Effect of Oligomer Length on Photophysical Properties of Platinum

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Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor-Acceptor-Donor Oligomers Seda Cekli, Russell W Winkel, and Kirk S. Schanze J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03977 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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The Journal of Physical Chemistry

Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor-AcceptorDonor Oligomers Seda Cekli, † Russell W. Winkel, † Kirk S. Schanze*,† †

Department of Chemistry and Center for Macromolecular Science and Engineering, University

of Florida, Gainesville, Florida 32611-7200, United States. Abstract: We report a systematic study that explores how the triplet excited state is influenced by conjugation length in a series of benzothiadiazole containing donor-acceptor-donor (DAD) type platinum acetylide oligomers and polymer. The singlet and triplet excited states for the series were characterized by an array of photophysical methods including steady-state luminescence spectroscopy and femtosecond-nanosecond transient absorption spectroscopy. In addition to the experimental work, a computational study using density functional theory was conducted to gain more information about the structure, composition and energies of the frontier molecular orbitals. It is observed that both the singlet and triplet excited states are mainly localized on a single donor-acceptor-donor unit in the oligomers. Interestingly, it is discovered that the intersystem crossing efficiency increases dramatically in the longer oligomers. The effect is attributed to an enhanced contribution of the heavy metal platinum in the frontier orbitals (HOMO and LUMO), an effect that leads to enhanced spin-orbit coupling.

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Introduction π-Conjugated polymers containing alternating electron rich donor (D) and electron deficient acceptor (A) structures are widely applied in organic electronics1-3 and photovoltaics4-6 as an approach to reduce the HOMO-LUMO energy gap.7,

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Heavy-metal containing donor-

acceptor-donor (DAD) polymers have also drawn significant attention in recent years.9-11 This interest derives from the fact that incorporation of a heavy metal atom (such as platinum, ruthenium, gold) into an organic π-conjugated system can significantly affect the electronic, magnetic, and optical properties and induce intersystem crossing from the singlet excited state to the triplet excited state.12 In this regard, platinum acetylides are usually preferred in structural designs because platinum promotes intersystem crossing due to enhanced spin orbit coupling (heavy atom effect)13, 14 and it extends π-conjugation via dπ(Pt)-pπ(C) orbital overlap.15 Our group has an ongoing interest in studying the triplet excited state properties of platinum incorporated π-conjugated structures.16-18 Very recently, we investigated the photophysical properties of a series of platinum acetylide containing π-conjugated donoracceptor-donor (DAD) type small molecules19 and showed that increasing acceptor strength results in localization of the excited state electrons on the acceptor unit which is located distal from the platinum center. As a result, the rate of intersystem crossing decreases with increased extent of charge transfer due to the increased average distance between the acceptor and the platinum. Although small molecules with uniform structure and monodisperse molecular weight are ideal candidates for fundamental studies, it is important to understand how oligomer length affects the photophysical properties to allow extrapolation to polymers, which are most often used in applications. Herein, in a continuation of our investigations, we focus on how the triplet


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excited state is affected by oligomer length in a series of benzothiadiazole containing DAD type platinum acetylide oligomers and polymer structures (Chart 1). The (4,7-di(thiophen-2yl)benzo[c][1,2,5]thiadiazole) chromophore (TBT) was chosen as the DAD chromophore because of the abundance of previous studies featuring this moiety in polymers and oligomers.2023

In addition, TBT containing platinum acetylide polymers have been extensively studied for

their photovoltaic applications.10,

24-26

In this study, the TBT chromophore is capped with

platinum(II) acetylide units on both sides to promote intersystem crossing.27 We report a full photophysical characterization of the singlet and triplet excited states by using steady-state luminescence spectroscopy and transient absorption spectroscopy on timescales ranging from picoseconds to microseconds. In addition to the experimental work, a computational study was conducted to provide more information about the spatial extent of the frontier orbitals. Our previous results19 showed that in DAD type structures, which featured a heavy atom located distal from the acceptor, the intersystem crossing efficiency decreased with increasing acceptor strength in part because the LUMO is localized on the acceptor unit. In the present study, we find that increased oligomer length results in a relatively delocalized LUMO throughout the π-conjugated DAD molecules. As a result, Pt localized orbitals, which contribute more to the LUMO, induce more efficient intersystem crossing in more extended DAD oligomer and polymer structures.


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Chart 1. Structures of the TBT containing Pt acetylide oligomers and polymer. Experimental Synthesis and Characterization. Complete details concerning the synthesis and characterization of the molecules and the polymer are provided in the SI (Supporting Information). The 1H/13C/31P NMR spectra and high-resolution mass spectrometry were recorded in the Chemistry Department at the University of Florida.19 The molecular weight of the polymer was characterized on a Gel permeation chromatography (GPC) system consisting of a Shimadzu SPD-20A photodiode array (PDA) detector and Shimadzu LC-6D pump. Instrumental Methods. Steady-state absorption and emission spectroscopy, fluorescence lifetimes measurements, nanosecond transient absorption spectroscopy and the cyclic voltammetry (CV)/differential pulse voltammetry (DPV) measurements were carried out as reported previously.19 Fluorescence quantum yields and singlet oxygen quantum yields were reported relative to Rhodamine B (ϕ = 0.69)28 and terthiophene (ϕ∆ = 0.84)29 respectively. The power dependent transient absorption experiments were carried out on a home-built transient absorption spectrometer as reported previously.30 The samples were excited by the third harmonic (355 nm) of a Continuum Surelite II-10 Nd:YAG laser and the optical density of the


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solutions were adjusted to ~0.60 at the excitation wavelength. The signal was monitored at 475 nm. Femtosecond-picosecond transient absorption spectroscopy was conducted on an Ultrafast Systems Helios transient absorption spectrometer equipped with UV-visible and nearinfrared detectors. 355 nm pump pulses were created in a Spectra Physics, Spitfire optical parametric amplifier. The fundamental output (800 nm, 100 fs, 1 kHz) of the Ti:sapphire fs regenerative amplifier was used to generate the white light continuum. Computational Details. All calculations were carried out using DFT as implemented in Gaussian 09 Rev. C.0131 according to the previous methods.19 To minimize computational cost, solubilizing PBu3 moieties were replaced with PMe3. These truncated molecules are designated with a ’, thus O2 becomes O2’. All starting conformations of O2’ were explored in the singlet state. Due to the very large size and number of conformations of O3’, only a select few conformations were run. T1 energies were calculated by taking the difference of the lowest energy singlet and lowest energy triplet states. Structures and orbitals were visualized using Chemcraft Version 1.7.32

Gaussian 09 was also used to generate natural atomic orbitals33

(NAOs) for analysis of atomic and acceptor contributions to the HOMO and LUMO via the program Multiwfn.34,

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Average percent contribution of platinum atoms was determined by

simply dividing the total contribution of platinum to the MO by the number of platinum atoms present in the molecule. Results and Discussion Structures, Synthesis and Characterization. In the present work, the effect of oligomer length on the photophysical properties is systematically studied in a series of four donoracceptor-donor (DAD) structures. Three of the compounds are oligomers which have one, two


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and three repeating DAD units that are referred to as O1, O2 and O3, respectively (Chart 1). For comparison, a polymer (p-TBT) was prepared with 10 repeating DAD units (average) that is most likely beyond the effective conjugation length.7 This study particularly focuses on how the triplet state properties, such as energetics and intersystem crossing efficiency, are affected by oligomer length in DAD chromophores. Therefore, in each compound TBT units are flanked on each end by trans-CC-PtII(PBu3)2(CCPh) to promote intersystem crossing due to the heavy-atom effect that originates from platinum.12 (The unmetallated TBT chromophore was previously studied25 and for ease of comparison here some data for this compound is included in the Supporting Information). The synthesis of the oligomers is presented in Scheme S1 and the synthetic details are discussed in the Supporting Information. The monomeric TBT unit (1, Scheme S1) and O1 were prepared following a published procedure.25 Oligomers O2 and O3 are obtained monodisperse by stepwise synthesis of acetylated TBT units and trans- or cis- Pt complexes. The polymer pTBT was prepared by coupling between 1 and cis-Pt(PBu3)2Cl2 under Hagihara conditions. The GPC chromatogram of p-TBT reveals that the number-average molecular weight (Mn) of 16.6 kDa and a polydispersity index (PDI) of 2.18. 1

H, 13C and 31P-NMR spectroscopy was used for the characterization of oligomers and p-

TBT, and the spectra of all the final compounds and intermediates are included in Supporting Information. The

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P-NMR spectra are shown in Figure 1 and they provide useful information

concerning the structures of O2, O3, and p-TBT. In particular, the spectrum of O1 exhibits a single peak at 3.27 ppm corresponding to the four equivalent phosphine ligands (a) bound to the two terminal Pt atoms. Pt-P spin-spin coupling leads to the appearance of satellite peaks and the coupling constant between Pt and P (2340 Hz) indicates a trans configuration of the phosphine


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ligands.36 In the

31

P-NMR spectrum of O2, two peaks are observed at 3.47 and 3.27 ppm that

belong to the phosphine ligands on the internal (b) and terminal (a) Pt centers, respectively. The internal phosphine ligand signal increases in relative amplitude as the oligomer length increases in O3. The same behavior was observed for the satellite peaks as well. As the oligomer length increases, the satellite peaks belonging to the external position (12.92 and 6.10 ppm) disappear. Note that in the p-TBT spectrum, there are two additional peaks observed at 7.14 and 0.37 ppm which are assigned to the phosphine groups at the end of the polymer chains. In particular, the peak at 7.14 ppm corresponds to the phosphine groups with a Pt-Cl terminus (see 31P-NMR of 2, Figure S3) whereas the peak at 0.37 ppm corresponds to the phosphine groups on Pt-I endgroups (Figure S3).37, 38

Figure 1. 31P-NMR spectra of O1-O3 and p-TBT. Electrochemistry. In an attempt to identify the effect of oligomer length on the redox properties and the energy gap, cyclic voltammetry (CV) and differential pulse voltammetry


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(DPV) experiments were performed. The electrochemical studies were conducted in dichloromethane/0.1 M TBAPF6 electrolyte solution by using platinum working and counter electrodes and a Ag/Ag+ pseudo-reference electrode. All potentials are reported relative to a Fc/Fc+ internal standard. The CV plots are included in the Supporting Information (Figure S14) and a summary of the electrochemical data is provided in Table 1. The CV results revealed that O1 and O2 exhibit two reversible oxidation waves in a narrow range (+0.27 V - +0.45 V), whereas the oxidation waves are quasi-reversible for O3 and the polymer. In the latter cases, DPV was used to estimate the oxidation potentials. The oxidation potentials of O3 and p-TBT are found to be 0.26 V. Turning to the negative potentials, interestingly, all of the molecules exhibit a single, reversible reduction wave at -1.74 V relative to Fc/Fc+. Table 1. Electrochemical data. E1/2/V a

a

Frontier Orbital Energies

HOMO-LUMO Gap ∆Eg,/eVe

∆Eopt/eVf

-5.40

2.03

1.96

-3.36

-5.37

2.01

1.95

0.26b

-3.36

-5.36

2.00

1.95

0.26b

-3.36

2.00

1.94

red

ox1

ox2

ELUMO/eVc

O1

-1.74

0.30

0.45

-3.36

O2

-1.74

0.27

0.38

O3

-1.74

p-TBT

-1.74

EHOMO/eVd

-5.36 +

b

CV data, conducted in dichloromethane and referenced to Fc/Fc as an internal standard. DPV data, conducted in

dichloromethane and referenced to Fc/Fc+ as an internal standard. cELUMO = -(E[red vs. Fc+/Fc] + 5.1) eV. dEHOMO = -(E[ox vs. Fc+/Fc] +

5.1) eV. eElectrochemical gap = Eox – Ered. fOptical gap is found from the onset of absorption spectra.

The HOMO and LUMO levels of the compounds were calculated from the oxidation and reduction potentials (Eox and Ered, respectively) according to the following equations: EHOMO = (Eox + 5.1) eV and ELUMO = -(Ered+ 5.1)39 and Figure 2 presents the trend in energetics for the compounds. As can be seen from Figure 2a, there is no change in the LUMO energies across the series. Density functional theory (DFT) calculations indicate that the LUMO is localized on the


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acceptor units in the DAD structures.40 Therefore, the consistency of LUMO energies is a direct result of the influence of the acceptor units. In addition, Figure 2b illustrates the DFT calculated HOMO and LUMO of the TBT dimer (O2’). The LUMO of O2’ is presented here to emphasize the localized nature of the LUMO.

Figure 2. a) Frontier orbital energy levels and HOMO-LUMO energy gap of O1-O3 and p-TBT calculated from electrochemical potentials. b) HOMO and LUMO of O2’. Over the entire series, the range of HOMO values is -5.40 to -5.36 eV, slightly increasing (0.04 eV) with oligomer length. In general, the reduction and oxidation potentials reported here are in close approximation to previously reported redox potentials of the free TBT unit41 which suggest that the oxidation and reduction process are concentrated on individual DAD segments of the π-conjugated system. Figure 2b also presents the HOMO concentrated on the DAD chromophores.


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The HOMO-LUMO gap (∆Eg) that is calculated from the oxidation and reduction potentials of the molecules are listed in Table 1, along with the optical band gap (∆Eopt) estimated from the onset of the long wavelength band in the absorption spectra which is in good agreement with ∆Eg (Figure 3). The HOMO-LUMO gap difference is only 0.03 eV between the shortest molecule O1 and p-TBT. This result further suggests that the HOMO and LUMO are mainly localized on a single DAD chromophore segment (TBT) rather than being delocalized throughout the entire π-conjugated system. Excited State Properties. UV-visible absorption spectroscopy was conducted to provide insight concerning the effect of oligomer length on the absorption spectra and the transition energies. The spectra were collected in THF and they are shown in Figure 3, and the absorption band maxima (λabs) are listed in Table 2. In general, all the compounds exhibited two strong absorption bands, which is typical for TBT containing DAD structures,42 and these bands are assigned to a high energy π-π* transition and a low energy charge transfer (CT) band. The unmetallated TBT chromophore (Figure S15) also features these two primary bands, which suggests that the main absorption bands are largely due to the transitions localized on the conjugated TBT unit. The absorption bands of the metallated chromophore (O1) are red-shifted relative to their positions in the unmetallated TBT chromophore, indicating the extended conjugation due to dπ-pπ orbital overlap with the Pt centers. A further red-shift in the absorption bands was also observed from monomer O1 to polymer p-TBT, likely due to an (slight) increase in the π-conjugation in the longer oligomers relative to the monomer. Although both bands were red-shifted at the lower energy region, the CT band showed the most significant shift, ~17 nm (0.07 eV) from O1 to p-TBT.


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The molar absorptivity (ε) values of the absorption bands are also listed in Table 2. In general, ε increases for both bands as the oligomer length increases. The ε of p-TBT is reported per repeat unit and was found to be 35,000 M-1cm-1 and 33,200 M-1cm-1 and the ε of O1 was found to be 51,700 M-1cm-1 and 34,400 M-1cm-1 for π-π* and CT bands, respectively. Closer inspection of the ε values reveal that the oscillator strength for the CT band increases 2 and 3 fold from O1 to O2 and O3, respectively. Interestingly, as can be seen in the normalized absorption spectra in Figure 3a, that ratio of the absorptivity of the π-π* band to the CT band (εππ*/εCT)

decreases with oligomer length (to the polymer). (The ratio επ-π*/εCT varies from 1.5 to

1.05 from O1 to p-TBT.) The origin of this effect is unclear, but it does signal that there is significant interaction, perhaps of an excitonic nature, between the individual TBT units interaction in the longer oligomers and polymers.

Figure 3. Normalized absorption (a) and normalized emission (b) spectra of O1-O3 and p-TBT. Absorption is normalized to 1 at the UV band maximum. Emission is normalized to reflect the relative quantum yields of the compounds.


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Table 2. Photophysical properties of O1-O3 and p-TBT. λabs, nma (ε, 104 M-1cm-1) 370, 546

O1

(5.17, 3.44) 372, 558

O2

(9.79, 7.80) 377, 560

O3

(11.5, 9.76) 380, 563 p-TBT

(3.50, 3.32)i

τfc

τTd

ns

µs

0.43

4.84

1.81

0.10

1.13

0.09

0.06

Φf

b

ESf

ETg

∆ES-Th

(eV)

(eV)

(eV)

0.15

2.01

1.17

0.84

1.69

0.49

1.99

1.15

0.84

0.93

1.59

0.50

1.98

1.13

0.86

0.62

1.30

0.50

1.98

1.08

0.90

Φ∆e

a

Measured in THF at room temperature. bMeasured in THF using Rhodamine B (Φf = 0.69)28 as an actinometer. Estimated error ±5% of value. cMeasured by TCSPC in dry THF. Estimated error ±2% of value. dMeasured by nanosecond transient absorption spectroscopy, in deoxygenated THF. Estimated error ±10% of value. eMeasured in benzene-d6 using terthiophene (Φ∆ = 0.84)29 as an actinometer. Estimated error ±5% of value. fS1 energy is found from the cross-section wavelength of absorption and emission spectra. gT1 energy is found from the energy gap law correlation. hSinglet triplet splitting energy, ∆ES-T= ES1 – ET1. i Absorptivity per repeat unit, which includes only a single -Pt(PBu3)2-CC- unit.

The photoluminescence spectra of the compounds were measured in THF at room temperature, and the spectra are illustrated in Figure 3. All of the compounds exhibit a featureless broad emission band which is typical of CT structures and interestingly they all have the same λmax for emission. Given the short lifetimes (Table 2) the emission is attributed to fluorescence from the singlet state. The fluorescence quantum yields (ϕf) and lifetimes (τf) were measured in THF and the results are listed in Table 2. As can be seen from Table 2, the fluorescence quantum yields of the compounds vary between 0.43 and 0.06 with the decreasing order of O1 > O2 > O3 > p-TBT. The decreasing trend can also be observed from the emission spectra in Figure 3b which is normalized to illustrate the variation in ϕf. The most dramatic decrease in ϕf occurs between O1 and O2; the ϕf of O1 is ~4 times larger than O2. A parallel trend is observed for the τf. The lifetime of the monomer O1 was found to be 4.84 ns, whereas


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lifetime of p-TBT is 620 ps. The dramatic decrease in both lifetimes and quantum yields suggests that the changes in ϕf and τf arise due to variation in the rate of a non-radiative decay channel across the series. This is supported by the fact that radiative decay rate, kr, is nearly constant across the series (8.8 – 9.6 x 107 s-1). In a previous study the decrease in ϕf and τf between O1 and p-TBT was noted and was attributed to enhanced non-radiative decay of the singlet state due to quenching by “defects” in the polymer chain and/or by interchain aggregates.25 However, our work and that of others has shown that in general singlet to triplet intersystem crossing (ISC) is the primary non-radiative singlet decay pathway operating in platinum containing π-conjugated structures.18,

43-46

In

addition, platinum acetylides with efficient ISC are expected to have very short fluorescence lifetimes (

Effect of Oligomer Length on Photophysical Properties of Platinum

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