Ï-Conjugated Organometallic Isoindigo Oligomer and Polymer

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#-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells Subhadip Goswami, Melissa Katherine Gish, Jiliang Wang, Russell W Winkel, John M. Papanikolas, and Kirk S. Schanze ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09041 • Publication Date (Web): 12 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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ACS Applied Materials & Interfaces

π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells Subhadip Goswami, † Melissa K. Gish, ‡, # Jiliang Wang, †, # Russell W. Winkel, † John M. Papanikolas,‡ and Kirk S. Schanze*,† †

Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States ‡

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599,United States *Corresponding author: e-mail [email protected], Tel 352-392-9133. #

These authors contributed equally to the work.

Abstract An isoindigo based π-conjugated oligomer and polymer that contain cyclometallated platinum(II) “auxochrome” units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission and picosecondnanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometallated units have a significant impact on HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with

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~100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a timescale of ~130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low. Keywords: Isoindigo, orthometallated platinum, triplet state, transient absorption, polymer solar cell Introduction The increasing demand of flexible, light weight and cost effective organic photovoltaic cells as an alternative renewable energy resource spurs the interest of the scientific community to develop novel materials. Efficient photocurrent generation requires a material to follow all the necessary steps (light absorption, exciton diffusion, charge separation and charge transport) effectively. One effective way of increasing absorption of the solar spectrum is to utilize donoracceptor π-conjugated chromophores as the donor materials for organic photovoltaic devices. In this approach, a π-electron rich donor is combined with a π-electron deficient acceptor, and due to the interaction of their frontier orbitals, the effective bandgap is reduced compared to their

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individual bandgaps.1-4 Utilizing this approach, the efficiency of polymer solar cells has been improved up to 10%.5 Compared to an abundance of π-electron donors, electron deficient acceptors are less common. Among them, benzothiadiazole (BTD),6-8 diketopyrrolopyrrole (DPP),9-11 naphthalene diimide (NDI)12 have gained popularity. However, to understand the structure property relationship for donor-acceptor (D-A) systems, it is necessary to search for novel acceptor materials. Isoindigo is a structural isomer of the well-known naturally occurring pigment indigo.13 Although the synthesis of isoindigo was first reported in 1988,14 it was not until 2010 when Reynolds and coworkers incorporated isoindigo into a π-conjugated D-A framework for application in organic photovoltaic devices.

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The electron deficient nature of isoindigo can be

attributed to the presence of electron poor lactam rings. When introduced into polymeric structures, isoindigo can have two different linkage isomers involving the 5,5’ and 6,6’ positions. Because the 6,6’-substituted isomer affords a conjugated quinoid mesostructure, this isomer gained popularity in organic optoelectronics over the other isomer.13 After the first report on isoindigo based small molecules for solar cells, there has been growing interest in using these materials for both photovoltaics as well as organic field effect transistors (OFETs). For example, Andersson and coworkers reported power conversion efficiency (PCE) of 6.3% with isoindigoterthiophene containing donor-acceptor copolymers,16 and in 2011 Bao and co-workers reported siloxane terminated isoindigo containing copolymers with hole mobility as high as 2.48 cm2 V-1 s -1.17 An isoindigo based polymer was also used as an acceptor in all polymer solar cell with P3HT as donor providing an overall PCE of 0.5%.18 Most studies on isoindigo chromophores have focused on singlet excited state properties; as such, the triplet excited state of this versatile acceptor remains unexplored to date. By contrast,

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the triplet excited state properties of indigo and thioindigo dyes were explored several decades ago.19-21 One approach to enhancing the efficiency for formation of the triplet excited state is to incorporate heavy metals into a π-conjugated electronic system to induce spin-orbit coupling (SOC) and enhance singlet-triplet intersystem crossing (ISC). Considering the importance of triplet excited states in optoelectronics,22 it is necessary to explore the triplet excited state properties of isoindigo based chromophores. Since their development in the 1970s,23 platinum acetylide containing π-conjugated chromophores have been of interest in the scientific community due to their high ISC efficiency (ɸISC) facilitated by strong interactions between the dπ orbitals of the metal and pπ orbitals of the chromophore. Recently, Wong and coworkers reported platinum acetylide containing isoindigo polymers and compared their photophysical properties with a diketopyrrolopyrrole analogue.24 However, triplet excited state photophysics was not explored in their study. It is important to mention that isoindigo-platinum acetylide derivatives were also synthesized in our lab, and the triplet excited state was not observed by nanosecond transient absorption spectroscopy.25 It was concluded that efficient non-radiative decay of the singlet state occurs via rapid twisting around the central olefinic bond, and because of this ISC is not competitive.26 Recently, we reported a comparative photophysical and electrochemical study of platinum acetylide and cyclometallated platinum complex auxochromes with diketopyrrolopyrrole πconjugated chromophores.27 This study found that the cyclometallated platinum auxochrome imparts stronger spin-orbit coupling and increased excited state delocalization compared to the platinum acetylide system. Inspired by this result, we have synthesized two cyclometallated platinum(II) isoindigo complexes with different solubilizing alkyl chains, namely Iin-C8Pt(acac) and Iin-C18-Pt(acac) (Chart 1). A structurally related polymer, Iin-C18-Poly was

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prepared with solubilizing octyldodecyl chains. (Note that the repeat unit structure of Iin-C18Poly is regio-random). Isoindigo precursors Iin-C8 and Iin-C18, which were used to compare the photophysical properties with metallated chromophore, are also shown in Chart 1. Chart 1

The metal complexes and the polymer were characterized by ground state absorption, steady state emission spectroscopy and picosecond-nanosecond

(ps-ns) transient absorption

spectroscopy. Finally, a preliminary study of solution processed, polymer photovoltaic cells with Iin-C18-Poly/PCBM blends gave a relatively low power conversion efficiency (PCE) of 0.22%. Analysis of the energetics of charge transfer suggest that the low PCE arises due to low driving force for Iin-C18-Poly to PCBM photoinduced charge transfer. This is the first detailed report of the photophysics of isoindigo containing organometallic chromophores, and it is the first study to provide direct observation of the triplet state in this important chromophore for materials chemistry.

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Experimental Synthesis. Synthetic schemes and details of the synthetic procedures along with the structural characterization data are provided in the supporting information. Instrumentation and Methods. 1H NMR spectra and

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C NMR spectra were recorded by

using Inova-500 FT-NMR instrument operating at 500 MHz for 1H NMR and 125 MHz for 13C NMR. Mass spectra analysis of the newly synthesized compounds were performed in mass spectrometry services, which is present in house at University of Florida. Elemental analysis of the metal complexes were performed at Complete Analysis Laboratories Inc. which is located in Parsippany, NJ. The molecular weight of the polymer was analyzed by Gel Permeation Chromatography (GPC) from a system containing Shimadzu SPD-20A photodiode array (PDA) detector and THF as eluent with a flow rate of 1 mL/ min. Before the measurement, the system was calibrated using linear polystyrene in THF. The photophysical measurements were performed with dry, HPLC grade THF, unless otherwise mentioned in 1 x 1 cm2 cuvettes. Ground state absorption spectra were obtained on a Shimadzu UV-1800 dual beam spectrophotometer. Steady state emission spectra were obtained on a Photon Technology International (PTI) spectrophotometer and the data were collected at 90° relative to the excitation beam. The optical density (O.D) of the sample solutions were ≤ 0.1 for optical measurements unless otherwise mentioned. For fluorescence quantum yield calculations, corrections for refractive index were included for both sample and standard solutions. Tetraphenylporphyrin in toluene (ɸF=0.09) was used as an actinometer for this measurement. Fluorescence lifetime data were obtained from PicoQuant FluoTime 100 Compact Fluorescence

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Lifetime Spectrophotometer by time-correlated single photon counting (TCSPC). Sample solutions were excited a 375 nm by PDL-800B Picosecond Pulsed Diode Laser. Nanosecond transient absorption spectroscopy data was obtained from an in-house instrument using the third harmonic of a continuum Surelite series Nd:YAG laser. The probe light was generated from a xenon flash lamp and the signal was detected by using gated intensified CCD mounted on a 0.18 M spectrograph (Princeton PiMax/Acton Pro 180). The optical density of the samples was kept at ~ 0.7 at 355 nm and the solutions were deoxygenated by purging argon for at least 45 minutes. The sample solutions contained in 1 cm path length flow cell (10 mL) and the irradiated volume was re-circulated through the pump-probe region to minimize sample degradation. Femtosecond transient absorption data were obtained using a pump-probe configuration with a 1 kHz Ti:sapphire chirped pulse amplifier (Clark-MXR CPA-2001). The 490 nm (600 nJ) pump pulse was generated via frequency doubling a portion of the 775 nm regenerative amplifier beam. A portion of the 775 nm fundamental was focused in a translating CaF2 window to generate a white light continuum, which was used as the probe pulse. A mechanical chopper synchronized to the laser chopped the pump at 500 Hz. Pump and probe polarizations were set to magic angle. The two beams were focused to a 150 µm spot size, and spatially overlapped at the sample. A fiber optic coupled multichannel spectrometer with a CMOS sensor collects the probe pulse, where pump-induced changes in the white light continuum were measured on a pulse-topulse basis with a sensitivity of 0.1 mOD. Time-resolved transient absorption spectra were collected with an approximate time resolution of 250 fs with a spectral window of 350 nm to 800 nm. The time delay between pump and probe are regulated using a computer controlled delay stage.

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The cyclic voltammetry studies were performed on a BAS CV-50W voltammetric analyzer (Bioanalytical Systems, Inc., www.bioanalytical.com) in dry dichloromethane (CH2Cl2) solution in the presence of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAH) as electrolyte. The measurements were performed with three electrodes: a platinum microdisk with area of 2 mm2 as working electrode, a platinum wire as the auxiliary electrode and a silver wire as the reference electrode. Sample concentrations were maintained to 1 mM and the scan rate of 100 mV/s was used for the study. The sample solutions were deoxygenated prior to the measurement and constant flow of argon was maintained during the data collection. All the potentials collected were corrected against ferrocene as internal standard (E(Fc/Fc+)) = 0.43 V vs SCE in CH2Cl2). Photovoltaic devices were fabricated by using pre patterned indium tin oxide coated glass slides obtained from Kintec Company. The ITO coated glass substrates are subsequently treated with aqueous sodium dodecyl sulfate (Fisher Scientific), deionized water (Milli-Q), isopropyl alcohol and acetone in an ultrasonic bath for 15 minutes. The substrates were then treated with oxygen plasma for 15 minutes in a Plasma Cleaner (Harrick PDC-32G). An aqueous solution of PEDOT:PSS (Baytron P VP Al 4083) was spin coated at a rate of 5000 rpm for 1 minute and subsequently annealed on a hot plate at 130°C for 20 minutes. A solution of the active layer with the desired ratio of polymer:PC61BM was prepared in chlorobenzene and stirred at 60°C overnight under argon filled M-Braun glove box. The active layer solution was then spin coated onto the PEDOT-PSS coated substrates at 1000 rpm in the glove box and the resulting films were dried under high vacuum overnight at room temperature. LiF (0.5 nm) and Al (100 nm) were deposited by thermal evaporation under high vacuum overnight on the active layer in sequence. The cathode pattern on the substrates was created by using copper mask, resulting pixel size of the cathodes of 0.07 cm2.

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The current-voltage (J-V) plot was measured with a Keithley 2400 source measurement unit (SMU) under illumination of A.M. 1.5 with an incident power density of 100 mW/cm2 supplied by a xenon arc lamp power supply (Oriel Instruments). The external quantum efficiency (EQE) of the devices were evaluated by measuring the IPCE (%). IPCE measurements were performed with monochromatic light generated by a monochromator (Oriel Instruments) with a xenon arc lamp as the light source. A power meter (S350, UDT Instruments), equipped with a calibrated silicon detector (221, UDT Instruments) was used to measure the intensity of the source at each wavelength (10 nm increment). A Keithley 2400 source meter was used with no voltage bias to record the current at each illumination wavelength. All the measurements were performed under air and without encapsulation. Atomic force microscopy (AFM) was carried out using a Veeco Innova Microscope in tapping mode with a silicon tip (radius ~ 8 nm) at 325 kHz with a force constant of approximately 40 N/m (Mikromasch USA). All calculations were carried out using DFT as implemented in Gaussian 0928 Rev. C.01. Geometries were optimized using the B3LYP functional along with the 6-31G(d) basis set for C, H, N, O, the 6-31+G(d) basis set for P, S, and the SDD basis set for Pt. To minimize computational cost, 4-(octyloxy)phenyl groups were replaced with methyl. The model complex is designated by the addition of a prime (’) to their names, thus the model for Iin-C18-Pt(acac) is termed as Iin-Pt(acac)’. Singlet optimizations were started from idealized geometries and were run without symmetry constraints. Triplet optimizations were started from the optimized singlet geometry and used the unrestricted B3LYP functional. All optimized structures were characterized by vibrational frequency calculations and were shown to be minima by the absence of imaginary frequencies. Time-dependent DFT calculations were performed for all optimized structures at the same level of theory with the same basis sets. Structures and orbitals were

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visualized using Chemcraft Version 1.8, which was also used to generate charge difference density plots.

Results and Discussion Electrochemistry. In an effort to understand the redox behavior of the isoindigo chromophores, electrochemical studies were performed with the precursor molecule Iin-C18, metal complex Iin-C18-Pt(acac) and the polymer Iin-C18-Poly in dry dichloromethane solvent. Cyclic and differential pulse voltammograms are shown in Figure S17 and the peak potentials are listed in Table 1 (vs Fc/Fc+ internal standard). Table 1. Summary of electrochemical properties for isoindigo chromophores.a Compound

E1/2/V Red1 Red2

E1/2/V Ox1

Iin-C18

-1.21

-1.69

–

Iin-C18-Pt(acac)

-1.25

-1.60

0.51b

Iin-C18-Poly

-0.90c

–

0.72b

Ox2 – – –

Measured in CH2Cl2 using 0.1 M NBu4PF6 as supporting electrolyte. It was scanned at 100 mV s−1. All the potentials are referenced to a Fc/Fc+ couple as an internal standard. b Oxidation potentials estimated from the optical bandgap and the reduction potentials derived from electrochemistry. c Onset of anodic wave in DPV. a

The model compound Iin-C18 features two reversible reduction potentials at -1.21 V and -1.69 V. The compound also features a poorly resolved, irreversible oxidative wave with an onset at ~ 1.00 V. The redox behavior of Iin-C18 is consistent with its electron deficient nature and is similar with a previous report for the same chromophore with different alkyl chains.29 The LUMO energy level was estimated at 3.89 eV on the basis of the first reduction potential. For the

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metal complex Iin-C18-Pt(acac) the cyclic voltammogram features a pair of reversible reduction waves at -1.25 V and -1.60 V. The LUMO energy was calculated at 3.85 eV. Note that there is minimal change in reduction potential from Iin-C18 to Iin-C18-Pt(acac), suggesting that the cyclometallated platinum unit has a negligible effect on the LUMO energy. The oxidation wave for Iin-C18-Pt(acac) was also poorly defined, likely due to participation of the metal center in the oxidation processes, leading to irreversible changes in the metal complex.30

Density

functional theory (DFT) calculations correlate well with the observed electrochemical data. While the LUMO is centered mostly on the isoindigo acceptor, the HOMO is delocalized over the entire molecule with significant contribution from the metal d-orbitals (Figure S14). For the polymer, the first reduction potential wave is shifted positive to -0.9 V (calculated from onset of DPV) and the LUMO energy level was estimated at 4.2 eV. The oxidation potentials for the polymer and the metal complex are estimated from the LUMO of the chromophores and the optical bandgap values. UV-Visible Absorption Spectroscopy. Ground state absorption spectra of the precursor molecules, metal complexes and polymer were recorded in THF solution and are presented in Figure 1A. In general the absorption spectra feature two bands, which is typical for D-A type chromophores. Previous reports on D-A based π-conjugated chromophores attribute the low energy absorption band to charge transfer and the high energy absorption to a π-π* transition.31 Precursors Iin-C8 and Iin-C18 exhibit similar absorption spectra (for clarity, only the spectrum of Iin-C18 is shown) consisting of an intense absorption at 300 - 425 nm and a moderately intense band at 425 - 600 nm. Both metal complexes Iin-C8-Pt(acac) and Iin-C18-Pt(acac) exhibit dual absorption bands between 300 - 450 nm and 500 - 700 nm. For the polymer, the

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absorption band is further red shifted compared to the metal complexes and the onset of absorption was observed at ~790 nm.

Figure 1. (A) Absorption and (B) emission spectra in THF solution at room temperature. (C) Magnified fluorescence spectrum of Iin-C18-Poly. The emission spectra were obtained with excitation wavelength at 600 nm and they are scaled with respect to their relative fluorescence quantum yields. Note that the molar absorptivity (ε) for Iin-C18-Poly is based on the repeat unit concentration, where the repeat unit contains a single Pt(II) center. The absorption maxima (λmax) and molar extinction coefficient values (ε) are shown in Table 2. Isoindigo precursors (Iin-C8 and Iin-C18) also feature a donor-acceptor interaction and this is believed to originate from a charge transfer (CT) interaction between both the central olefinic double bond and the nitrogen atom with the carbonyl oxygen atom. In general, the molar absorptivity (ε) for the CT band is less than the π-π* band. The λmax for the CT band red shifts from 496 nm for the isoindigo precursor molecules to 596 nm for the metal complexes and further to 690 nm for the polymer. This result can be attributed to increased delocalization imparted by the cyclometallated platinum chromophores. The molar absorptivity for the visible CT band increases from 12,800 M-1cm-1 for the precursor molecule to 58,000-60,000 M-1cm-1 for the metal complexes. This finding is in agreement with a study by Würthner and co-workers on thiophene containing isoindigo derivatives where a significant increase in molar absorptivity was 12


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observed with increased conjugation from Iin-C8.32 This enhanced molar absorptivity of the charge transfer band can be attributed to the increased electron donation from the cyclometallated units to isoindigo in the metal complexes, and increase in the oscillator strength due to the increased length of the π-conjugated system. Note here, the increased molar extinction coefficient and bathochromic shift in absorption by electron rich substituents is characteristic for 6,6’-substituted isoindigo derivatives and in contrast to 5,5’-substituted derivatives where the ε value remains almost unchanged with increased electron density from the substituents.29 In addition, the enhanced molar absorptivity of the metal complexes and the polymer compared to the isoindigo precursors in the region 300 - 500 nm can be attributed to the localized absorption imparted by the cyclometallated platinum “auxochromes”.27 The low ε value (31,000 M-1cm-1) for the polymer is an in part artifact, as the polymer repeat unit (PRU) used for calculation has one cyclometallated unit, whereas the metal complexes contains two. Table 2. Summary of photophysical properties.a Compound

λmax

ε x 104

λem

(nm)

(M-1cm-1)

(nm)

ɸfl b

τfl

τTA

(ns)

(µs) -

Iin-C18

498

1.3

-

-

-

Iin-C8-Pt(acac)

595

5.8

748

0.04

Ï-Conjugated Organometallic Isoindigo Oligomer and Polymer

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Ï-Conjugated Organometallic Isoindigo Oligomer and Polymer