Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays

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Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays Zhuo Chen, Hsien-Yi Hsu, Mert Arca, and Kirk S. Schanze J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp509130b • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 27, 2014

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The Journal of Physical Chemistry

Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays Zhuo Chen,†,‡ Hsien-Yi Hsu, †,‡ Mert Arca§ and Kirk S. Schanze*,† †

Department of Chemistry and Center for Macromolecular Science and Engineering and §

Department of Chemical Engineering,

University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States

ABSTRACT. Light harvesting and triplet energy transport is investigated in chromophore-functionalized polystyrene polymers featuring light harvesting and energy acceptor chromophores (traps) at varying loading. The series of precision polymers was constructed via reversible addition-fragmentation transfer (RAFT) polymerization and functionalized with platinum acetylide triplet chromophores by using an azide-alkyne “click” reaction. The polymers have narrow polydispersity and degree of polymerization ~60. The chromophores have the general structure, trans-[-R-C6H4-C≡C-Pt(PBu3)2-C≡CAr], where R is the attachment point to the polystyrene backbone, and Ar is either -C≡CC6H4-C≡C-Ph or -C≡C-pyrenyl (PE2-Pt and Py-Pt, respectively, with triplet energies of 2.35 and 1.88 eV). The polychromophores contain mainly the high energy PE2-Pt units (light absorber and energy donor), with randomly distributed Py-Pt units (3 - 20% loading, energy acceptor). Photophysical methods are used to study the dynamics and 1 ACS Paragon Plus Environment


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efficiency of energy transport from the PE2-Pt to Py-Pt units in the polychromophores. The energy transfer efficiency is >90% for copolymers that contain 5% of the Py-Pt acceptor units. Time-resolved phosphorescence measurements combined with Monte Carlo exciton dynamics simulations suggest that the mechanism of exciton transport is exchange energy transfer hopping between PE2-Pt units.

KEYWORDS. Light harvesting polymers, triplet energy transfer, side-chain conjugated polymer, platinum acetylide arrays, controlled radical polymerization

Introduction Triplet-triplet energy transfer is a fundamental photophysical process which occurs by the Dexter electron-exchange mechanism.1,2 This process plays an important role in chemistry (triplet photosensitization),1 biology (photosynthesis, photodynamic therapy)3,4 and materials science (organic light-emitting diodes and organic solar cells).5,6 Triplet-triplet energy transfer has been studied in many bimolecular and covalently linked donor-acceptor systems.

Classical studies include non-vertical energy transfer in

bimolecular trans-stilbene/quencher systems7-9 and long-range energy transfer between saturated hydrocarbon bridged triplet donor-acceptor dyads.10-12

More recent

investigations have examined ultrafast triplet energy transfer in strongly coupled bichromophores such as a Pt(II)-polypyridine-acetylide-pyrene complex, in which energy transfer occurs from a triplet Pt → dimine metal to ligand charge transfer (3MLCT) state to a pyrene localized triplet state in ~200 femtoseconds.13,14

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While the process of triplet energy transfer between discrete molecular units is fundamentally interesting, in nature and technology the process of exciton transport within polychromophore arrays and in solids is probably more significant. For example, it is well-known that exciton transport within light harvesting proteins is key to the overall efficiency of photosynthesis.15 Moreover, the role of exciton diffusion in organic solar cells and organic light emitting diodes is key to the efficiency in the devices, and ultimately may determine whether such technology is economically feasible.16,17 The study of exciton transport within model polychromophore assemblies and in molecular solids has been the subject of much interest for several decades; however, in most cases emphasis has been placed on the investigation of singlet exciton transfer.18-23 Significant insight has been gained from the study of singlet exciton transfer in polystyrene chromophore arrays,24,25 in multiblock co-polymer assemblies,26 and in supramolecular porphyrin chromophore assemblies.27-30 Understanding of triplet exciton transport in polychromophore arrays and in solids has developed more slowly, in part due to the difficulty of probing the photodynamics of the triplet excited state, because phosphorescence is weak (or non-existent) from most triplet excited chromophores at ambient temperatures. Notable exceptions to this are studies that utilize polypyridine Ru(II) and Os(II) complexes which display efficient 3

MLCT luminescence. In particular, Meyer, Papanikolas and coworkers studied triplet

exciton transport in polystyrene based light-harvesting assemblies containing Ru(II)- and Os(II)-complexes.31,32

This work highlighted how efficient long-distance energy

migration can occur when long-lifetime chromophores are involved. More recently, the same groups reported exciton transport in metal-organic frameworks (MOF) that consist



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of arrays of Ru(II) and Os(II) units incorporated at different relative stoichiometry.33 In both the polymeric and MOF-arrays, triplet exciton migration over significant distances involving exciton hopping between Ru(II) units with hopping times ~1 ns is involved. Platinum-acetylide complexes, comprised of a four coordinate square planar platinum(II) center with the general formula trans-PtL2(C≡CAr)2 (where L is a phosphine or carbene ligand, and Ar is a π-conjugated arylene unit), provide a versatile platform for the study of triplet exited states and related photophysical processes.14,34-38 This family of complexes typically exhibits a long-lived excited state having predominantly 3π,π* character, often characterized by efficient room temperature phosphorescence. Intersystem crossing and radiative decay from the triplet state are facilitated by the spinorbit coupling induced by the heavy metal Pt(II) center.39-43 Intramolecular triplet-triplet energy transfer in Pt-acetylide oligomers and polymers has been explored, where these studies are facilitated by observing phosphorescence from specific platinum-acetylide energy donor and acceptor units.39,40,42,43 In the present Article we report the synthesis and time-resolved photophysical investigation of triplet exciton transport in polystyrene-based chromophore arrays containing

Pt-acetylide

chromophore

units

(poly-Pt-Ar,

Chart

1).

These

polychromophores feature a narrow polydispersity polystyrene backbone that is prepared by reversible-addition fragmentation transfer (RAFT) controlled radical polymerization. The Pt-acetylide chromophores are subsequently grafted onto the polystyrene backbone by using an azide-alkyne “click” reaction, affording polymeric arrays containing on average 60 individual chromophore units. The Pt-acetylides are designed to feature two distinct triplet chromophores, one based on phenylene ethynylene (PE2-Pt, donor) and 4 ACS Paragon Plus Environment

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Chart 1. Structures of polymers and model compounds

the other on ethynyl pyrene (Py-Pt, acceptor).

By varying the ratio of these two

chromophores in the co-polymers it is possible to utilize time-resolved absorption and phosphorescence spectroscopy to study triplet exciton migration among the PE2-Pt donor units and trapping on the Py-Pt acceptor. The results give clear evidence for efficient energy migration in the arrays, wherein triplet exciton transfer within a polymer array containing 60 chromophores with a single acceptor (trap) site occurs with > 85% efficiency. Modeling of the experimental exciton dynamics with a Monte-Carlo 1-D random walk simulation reveals that transfer is more rapid than predicted for low acceptor (trap) loadings, suggesting that transport involves trajectories that are more efficient than those based on solely next-nearest neighbor triplet exchange. Experimental Methods Materials and Synthesis. All reagents were used as received unless otherwise specified. 4-Vinylbenzyl chloride was passed through a short neutral alumina column to remove inhibitor prior to use. Cuprous bromide was purified by washing three times with glacial acetic acid and three times with absolute ethanol. Detailed synthetic procedures 5 ACS Paragon Plus Environment


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and structural characterization of polymers and compounds are provided in the Supporting Information. Instrumentation and Methods. NMR spectra were measured on a Varian Gemini-300 FT-NMR, Mercury-300 FT-NMR, or Inova-500 FT-NMR spectrometers. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier transform ion cyclotron resonance mass spectrometer (Bruker Daltonics, Billerica, MA). Gel permeation chromatography (GPC) analyses were carried out on a system comprised of a Shimadzu LC-6D pump, Agilent mixed-D column and a Shimadzu SPD-20A photodiode array detector, with THF as eluent at 1 mL/min flow rate. The system was calibrated against linear polystyrene standards in THF. For UV-visible absorption measurements, samples were dissolved in THF and were carried out on a Shimadzu UV-1800 dual beam absorption spectrophotometer using 1 cm pathlength quartz cells. Photoluminescence measurements were obtained on a QuantaMaster 300 fluorimeter (Photon Technology International) using 1 cm square quartz cells. For phosphorescence measurement the sample solutions were deoxygenated via freeze-pump-thaw for three cycles. Luminescence lifetimes were obtained with a multichannel scaler/photon counter system

on

a

PicoQuant

FluoTime

100

Compact

Luminescence

Lifetime

Spectrophotometer. A Coherent CUBE diode laser provided the excitation at 375 nm (< 16 mW CW power). The laser was pulsed using an external Stanford Research Systems DG535 digital delay and pulse generator with four independent delay channels. The emission wavelength (typically 520 nm) was selected by using a 10 nm bandpass dichroic


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filter on the emission side.

The phosphorescence decay kinetics was fitted to a

biexponential function using FluorFit software (PicoQuant). Nanosecond triplet-triplet transient absorption measurements were acquired with excitation at 355 nm (10 mJ/pulse) using the third harmonic of a Continuum Surelite II10 Nd:YAG laser. A Perkin-Elmer LS1130-3 pulsed xenon lamp was used as a probe source and the transient absorption signal was detected with a gated–intensified CCD mounted on a 0.18 M spectrograph (Princeton PiMax/Acton Pro 180). Samples were prepared with an optical density of 0.7 at the excitation wavelength in a continuously circulating 1 cm path length flow cell (9 mL), and were deoxygenated via bubbling argon for 45 minutes before measurement. Triplet decay kinetics were calculated with a singleexponential global fitting of the transient absorption decay data using SpecFit analysis software. Single-wavelength transient absorption kinetics measurements were done on a home-built system that has been previously described.44 The excitation was provided by a Continuum Surelite I laser operating at 355 nm (3rd harmonic). Random-Walk Numerical Simulation.42 To simulate the random-walk process numerically, a program was written within MatLab (see Supporting Information for MatLab .vi). Briefly, a triplet exciton was assumed to travel in one-dimension along a 1dimensional lattice. The lattice has a length of 103 sites and contains a designated concentration of randomly distributed trap sites (0.03, 0.05, 0.1, and 0.2 fraction). At the start of the trajectory, the exciton is assigned to a random position on the lattice. In each step the exciton moves left or right with equal probability until it arrives at a trap site, where it is annihilated and the trajectory is terminated. Periodic boundary conditions are



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used to prevent annihilation at the lattice ends. For each trajectory the total number of steps is measured for an exciton to reach a trap site.

The ensemble averages are

calculated over 106 trajectories. A step size of 2.9 ns is used to compare with the experimental findings in Figure 8 below. Results and Discussion Polymer Design, Synthesis and Characterization. A series of well-defined polystyrene-based macromolecular structures (poly-Pt-Ar, Chart 1) were designed and prepared to study triplet-triplet energy transfer in platinum-acetylide chromophore arrays. These poly-Pt-Ar feature a narrow polydispersity, atactic polystyrene backbone, substituted with two different pendent Pt-acetylide chromophores in varying ratios. As described below, the number-average degree of polymerization (DP) of the polymers is 60. The two platinum-acetylide chromophores were carefully designed for triplet energy transfer, and they feature either a phenylene-ethynylene (PE2) or pyrenyl (Py) ligand attached to the platinum center, respectively (these chromophores are referred to as PE2Pt and Py-Pt hereafter). In these Pt-acetylide chromophores, the platinum center induces spin-orbit coupling, and consequently intersystem crossing is very rapid and efficient (φisc = 1) following excitation. The triplet energy of the PE2-Pt chromophore is higher than that of Py-Pt (2.35 eV vs. 1.88 eV, determined from the phosphorescence emission, vide infra), thus in the polymer assemblies the PE2-Pt units act as the light harvester and triplet energy donor, while the Py-Pt units act as the triplet energy acceptor. As shown by the structures in Chart 1, a series of poly-Pt-Ar arrays were prepared with different PE2Pt (donor)/Py-Pt (acceptor) ratios.

The polymers are referred to as P-x, where x

represents the percentage of Py-Pt chromophores in the polymer. Homopolymers that


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contain only the PE2-Pt donor (P-0) and Py-Pt acceptor (P-100) were also examined. Finally, monomeric model compounds (M1 and M2) were prepared, in order to examine the properties of the individual chromophores in the absence of chromophorechromophore interactions that may occur in the polymer assemblies. To avoid confusion regarding possible energy transfer mechanisms, it is important to point out that the degree of spin-orbit coupling induced by the Pt(II) centers in the Pt-acetylide chromophores is sufficient to enable efficient intersystem crossing, but not large enough to significantly enhance the probability of the S-T radiative transitions (e.g., S0-T1 absorption and T1-S0 radiative decay).45,46 As such, energy transfer in the poly-Pt-Ar arrays occurs by the Dexter exchange mechanism. The efficiency of Förster transfer is low because the S-T radiative transitions are only very weakly allowed and consequently the Förster overlap integral is very small. The poly-Pt-Ar assemblies were constructed via a “RAFT-SN2-click” route,25,47 as illustrated in Scheme 1. 4-Vinylbenzyl chloride (VBC) was polymerized via reverse addition-fragmentation transfer polymerization (RAFT) with 2-cyanoprop-2-yl 1dithionaphthalate (CPDN) as chain transfer agent. The resulting poly(vinylbenzyl chloride) (PVBC) has a molecular wright of 9800 (DP ~ 60) with a PDI of 1.22. Then azide groups were then introduced to make a “clickable” polymer precursor, poly(vinylbenzyl azide) (PVBA), via SN2 reaction. PVBA has a molecular weight of 10,100, and retains a relatively low PDI of 1.31 (see GPC, Figure 1). Finally the polychromophores (poly-Pt-Ar) were obtained by grafting the Pt-acetylide complexes (1 and 2; the detailed synthetic routes of which are shown in Scheme S1 in the Supporting Information) onto the polystyrene backbone via copper(I)-catalyzed azide-alkyne



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Scheme 1. Synthetic Route to Poly-Pt-Ar S

S CN

(CPDN)

Cl

S n

NC

NaN3

N3

Cl

Vinylbenzyl Chloride (VBC)

Poly(vinylbenzyl chloride) (PVBC) Mn = 9800, PDI = 1.22

PBu3 Pt PBu3

H

Poly(vinylbenzyl azide) (PVBA) Mn = 10100, PDI = 1.31

(1) SH

H

x n

1-x

NC

CuBr, PMDETA, THF RT, 24h

SH

DMF RT, 24h

AIBN Tolune, 90oC 8h

PBu 3 Pt PBu 3

n

NC S

(2) N PBu3 Pt PBu3

N

N N

P-0: x = 0 P-3: x = 0.03 P-5: x = 0.05

N N

PBu3 Pt PBu3

P-10: x = 0.10 P-20: x = 0.20 P-100: x = 1

Figure 1. Gel permeation chromatography traces of Poly-Pt-Ar and PVBA. P-0 (black, Mn = 31900, PDI = 1.31), P-3 (dark yellow, Mn = 32000, PDI = 1.30), P-5 (blue, Mn = 34300, PDI = 1.24), P-10 (red, Mn = 33800, PDI = 1.26), P-20 (brown, Mn = 30600, PDI = 1.31), P-100 (green, Mn = 30000, PDI = 1.23) and PVBA (black dash, Mn = 10100, PDI = 1.31). cycloaddition (CuAAC “click” reaction) between the azide functionality of PVBA and the terminal alkyne unit in 1 and 2. By adjusting the relative stoichiometry of 1 and 2 in


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the “click” reaction, poly-Pt-Ar samples with different triplet acceptor (Py-Pt) content (P0 to P-100) were prepared. The “click” reaction can be monitored by gel permeation chromatography (GPC) as illustrated in Figure 1. The GPC traces of all polymers display nearly monomodal chain length distributions, indicating that no branching or crosslinking occurs during the SN2 or the “click” grafting reactions. After click grafting, the polymer molecular weights increase significantly, with GPC traces shifting to lower retention time (i.e., higher molecular weight) from PVBA to P-x polymers (Figure 1). The 1H NMR spectra of the poly-Pt-Ar samples provide clear evidence for the structure and composition of the polymers. As shown in Figure 2, the resonances in the 1

H NMR spectrum of P-0 can be divided into three groups. The first group arises from

the polystyrene backbone, including two peaks (6.35 and 6.82 ppm) assigned to phenyl protons in the backbone (a and b in Figure 2) and a peak (5.42 ppm) corresponding to triazolemethylene protons (c in Figure 2). The complete disappearance of a resonance at 4.27 ppm for the azidomethylene protons in PVBA and appearance of the peak at 5.42 ppm in the P-x polymers is evidence of essentially quantitative chromophore grafting (> 95%) in the azide-alkyne “click” cycloaddition reaction.25,47 The second group of peaks arises from the n-butylphosphine (PBu3) protons in the PE2-Pt and Py-Pt complexes, at 2.11, 1.60, 1.42, and 0.90 ppm (d, e, f and g in Figure 2). Note that in the spectra of P-3 P-20, small shoulders are evident on the low field side of the PBu3 resonances d and e; these features are due to the PBu3 ligands on the Py-Pt chromophores, which are shifted slightly downfield relative to those for the PE2-Pt units (compare peaks d and e in spectra of P-100 and P-0). Finally, the third group of resonances correspond to the aromatic



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protons arising from the from PE2-Pt and Py-Pt aryl units (6.35 to 8.71 ppm, Figure 2). The peaks from 6.35 to 7.72 ppm (h – n) of the spectrum of P-0 arise from the aromatic protons on the PE2-Pt units. Additional small peaks appear in downfield region for sample P-3, which arise from pyrene protons in Py-Pt side chains (7.80 - 8.80 ppm, o – w in Figure 2). From P-3 to P-100, these signals become stronger as the Py-Pt content increases, whereas the signals from k to n become relatively weaker. In P-100, the Py-Pt homopolymer, signals k to n are not present. The first group of resonance signals arising from the polymer backbone protons (a – c) are broad, because the grafted side chains limit the rotational freedom of polystyrene backbone. By contrast, the second and third groups of peaks assigned to PE2-Pt and PyPt side units are relatively sharp. Of note is the fact that an indicator for Py-Pt content is the single peak at 8.71 ppm, which is assigned to the 10-position proton of the pyrene moiety (w in Figure 2). This peak increases in amplitude along with the Py-Pt content and the peak integration is used to estimate the Py-Pt content in the copolymers. However, the calculation has some error in P-3, P-5 and P-10, because this signal is relatively weak, introducing error in the integration. As described below, more accurate calculation of the chromophore loadings comes from analysis of the UV-visible absorption spectra of the polymers.


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Figure 2. 1H NMR spectra of Poly-Pt-Ar (P-0 to P-100).



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UV-Visible Absorption Spectroscopy of Poly-Pt-Ar in Solution. Absorption spectra of poly-Pt-Ar (P-0 to P-100) and the two model compounds (M1 and M2) were measured in THF solution at room temperature (Figure 3a). The energy donor (PE2-Pt)only polymer P-0 has a similar absorption spectrum as the corresponding model compound M1. The spectrum, which is characteristic of the π,π* transition of the PE2-Pt chromophore,36,37 is dominated by an intense near-UV band with λmax = 350 nm and a shoulder at λ ~ 300 nm. Likewise, the acceptor (Py-Pt)-only polymer P-100 and the PyPt model compound M2 have similar absorption spectra. Both exhibit a UV absorption band at 292 nm and a single transition with vibronic structure in the near-UV with peaks at 368 nm, 387 nm and 398 nm; these transitions are π,π* transitions of the Py-Pt moiety.13,14,38 Note that there is also moderate underlying absorption from 300 to 350 nm. The comparison of extinction coefficients between model compound and corresponding polymer (i.e., M1 vs. P-0 and M2 vs. P-100) are consistent with nearly quantitative grafting during the “click” reaction, as the difference between the extinction values are less than 5% (summarized in Table 1).

Figure 3. (a) Comparison of ground-state absorption of model compounds (M1 and M2), donor-only and acceptor only polymers (P-0 and P-100); (b) Ground-state absorption of poly-Pt-Ar in THF solution. Arrow shows increase in absorption due to Pt-Py repeat unit chromophore. 14 ACS Paragon Plus Environment

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As shown in Figure 3b, the absorption of the co-polymers P-3 to P-20, exhibit features for the PE2-Pt and Py-Pt chromophores; however, due to the fact that the former chromophore is more abundant, the spectra are dominated by the PE2-Pt transition at 350 nm. Nonetheless, features arising from the Py-Pt chromophore are evident, especially in the region between 380 and 425 nm, where the absorptivity increases with Py-Pt loading in the sequence P-3 to P-20. The presence of this clear absorption band due to the Py-Pt chromophore allows straightforward determination of its loading in the copolymer by measuring the ratio of the absorbance at 350 and 398 nm. The calculation results (summarized in Table 1) indicate that the fractional loading of PE2-Pt and Py-Pt corresponds closely to the stoichiometry used in the click reaction. In addition, a simulation of the absorption spectrum of P-20 was carried out by making a linear combination of the absorption spectra of P-0 and P-100 (weighted by the chromophore loadings and molar absorptivity values), and the simulated spectrum corresponds very well with the experimental absorption spectrum of the polymer (Figure S1). To summarize, the absorption data for the model compounds and polymers allow several conclusions.

First, the fractional loading of PE2-Pt and Py-Pt units in the

polymers closely corresponds to the stoichiometry used in the reaction mixture for the click reactions, which indicates that the “click” reaction rates of the two platinum acetylide are similar. Second, the co-polymer absorption at 348 nm is due almost exclusively to PE2-Pt (donor), making it possible to selectively excite this chromophore in the energy transfer studies. Finally, the co-polymer absorption spectra are nearly quantitatively simulated as a linear combination of the spectra of the PE2-Pt and Py-Pt



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Table 1. Photophysical characteristics of model compounds and poly-Pt-Ar. ϕ(PE2-Pt) (480-628 nm)

ϕphc ϕ(PE2-Pt) (628-850 nm)

ϕ(total) (480-850 nm)

Energy Transfer Efficiencyc (%)

527

0.18

--

0.18

--

0.002

660 737

--

0.015

0.015

--

388

Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays

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