Chee Avenue, Kowloon Tong, Hong Kong S.A.R., China #

Areas of Excellence Scheme, University Grants Committee (Hong Kong)

Corresponding Authors * [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT. We report the synthesis and characterization of a conjugated polymer incorporated with cyclometalated platinum complexes on the main chain. The polymer may serve as an efficient triplet sensitizer in light harvesting systems. The photophysical properties of the polymer was studied by nanosecond and femtosecond time resolved transient absorption spectroscopies. After excitation, an energy transfer process from the thiophene units on the conjugated main chain to the singlet excited state of the Pt complex moieties occurred in less than 150 fs. The subsequent intersystem crossing process resulted in the formation of a triplet excited state at the Pt complex moieties in ~3.2 ps, which was then followed by an efficient triplet diffusion process that led to the formation of triplet excitons on the polymer main chain in ~283 ps. This proposed efficient triplet sensitized polymer system not only enhances the exciton diffusion length, but also reduces energy loss in the process, which displays remarkable implications in the design of novel materials for triplet sensitized solar cells.

TOC GRAPHIC

KEYWORDS. Transient absorption, platinum complex, conjugated polymer, light harvesting


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Conjugated polymers have been extensively used as key materials in a variety of optoelectronic devices such as transistors, photovoltaic cells, and light emitting diodes.1-3 The operation of all of these devices involves the generation and migration of excitons (electronhole pairs) in polymer molecules, and the properties and lifetime of these excited species are critical to the performance of the devices concerned. For example, in a typical organic photovoltaic cell, excitons are generated upon photoexcitation of the sensitizers. The excitons may undergo charge recombination or separation, which may be singlet or triplet in nature. It has been reported that organic photovoltaic devices based on materials with a large triplet yield exhibited good device performance,4-6 due to the longer lifetimes and exciton diffusion length. In order to achieve an efficient charge transport process in the conjugated main chain, it is essential to maintain a rapid intra- and inter-chain transfer of excitons, because the geometric disorder and chemical defects present in the main chain could have a detrimental effect on the device performance.7-9 Transition metal complexes, such as those based on ruthenium (II) and platinum (II), have been used as photosensitizers extensively due to their unique photophysical and electrochemical properties. Their singlet excited states undergo rapid intersystem crossing (ISC) process due to the strong spin-orbit coupling imparted by the heavy metal centers, which results in the formation of triplet excited states.10 Therefore, an enhancement in the light harvesting ability of the conjugated polymers with metal complex sensitizers may be achieved by making use of the high ISC yields of the transition metal complex. In such systems, the singlet excitons formed on the polymer main chain may migrate to the metal complexes, which then undergo rapid ISC process to yield longer-lived triplet excitons. Polymers that contain transition metal complexes with efficient singlet to triplet exciton sensitization have been used for light harvesting applications.11,

12

Recently, the

photophysical properties of a polythiophene incorporated with a platinum porphyrin complex


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was presented.13 After excitation, the singlet exciton formed on the polymer main chain underwent an ISC process via the platinum complex. Subsequently, 60 % of the triplet excitons formed were transferred back to the polymer main chain. Comparison to the approach by which metal complexes are blended with a polymer in a host-guest system, the use of a metalloconjugated polymer gives a much more homogeneous morphology and less defects. This may inhibit the energy loss in the exciton migration process and show an improvement in the photovoltaic performance. In this letter, we present the synthesis and characterization of a new metalloconjugated polymer that contains C^N^C type platinum cyclometalated complex as part of the polymer main chain. The complex 1 was achieved by the reaction between the K2PtCl4 and 2,6-Bis(5bromothiophen-2-yl)-4-phenylpyridine according to the procedure reported.14 Cyclometalated Pt complexes exhibit high thermal stability and have widely been used as phosphorescent emitters in light emitting devices due to the high quantum efficiency from the triplet excited states.14,

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We have employed femtosecond transient absorption (fs-TA) and nanosecond

transient absorption (ns-TA) spectroscopic techniques to study the formation and diffusion of the singlet and triplet excitons directly. These results provide a fundamental understanding of the photophysical properties of the metalloconjugated polymers, as well as the role of the Pt complexes in the formation and migration of triplet excitons within the polymer system. Better understanding this will be crucial in designing new metal complex based polymers for applications in triplet sensitized light harvesting systems. The target polymer 2-Pt was synthesized by the palladium-catalyzed coupling reaction between bithiophene monomer 12 and platinum (II) complex 1, synthesized by a modified literature procedure.18,

19

11, 16, 17

which was

A metal free polymer 2 was also

synthesized as the model polymer for photophysics properties studies. Detailed synthetic procedures and characterization data are presented in the Supporting Information (SI).


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Scheme 1. Structures of complex 1, polymer 2 and polymer 2-Pt. complex 1 polymer 2 polymer 2-Pt

Normalised Intensity

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300

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Wavelength (nm)

Figure 1. UV-Vis absorption (solid line) and fluorescence spectra (dot line) of complex 1, polymer 2, and polymer 2-Pt measured in THF solutions. The UV-vis absorption and emission spectra of complex 1, polymer 2, and polymer 2-Pt are shown in Figure 1. Polymer 2 exhibits an intense absorption band centered at 400 nm due to the π-π* transition of the polymer backbone.14, 20 For complex 1, its absorption features are similar to the analogous Pt complex reported previously.14 It exhibits two strong absorption bands centered at 310 and 340 nm, which are assigned to the intraligand (IL) transitions of the C^N^C ligand. In addition, two low-intensity absorption bands centered at ca. 520 and 570 nm are assigned to the mixed metal-to-ligand charge transfer-intraligand (MLCT-IL) transitions.14 TDDFT calculation results also support these spectroscopic assignments (Table S1). For 2-Pt, a notable difference in its electronic transitions is observed compared to the metal free polymer 2 and the monomeric complex 1. Two strong absorption bands at 315 and


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367 nm are assigned to the C^N^C ligand based IL transitions, which is similar to those observed in complex 1. In addition, a shoulder band is seen at ca. 420 nm, which is assigned to the absorption due to the conjugated main chain. For the electronic transitions at lower energy, the polymer 2-Pt shows a significant red shift in the absorption maxima as well as an increase in intensity compared to complex 1. In order to assign these absorption bands, TDDFT calculations were performed on complexes 13, 14 and 15 (serving as the models for polymer 2-Pt), and their structures are shown in Figure S1. It can be seen that the HOMO of model complex 13, complex 14 and complex 15 are mainly contributed from the Pt center, π orbitals of the C^N^C ligand, and the thiophene units (Table S1). On the other hand, the LUMOs has mainly contributions from the pyridine and isocyanide ligands, while the electron densities of the thiophene units and the Pt center are significantly reduced. This strongly suggests that the HOMO-LUMO electronic transitions have substantial charge transfer character, and these absorption bands can be tentatively assigned to mixed intraligand charge transfer (ILCT) and MLCT-IL transitions as discussed above. The calculated HOMO energies (Table S1) for the model complexes 13-15 agree quite well with that of polymer 2-Pt obtained by CV measurements (Table S2). However, the LUMO energies calculated for these model complexes deviate from the experimental values. The lower LUMO energy achieved for polymer 2-Pt could be due to its more extended π electron delocalization for the polymer. Polymer 2 exhibits a strong emission band at 520 nm with a shoulder at 550 nm (Figure 1), which is assigned to the fluorescence of the conjugated main chain.21 However, polymer 2-Pt is non-emissive, even though it contains both the conjugated main chain and the cyclometalated Pt complex moieties. The quenching of the emission is suggested to be due to an energy transfer22 process from the singlet excited state of polymer backbone to the Pt complex moieties, which then underwent a series of processes to a non-emissive excited state


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(see discussion below). An efficient light harvest system can be designed by taking advantage of such an energy transfer process in polymer 2-Pt. The photophysical properties of complex 1, polymer 2, and polymer 2-Pt were studied by fsTA and ns-TA spectroscopy experiments. The fs-TA spectra of the polymer 2 are displayed in Figure 2. With reference to its fluorescence spectrum (Figure 1), the appearance of a TA absorption band at 550 nm at 40 ps (Figure 2a) is assigned to the stimulated emission from the singlet exciton of the thiophene units.13 Therefore, the TA bands observed from 0.7 ps to 40 ps could be due to the transition from higher singlet excited states Sn to the lowest singlet excited state S1. Subsequently, a new band appears at 710 nm in 630 ps, and a distinct isosbestic point at 740 nm is observed (Figure 2b). This is assigned to the ISC process. The formation of triplet excited states was confirmed by measuring the ns-TA spectra in degassed solution (Figures S2a-b). The decay time constants in both air-saturated and degassed solutions are identical (222 ns and 13.4 µs), indicating that the band observed at late delay times in the fs-TA spectra corresponds to formation of triplet exciton of the thiophene units on the conjugated main chain.

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8 742fs 897fs 1.16ps 1.76ps 3.47ps 6.41ps 11.1ps 26.2ps 39.9ps

0

-4

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∆ OD /10

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τ=630 ps

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39.9ps 59.9ps 119ps 196ps 320ps 505ps 911ps 2.9ns 700 750

Wavelength (nm)

Figure 2. (a,b) fs-TA spectra of polymer 2 in THF solution after excitation at 400 nm. The kinetics of the growth process observed at 710 nm are shown in the inserted diagram in (b). The solid line indicates the fitting to the experimental data.


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For complex 1, the fs-TA (in air-saturated THF solution) and ns-TA (in degassed THF solution) spectra are shown in Figures S3a and S4a, respectively. The decay of the band at 450 nm and the growth of the band at 650 nm from 1 ps to 64 ps (Figure S3a) correspond to the ISC process from the singlet MLCT-IL to triplet MLCT-IL excited states (with a time constant of 10 ps, see Figure S3b). The TA spectrum observed at 64 ps in Figure S3a can also be observed in Figure S4a by the ns-TA experiment, indicating that they are the same species. However, the kinetics of the decay of the ns-TA spectra measured in air-saturated and degassed solutions show a significant difference (Figure S4b), and the decay time constants are 232 ns and 6.1 µs, respectively. The oxygen quenching study clearly suggests the excited species observed at 64 ps is triplet in nature.

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4 3

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628fs 1.17ps 1.83ps 2.32ps 4.38ps 7.87ps 10.1ps

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τ1= 3.2 ps

10.1ps 18.6ps 25.6ps 37.2ps 61.5ps 88.4ps 144ps 263ps 457ps 698ps 2.92ns

3

∆ OD /10

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2

τ2= 283 ps

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Figure 3. (a, b) fs-TA spectra of polymer 2-Pt in THF solution after excitation at 400 nm. (c) Kinetics of the decay process observed at 480 nm. The solid lines indicate the fitting of the experimental data points. The fs- and ns-TA spectra of polymer 2-Pt are shown in Figures 3 and 4. Within the first 10 ps, the TA spectra are similar to those of complex 1, except that the initial absorption band shifts from 450 to 480 nm with an enhanced intensity, and there appears to be no shift in the absorption band at 650 nm. It is suggested that the absorption is due to an excited state with mixed singlet MLCT-IL as well as ILCT characters of the Pt complex moieties as discussed above. On the other hand, the singlet emission band observed in the fs-TA spectra of polymer


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2 (Figure 2) is not observed in polymer 2-Pt (Figure 3a). This suggests that there is a very fast energy transfer process from the thiophene units on the polymer backbone to the Pt complex moieties, and the process occurs within the instrument response time (

Direct Observation of an Efficient Triplet Exciton ... - ACS Publications

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