Synthesis and Photophysics of Coaxial Threaded Molecular Wires

Feb 3, 2014 - ... and Department of Physics and Astronomy, University College London, ... of California, Santa Barbara, California 93106-5121, United ...
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Synthesis and Photophysics of Coaxial Threaded Molecular Wires: Polyrotaxanes with Triarylamine Jackets Giuseppe Sforazzini,† Axel Kahnt,‡ Michael Wykes,§ Johannes K. Sprafke,† Sergio Brovelli,∥,⊥ Damien Montarnal,# Francesco Meinardi,⊥ Franco Cacialli,*,∥ David Beljonne,*,§ Bo Albinsson,*,‡ and Harry L. Anderson*,† †

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford OX1 3TA, United Kingdom Department of Chemical and Biological Engineering, Chalmers University of Technology, 41296 Göteborg, Sweden § Laboratory for Chemistry of Novel Materials, University of Mons, Hainaut Place du Parc 20, B-7000 Mons, Belgium ∥ London Centre for Nanotechnology, and Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom ⊥ Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 53, I-20125 Milano, Italy # Materials Research Laboratory, University of California, Santa Barbara, California 93106-5121, United States ‡

S Supporting Information *

ABSTRACT: Conjugated polyrotaxanes jacketed with holetransport groups have been synthesized from water-soluble polyrotaxanes consisting of a polyfluorene-alt-biphenylene (PFBP) conjugated polymer threaded through β-cyclodextrin macrocycles. The hydroxyl groups of the oligosaccharides were efficiently functionalized with triphenylamine (TPA) so that every polyrotaxane molecule carries a coat of about 200 TPA units, forming a supramolecular coaxial structure. This architecture was characterized using a range of techniques, including small-angle X-ray scattering. Absorption of light by the TPA units results in excitation energy transfer (EET) and photoinduced electron transfer (ET) to the inner conjugated polymer core. These energy- and charge-transfer processes were explored by steady-state and time-resolved fluorescence spectroscopy, femtosecond transient absorption spectroscopy, and molecular modeling. The time-resolved measurements yielded insights into the heterogeneity of the TPA coat: those TPA units which are close to the central polymer core tend to undergo ET, whereas those on the outer surface of the polyrotaxane, far from the core, undergo EET. Sections of the backbone that are excited indirectly via EET tend to be more remote from the TPA units and thus are less susceptible to electron-transfer quenching. The rate of EET from the TPA units to the PFBP core was effectively modeled by taking account of the heterogeneity in the TPA−PFBP distance, using a distributed monopole approach. This work represents a new strategy for building and studying well-defined arrays of >100 covalently linked chromophores.

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INTRODUCTION

cylindrical self-assembled rod antenna complexes exist in green sulfur bacteria (GSB).7,8 The high density and tight packing of chromophores in these rod-like structures provide a large light absorption cross section for GSB, enabling these organisms to live in extremely low-light environments, such as at depths of 100 m in the Black Sea.9 Several approaches have been reported for the preparation of artificial rigid-rod multichromophoric systems, including dendritic polymers,10−12 supramolecular selfassemblies,13−20 and dye-functionalized polymeric scaffolds.18,19 Among these strategies, the use of a rigid scaffold with a welldefined geometry can lead to structures with a precise positioning of the chromophores.20 Foldamers and helical

Plants and photosynthetic bacteria have evolved highly efficient strategies for gathering energy from sunlight. They use lightharvesting complexes consisting of large numbers of chromophores, which act synergistically to absorb light and funnel the energy into the reaction center.1,2 The geometrical organization of the chromophores guarantees efficient energy transfer and subsequent electron transfer over distances greater than 10 nm, illustrating the important relationship between architecture and function. Artificial light-harvesting antenna systems are widely investigated as components of photochemical and photoelectronic devices,3−5 and inspiration for the design of these synthetic systems is often provided by their counterparts in nature. Circular photosynthetic antenna complexes are found in the purple bacteria light-harvesting complex 2 (LH2),6 whereas © 2014 American Chemical Society

Received: January 19, 2014 Revised: February 2, 2014 Published: February 3, 2014 4553

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polyrotaxane were investigated by steady-state and timeresolved spectroscopy, combined with computational simulations. We have previously reported the functionalization of watersoluble cyclodextrin conjugated polyrotaxanes with lipophilic groups,33−38 to produce polyrotaxanes with high solubility in nonpolar solvents. Insulated molecular wires soluble in organic solvents have also been prepared using methylated cyclodextyrins.39,40 Molecular mechanics calculations show that in these highly functionalized polyrotaxanes, the backbone is fully covered with a hydrophobic shell. The densely packed solubilizing functionality yields rod-shaped structures that are straighter than the analogous unthreaded polymer.41 Replacement of the alkyl chains with hole-transport groups, such as triarylamines, leads to a spatial organization of the chromophores to form an outer cylindrical hole-transport sheath around the backbone (Figure 1). Here the conjugated polymer not only acts as rigid-rod scaffold but also is an active photoelectronic component in the system. Appropriate selection of the electronic and optical properties of the shell and core components provides ideal photoactive coaxial systems for the investigation of intramolecular transfer of charge and energy in a multichromophoric architecture. Moreover, these coaxial polyrotaxane are in principle architectures of interest for solar cell, organic light-emitting diodes, batteries, and photodetectors.30−43

polymers are often used to organize arrays of dyes into wellordered assemblies.3−5 Carbon nanotubes can also be used as frameworks to arrange many functionalities along their lengths.21 Cyclodextrin polyrotaxanes with conjugated polymer backbones can align the oligoglucose macrocycle rings along the polymer chain, leading to a supramolecular shape-persistent multicomponent architecture.22−25 Attachment of dyes onto the rims of unthreaded cyclodextrins has been used to produce circular chromophore arrays in which excitation energy hops among the antenna units to arrive ultimately at an included guest in the central cavity of the cyclodextrin.26,27 Lightharvesting polyrotaxanes consisting of functionalized cyclodextrins on polyethylene glycol chains have also been investigated.28,29 Cyclodextrin polyrotaxanes with conjugated polymer cores have been synthesized as insulated molecular wires by the aqueous Suzuki polymerization of a cyclodextrin/ monomer inclusion complex, with concomitant chain-endcapping with bulky stopper units.22−25,30−32 Functionalization of the peripheral polyrotaxane OH groups with a large number of chromophores should lead to a cylindrical rod-like multichromophoric structure where both outer and inner moieties are photoelectronically active. Here we report a general procedure for the synthesis of rod-like coaxial multichromophoric supramolecular photosystems as bioinspired architectures. The structure under consideration consists of a conjugated polyfluorene-alt-biphenylene (PFBP) threaded through a series of cyclodextrins functionalized with charge-transport units. Bulky terminal groups prevent unthreading of the cyclodextrin rings. The resulting polyrotaxane possesses a densely packed outer hole-transport sheath and an inner rod-like conjugated polymer core (Figure 1). Energy and photoinduced charge-transfer processes in this modified

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RESULTS AND DISCUSSION Synthesis and Chemical Characterization. Three different strategies have previously been used to functionalize the hydroxyl groups of cyclodextrin polyrotaxanes: alkylation, silylation, and acylation.33−38 Acylation was found to be the most versatile and efficient approach for incorporating holetransport units onto the polyrotaxane. The reaction conditions for functionalizing cyclodextrin polyrotaxanes were first optimized using native β-cyclodextrin (β-CD). As a model reaction, β-CD was reacted with pentafluorophenyl butyrate PrCO-PFP (4 equiv per OH group) in the presence of DMAP (3 equiv per OH group) in pyridine (85 °C, 24 h) to give CDCOPr (69% yield, Scheme 1). This reaction gave clean Scheme 1. Synthesis of Functionalized Cyclodextrinsa

a

Reagents and conditions: (a) PrCO-PFP (for CD-COPr) or TPAPFP (for CD-TPA), DMAP, pyridine, 85 °C.

acylation at all 21 hydroxyl groups, after 24 h, as demonstrated by matrix-assisted laser desorption ionization (MALDI) MS analysis (Figure 2a). 1H NMR analysis also confirmed the complete functionalization of all 21 hydroxyl residues. Synthesis of the desired triarylamine hole-transport group active ester (TPA-PFP) was carried out as shown in Scheme 2. Heck coupling of 4-bromo-N,N-diphenylaniline (TPA-Br) with methyl acrylate followed by hydrogenation and hydrolysis of the methyl ester gave the carboxylic acid, which was then converted to the active ester TPA-PFP using pentafluorophenol and dicyclohexylcarbodiimide.

Figure 1. Molecular design of the triphenylamine functionalized polyfluorene-alt-bisphenylene polyrotaxane. (a) Energy-minimized geometry calculated for the nominal decamer chain length with one functionalized cyclodextrin per polymer repeat unit. (b) Chemical structure with the β-cyclodextrins represented as green cones. 4554

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Figure 3. 1H NMR spectrum of the TPA functionalized CD-TPA (R = COCH2CH2C6H4NPh2) in CDCl3 (400 MHz, 300 K) with assignments from the COSY spectrum.

Water-soluble polyfluorene-alt-biphenylene (PFBP) polyrotaxane Rtx was prepared as previously reported (with n̅ = 10).22−25,33−35 The number of threaded β-cyclodextrin rings in a polyrotaxane is calculated as the average number per polymer repeat unit; the threading ratio (y) is defined as y = x/(n + 1), where x is the number of β-CD per polymer backbone and n is the degree of polymerization of the conjugated core. From analysis of the 1H NMR spectrum of Rtx, we find an average y ̅ = 1.1, which is consistent with the elemental analysis of this compound. Functionalization of polyrotaxane Rtx was carried out as summarized in Scheme 3. Methylation of the carboxylic groups was carried out with diazomethane.33−35 The methyl ester polyrotaxane RtxMe is insoluble in most solvents, except DMSO and pyridine. Reaction of RtxMe with an excess of triphenylamine active ester TPA-PFP gave the triarylamine functionalized polyrotaxane RtxMe-TPA in 45% yield; this modified polyrotaxane is soluble in common organic solvents such a THF, dichloromethane, chloroform, and toluene. We define the degree of functionalization, z,̅ of a βcyclodextrin unit as the fraction of hydroxyl groups that are functionalized, so that z̅ = m̅ /21, where m̅ is the number of functionalities per cyclodextrin. The broadness of some 1H NMR signals made it difficult to measure an accurate value of m̅ by NMR integration. The percentage of nitrogen in the functionalized polyrotaxane, from combustion analysis, provides a useful measure of the degree of functionalization. If all the OH groups in RtxMe-TPA were functionalized with TPA units (z̅ = 1, m̅ = 21), assuming threading ratio y = 1.1, the elemental composition would be C, 78.33; H, 5.79; N, 3.72. The experimental percentages of C, H, N were found to be

Figure 2. MALDI time-of-flight (TOF) MS of the functionalized cyclodextrin. (a) CD-COPr in dithranol matrix. (b) CD-TPA in DCTB matrix. The integer n indicates the number of functionalities attached per cyclodextrin. Inserts: percentage of the species with different number of functionality.44 Samples were prepared according to Scheme 1 with reaction times of 24 h (a) and 60 h (b).

Treatment of native β-CD with TPA-PFP in pyridine and 4dimethylaminopyridine for 24 h gave the corresponding triphenylamine functionalized β-cyclodextrin CD-TPA (Scheme 1). However, analysis by MALDI MS suggested the presence of multiple species with 19, 20, or 21 triphenylamine substituents per β-CD. The lower degree of functionalization may be a consequence of the steric hindrance of the triarylamine group. The reaction was repeated extending the reaction time to 60 h, and MALDI MS showed complete functionalization of all 21 hydroxyl groups, though a trace of a cyclodextrin with 20 triarylamine groups functionality were still present (Figure 2b).44 The triphenylamine functionalized β-CD (CD-TPA) was isolated in 80% yield. This modified β-CD is soluble in common organic solvents such as THF, DCM, chloroform, and toluene. The 1H NMR spectrum (Figure 3) is consistent with complete functionalization of the OH groups to attach 21 TPA units. Scheme 2. Synthesis of TPA Active Estera

Reagents and conditions: (a) Pd(OAc)2 (10 mol %), Et3N, methyl acrylate, tri-p-tolylphosphine, DMF, 100 °C, 16 h. (b) Pd/C (10%) methanol/ ethyl acetate and H2. (c) LiOH (aq), THF. (d) Pentafluorophenol, DCC, DMF. a

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Scheme 3. Synthesis of Functionalized Polyrotaxanesa

Figure 4. Absorption spectra of RtxMe-TPA (solid red), TPA-Me (solid blue), and RtxMe-COPr (solid black). Normalized emission spectra of RtxMe-TPA excited at 303 nm (dashed green) and excited at 370 nm (dashed cyan), TPA-Me excited at 303 nm (dashed brown), and RtxMe-COPr excited at 370 nm (dashed purple). The molar absorption coefficient of RtxMe-TPA is plotted per polymer repeat unit, assuming that each cyclodextrin bears 17 TPA groups, giving values of 1.21 × 105 M−1 cm−1 at 372 nm and 6.68 × 105 M−1 cm−1 at 303 nm. All the spectra were recorded in CH2Cl2.

a Reagents and condition: (a) (i) HCl (aq), (ii) CH2N2, DMSO, Et2O, EtOH; (b) TPA-PFP or COPr-PFP, DMAP, pyridine, 85 °C, 60 h.

where C is the sample concentration in the cuvette and εD(303) is the molar decadic absorption coefficient of a TPA unit at 303 nm estimated from methyl ester TPA-Me (εD(303) = 0.38 × 105 M−1 cm−1); εA(372) and εA(303) are the absorption coefficients of the backbone at 372 and 303 nm, respectively, estimated from RtxMe-COPr (εA(372) = 1.21 × 105 M−1 cm−1, εA(303) = 0.19 × 105 M−1 cm−1, per polymer repeat unit, in dichloromethane). The absorption coefficient of the PFBP conjugated polymer core was calculated by counting the group bound to the terminal triaryl stopper as a repeat unit; hence, for a polyrotaxane with a degree of polymerization n = 10, the number of polymer repeat unit is counted as 11 (n + 1). Typical batches of RtxMe-TPA give ADA(303)/ADA(372) = 5.66 in dichloromethane, which by eq 3 correspond to m̅ = 16 (z̅ = 0.76), in good agreement with the value obtained by elemental analysis (m̅ = 16; z̅ = 0.75). Gel permeation chromatography (GPC) analysis of RtxMeTPA was performed in THF against polystyrene narrow standard (Figure 5). The weight average molecular weight (M̅ w) and the number average molecular weight (M̅ n) were found to be 61 600 and 28 000 Da, respectively, with a polydispersity (M̅ w/M̅ n) of 2.2. Considering the observed z̅ value, the threading ratio y = 1.1, and average degree of polymerization (n̅ = 10), RtxMe-TPA is expected to have an average molecular weight (M̅ n(calc.)) of 68 000 Da. The M̅ n measured is 2.4 times smaller than the calculated value, which is consistent with the behavior reported for organic-soluble polyrotaxanes such as RtxMe-COPr.33−35 GPC typically underestimates the molecular weight of compact structures, as exemplified by dendronized polystyrene, which has been shown to give a M̅ n smaller by a factor of 1.6−3.9 when compared to small-angle neutron scattering (SANS) data.45,46 Small angle X-ray scattering (SAXS) analysis of the CD-TPA reference compound was carried out to acquire information on the structure of the macrocycle in solution. Linearity over a wide q-range in the Guinier region confirms monodispersity of this functionalized cyclodextrin and gives a radius of gyration Rg = 11.9 Å (Figure S1 of Supporting Information), which matches closely with the value from a molecular dynamics (MD) minimized structure (Figure 6b, Rg = 12.1 Å). We also

slightly lower (C, 74.39; H, 5.75; N, 3.32). A good agreement between the measured and calculated nitrogen analysis is achieved assuming that 17 TPA units were attached per cyclodextrin, z̅ = 0.79 (C, 75.91; H, 5.68; N, 3.32). Optimization of the elemental percentages for the amount of carbon, which depends on y and z, was performed changing only the amount of triphenylamine residues. The best agreement between expected and observed values was obtained for a degree of functionalization z̅ = 0.70 (C, 74.45; H, 5.62; N, 3.08). Therefore, the elemental analysis indicates that the acylation was accomplished on 75 ± 5% of the polyrotaxane hydroxyl groups (m̅ = 16, z̅ = 0.75). The degree of functionalization of RtxMe-TPA was also evaluated by UV−visible spectroscopy. The spectrum of RtxMe-TPA in dichloromethane solution is shown in Figure 4. The absorption spectrum is a superposition of components from the individual triarylamine and backbone units: the first maximum at 304 nm is attributed to the triphenylamine units (λmax = 303 nm), whereas the second maximum at 370 nm is due to the conjugated polymer core (RtxMe-COPr, λmax = 372 nm).33−35 Assuming that the conjugated polyrotaxane backbones in RtxMe-COPr and RtxMe-TPA have the same absorption coefficient per repeat unit and that each TPA group in RtxMeTPA has an absorption coefficient that is the same as that in the methyl ester TPA-Me, then we can determine the degree of functionalization. The average number of TPA functionality attached per cyclodextrin (m̅ ) in RtxMe-TPA was calculated from the ratio of the absorption at 303 nm (ADA(303), mainly TPA absorption, eq 1) and 372 nm (ADA(372), entirely backbone absorption, eq 2) using eq 3: ADA(303) = C[(εD(303)my̅ ) + εA(303)]

(1)

ADA(372) = CεA(372)

(2)

εA(372) ⎡⎛ ADA(303) ⎞ ⎛ εA(303) ⎞⎤ ⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟⎥ m̅ = εD(303)y ⎢⎣⎝ ADA(372) ⎠ ⎝ εA(372) ⎠⎥⎦

(3) 4556

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Figure 5. Molecular weight distributions for RtxMe-TPA (solid line) and RtxMe-COPr (dashed line) obtained by GPC in THF against polystyrene narrow standards with UV detection at 254 nm.

found perfect matches of the experimental scattering data (Figure 6a) as well as the pair distribution function (Figure 6b) with data based on an MD simulation (averaged over 10 000 frames). The pair distribution function gives a longest intramolecular distance of 35 Å, which corresponds to the external diameter. SAXS data for the polyrotaxane RtxMe-TPA were modeled with a randomly oriented cylindrical stack of N CD-TPA units separated from each other by a distance D. The contribution of the polymer backbone to the scattering signal was neglected because its contribution to the overall molecular weight is small. The 1D averaged scattering intensity of the polyrotaxane can therefore be written in the following way: IPR (q) = I0

∫0

π /2

FCD(q)2 Sα(q) sin(α) dα

(4)

where FCD(q) is the form factor of a single CD-TPA unit and the structure factor Sα(q) corresponds to the cylindrical stack forming an angle α with the wave-vector q.47 The angle-averaged structure factor S(q) can therefore be obtained by directly dividing the scattering intensities for the polyrotaxane and the isolated cyclodextrin S(q) =

IPR (q) = ICD(q)

∫0

π /2

Sα(q)sin(α)dα

(5)

To reflect the dynamics of the cyclodextrin rings on the polymer backbone, the distance between CD-TPA units in the stack was modeled by a Gaussian distribution (average D and standard deviation σD). The corresponding structure factor is48 Sα(q) = 1 +

2 N

Figure 6. (a) Experimental small-angle X-ray scattering (black circles) of CD-TPA in toluene and simulated scattering from MD (red line). (b) CD-TPA pair distribution function from experiment (black circles) and the model (red line) and an optimized high-symmetry structure of CD-TPA. (c) Experimental scattering (black circles) of RtxMe-TPA in toluene and simulated scattering of a cylindrical stack of 10 CD-TPA rings with D = 20 Å (blue line).

N−1

∑ (N − k)cos(kDq cos(α)) k=1

exp[−k /2(q cos(α) × σD)2 ]

(6)

The fitting of the structure factor S(q) with eqs 5 and 6 was conducted with three parameters (N, D, and σD) and did not require the use of a scaling factor. The best fits were obtained for D = 20 Å and σD = 3 Å (Figure S2 of Supporting Information), which is in excellent agreement with the distance found in a molecular mechanics minimized model (D = 17 Å) of RtxMe-TPA. In the q-region that we are probing, the

structure factor is weakly dependent on N, and we can confirm only that N ≥ 6 (Figure S3 of Supporting Information). The full reconstruction of the scattering intensity for the polyrotaxane (Figure 6c) is obtained by using the form factor of CD-TPA obtained from the molecular dynamics simulations (Figure 6a) and N = 10 as determined for the unfunctionalized polyrotaxane by NMR. While the agreement between 4557

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Table 1. Fluorescence Quantum Yields (Φ) for TPA Functionalized Polyrotaxanes, Organic-Soluble Polyrotaxane, and TPABased Molecules and Energy Transfer Quantum Yield (ΦEET) for Polyrotaxane in a Range of Solventsa RtxMe-COPr

CD-TPA

TPA-Me

solvent (ε)

ΦDA(ex:303 nm)

RtxMe-TPA ΦDA(ex:370 nm)

ΦEET

ΦA(ex:370 nm)

ΦD(ex:303 nm)

ΦD(ex:303 nm)

THF (7.6) dioxane (2.2) benzene (2.3) DMF (36.7) DMSO (46.7) CH2Cl2 (8.9) CHCl3 (4.8) CCl4 (2.2)

0.23 0.26 0.25 0.07 0.08 0.14 0.03 0.004b

0.72 0.75 0.76 0.18 0.23 0.50 0.27 0.20

0.28 0.29 0.28 0.33 0.28 0.27 0.10 0.02b

0.78 0.78 0.81 0.80 0.72 0.73 0.77 0.69

0.028 0.022 0.028 0.026 0.026 0.013 1.8 × 10−4 8.6 × 10−5

0.027 0.028 0.032 0.032 0.028 0.014 2.2 × 10−4 9.5 × 10−5

a

The absorbance and the emission spectra were recorded in the solvents indicated in column 1. Fluorescence quantum yields (Φ) are measured relative to 9,10-diphenylanthracene in cyclohexane.51 Energy transfer quantum efficiency (ΦEET) are calculated by comparison of the absorption and the excitation spectra (eq 7). The error in the reported values is within 10%. bError >10% because of weak luminescence signal. ε is the relative dielectric constant of the solvent.

calculated and experimental scattering data is generally good, we observe a slight deviation at low q-values. This difference may be explained by the simplicity of our model but also the incomplete functionalization of the polyrotaxane with TPA units (75%). Steady-State Fluorescence. The steady-state fluorescence spectra of RtxMe-TPA excited either on the TPA units (303 nm) or on the conjugated polymer core (372 nm) are compared with those of the reference compounds RtxMeCOPr and CD-TPA in Figure 4. The triarylamine-functionalized polyrotaxane RtxMe-TPA exhibits an emission spectrum that is independent of the excitation wavelength (303 or 372 nm). The emission spectrum of RtxMe-TPA is identical to that of the organic-soluble polyrotaxane with butanoate chains RtxMe-COPr (λmax = 408 nm)33−35 and shows no significant contribution from direct fluorescence of the TPA at 365 nm. The absorbance and emission spectra of triarylamine functionalized β-cyclodextrin CD-TPA match closely those of TPA methyl ester TPA-Me. The lack of any detectable emission at 365 nm from RtxMe-TPA proves the absence of any contamination from TPA units not connected to the backbone. Despite the high number of triarylamine units closely packed on the cyclodextrin periphery of CD-TPA, no evidence of excimer49 formation in the PL spectrum and no significant difference in quantum efficiency with TPA-Me are observed. The large spectral overlap between the emission spectrum of the TPA unit (λmax = 365 nm) and the absorption spectrum of the conjugated polymer core (λmax = 372 nm), as is evident from Figure 4, combined with the proximity between donor and acceptor, assists energy transfer from the triarylamine units to the backbone of the polyrotaxane. The luminescence quenching of the triphenylamine moieties of RtxMe-TPA in THF (i.e., quenching of emission at 365 nm) was estimated to be close to quantitative (≥99%, Figure S4 of Supporting Information). The absence of a significant contribution in the emission from TPA units in RtxMe-TPA indicates that energy is transferred efficiently to the acceptor; however, photoinduced electron transfer also quenches the fluorescence of the TPA units (as discussed below). The energy-transfer efficiency was probed by comparison of the absorption and excitation spectra of RtxMe-TPA. When absorption and excitation spectra are normalized at 370 nm (the absorption maximum of the PFBP backbone), the intensity of the excitation peak at 303 nm (the absorption maximum of the TPA) reveals the amount of excitation transfer to the backbone (Figure S5 of Supporting

Information). Accounting for the small fraction of light absorbed at 303 nm by the backbone, the energy-transfer efficiency (ΦEET) on excitation of the donor TPA unit of RtxMe-TPA at 303 nm can be described as

ΦEET

( )−( = I303 A303

(1 −

εA(303) εDA(303)

εA(303)

εDA(303)

)

) (7)

where I303 and A303 are the magnitude of the excitation spectrum and the absorbance at 303 nm, respectively, when the spectra are normalized at 370 nm (Figure S5 of Supporting Information).50 This equation was used to determinate values of ΦEET in various solvents, which are in the range of 0.33−0.02 (Table 1). The photoluminescence quantum yield of RtxMe-TPA was measured in solvents with different polarities, see Table 1. The absorption and emission spectra recorded in these different solvents were all essentially identical, indicating the absence of aggregation. When the TPA-functionalized polyrotaxane RtxMe-TPA is excited on the TPA units at 303 nm, the fluorescence quantum yield (ΦDA(ex:303)) is lower, in all solvents, than the quantum yield measured for direct excitation of the conjugated polymer core at 370 nm (ΦDA(ex:370)). Excitation of RtxMe-TPA on the conjugated polymer backbone at 370 nm (ΦDA(ex:370)) results in a fluorescence quantum efficiency that strongly depends on solvents. Nonpolar solvents (THF, dioxane, and benzene) give luminescence quantum yields similar to those of RtxMe-COPr (see Table 1, ΦDA(ex:370) ≈ ΦDA(ex:370) ≈ 0.75). In contrast, polar solvents (DMF and DMSO) and chlorinated solvents (CH2Cl2, CHCl3, and CCl4) dramatically reduce the luminescence efficiency. The photoluminescence quantum yield of RtxMe-COPr was found to be 0.69−0.81 regardless of the solvent (see Table 1, ΦA(ex:370)). In contrast, the triphenylamine derivatives, TPA-Me and CDTPA, show a decreased luminescence quantum yield in chlorinated solvents, with a trend CH2Cl2 > CHCl3≫CCl4 that correlates with the ability of the solvent to act as an electron acceptor (see Table 1, ΦD(CD‑TPA), Φ(TPA‑Me)). The reduced photoluminescence of electron-rich molecules, such as triphenylamines,52,53 in chlorinated solvents has been previously attributed to photoinduced electron transfer to the solvent.54,55 4558

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The energy transfer efficiency (ΦEET) was found to be independent of the solvent polarity, and has an average value of 0.29 (from eq 7, considering ΦEET in THF, dioxane, benzene, DMF and DMSO; Table 1). However, ΦEET is significantly reduced in chloroform and carbon tetrachloride, where charge transfer to the solvent competes with energy transfer to the backbone. More importantly, the contrast between the very efficient quenching of fluorescence for the TPA units in RtxMeTPA (>0.99) and the modest efficiency of energy transfer (ΦEET ≈ 0.3) indicates that another nonradiative decay channel also quenches the fluorescence of the TPA units with an efficiency of about 0.7. This process is ascribed to photoinduced electron transfer from excited TPA units to the polyrotaxane PFBP backbone. The PFBP polymer and TPA units have first oxidation potentials of 0.84 and 0.50 V, respectively (relative to internal ferrocene56−59), and optical HOMO−LUMO gaps of 3.1 and 3.6 eV, respectively, implying that photoinduced electron transfer from the S1 excited state of TPA to PFBP should be thermodynamically favorable by about 0.8 eV. The lower fluorescence quantum yield of RtxMe-TPA in polar solvents (DMF and DMSO) can be attributed to increased formation of charge-separated states in a more polar environment. Further insights into the photophysical processes occurring in this system were provided by time-resolved fluorescence and picosecond transient absorption experiments. Time-Resolved Photophysics. A schematic energy diagram with the lowest electronic states of the donor and acceptor moieties along with the charge-separated state is displayed in Figure 7. To interpret the various steady-state and

Figure 8. Time-resolved fluorescence profile for RtxMe-TPA in THF excited at 300 nm, monitoring emission at 410−460 nm. Inset: corresponding fluorescence time profile measured in the 345−360 nm range. Red lines show biexponential fits to the experimental decays. Note that the 40 ps decay from the strongly quenched TPA emission at 350 nm is observed as a rise-time of the polyrotaxane emission at 430 nm, strongly signaling that the emission is sensitized through singlet excitation energy transfer.

ments exciting at 315 nm reveal four spectral components for the decay of 1*D−A to the ground state (Figure 9). The time and wavelength data were analyzed by singular value decomposition (SVD) of the data matrix and are well-described by four distinct spectral components, as judged by the absolute value of the singular values and the residual matrix. Adding a fifth component did not significantly alter the quality of the decomposition. The four orthogonal component spectra were linearly combined by imposing a kinetic model (Supporting Information) consistent with the reaction scheme in Figure 7, and the related rate constants were found through nonlinear optimization. In Figure 9, the four spectral components found from this procedure are displayed together with the normalized time-evolution of the concentrations of these species and the fitted data matrix. As found from comparison of transient absorption spectra of the reference CD-TPA (Figure S11 of Supporting Information) and polyrotaxane RtxMe-COPr (Figure S12 of Supporting Information), the dominant spectral component at short times (4 ns), the fourth component grows in, and this is tentatively ascribed to triplet energy transfer from the TPA units to the polyrotaxane backbone, thus forming triplet excited polyrotaxane (blue curves, D−3*A). No spectral component is observed corresponding to the charge-separated state (D•+− A•−), indicating that when this charge-separated state is formed from 1*D−A, it must decay rapidly, probably to 3*D−A. This interpretation is consistent with the rapid growth of the 3*D signal in Figure 9. Note that 3*D−A could not be formed through direct intersystem crossing from 1*D−A because this state is heavily quenched by energy and electron transfer. The reaction scheme in Figure 7 contains 10 rate constants, but only a small fraction of them are independently fitted. First, from steady-state arguments, the relative fraction of 1*D−A that decays via electron (k3) or energy (k2) transfer is locked by the observation of close to 100% quenching but only 28%

Figure 7. Energy diagram for excitation of the triphenylamine functionalized polyrotaxane RtxMe-TPA. (The donor, D, is the TPA unit, and the acceptor, A, is the PFBP conjugated polymer core; superscripts 1 and 3 indicate the singlet and triplet excited states, respectively; dashed arrows indicate slow processes that are not observed.

time-resolved measurements, the interplay between these states needs to be elucidated. To this end, the photophysics of RtxMe-TPA was studied by time-resolved fluorescence and transient absorption measurements. When RtxMe-TPA is excited at 300 nm, the emission spectrum exhibits two distinct spectral components: a strong fluorescence signal at 410−460 nm due to emission of the PFBP backbone and a short-lived (40 ps) band at 345−360 nm due to quenched TPA emission (Figure 8). The intense backbone luminescence rises with a lifetime (40 ps) identical to that of the TPA fluorescence decay, providing evidence of an efficient singlet−singlet excitation energy transfer (EET) from the TPA units to the polyrotaxane backbone. The sensitized polyrotaxane emission decays with a lifetime similar to the lifetime of the reference compound RtxMe-COPr (about 400 ps). However, as previously observed in the steady-state analysis, EET is not the only additional nonradiative process occurring from the first excited singlet state of TPA. Complementary transient absorption measure4559

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Article

Table 2. Fitted Rate Constants (All in Reciprocal Nanoseconds) from Transient Absorption of RtxMe-TPA in THF Solution upon Excitation at 315 nma

a

k1

k2

k3

k4

k5

k6

k10

1.2

6.1

15.4

2.5

0.25

>k3+k5

0.19

k7 = k8 = k9 = 0; k3 = 2.5k2; see text for explanation.

COPr. The acceptor moiety has biexponential fluorescence decay: one short (about 40 ps) probably due to a combination of charge separation and singlet−singlet annihilation from multiple excited polymer units and a second around 440 ps due to emission from the unquenched polyrotaxane backbone (Figure S9 of Supporting Information). Transient absorption measurements exciting at 390 nm give a spectrum similar to that of RtxMe-COPr (Figure S12 of Supporting Information), and as observed for the fluorescence, the decay is biexponential: one short lifetime (about 20 ps) due again to a combination of charge separation and the deactivation of multiple excited units of the backbone and one long lifetime for the de-excitation of unquenched acceptor units to the ground state (396 ps). The amount of charge separation originating from excitation into the polyrotaxane backbone is difficult to estimate because of the expected annihilation process occurring on a similar time scale. Comparison with the steady-state emission results indicates a low efficiency for charge separation from the singlet excited polyrotaxane. In addition, no long-living transients are observed in the transient absorption, suggesting both that any chargeseparated state formed upon acceptor excitation does not recombine via 3*D−A and that D−3*A is not directly formed from the first excited singlet state of the backbone but can be formed only by sensitization via triplet energy transfer from 3 *D−A. Most of the steady-state and time-resolved photophysical results are consistent with the kinetic model in Figure 7 and can be summarized by the following three conclusions: (1) Charge separation from TPA-excitation (at 300 nm) is efficient [ΦET = k3/(k1 + k2 + k3) ≈ 0.7], and leads to rapid formation of the TPA triplet (3*D−A). (2) Energy transfer from TPA units to the PFBP conjugated polymer backbone is quite efficient, generating D−1*A, and competes with charge separation [ΦEET = k2/(k1 + k2 + k3) ≈ 0.3]. (3) Direct excitation of the polyrotaxane backbone (at 370− 390 nm) leads to little formation of the charge-separated state ([k5/(k4 + k5) ≈ 0.09]. The RtxMe-TPA polyrotaxane is a polydisperse system in three respects: it has distributions of chain lengths (n̅ = 10), levels of threading (y ̅ = 1.1), and extents of functionalization (m̅ = 16). The orientation of the cyclodextrins (head-to-head or head-to-tail) on the backbone is also random, and the TPAfunctionalized cyclodextrin can adopt a range of conformations. Despite this complexity, many of the photophysical characteristics of this system are surprisingly simple; for example, the rise and decay of fluorescence in Figure 8 fit well to monoexponentials. However, the time-resolved data also provide evidence for heterogeneity. For example, backbone singlet excited states (D−1*A) can be generated in two ways: either directly, by excitation at 380 nm, or indirectly, via excitation of TPA units at 300 nm and EET. The average fluorescence decay times for D−1*A states generated in these two ways are different, with those from indirect excitation being

Figure 9. Transient absorption spectra obtained upon femtosecond laser flash photolysis (315 nm) of RtxMe-TPA in argon-saturated THF. (a) Raw data measured between 500 and 750 nm for delay times between 0 and 9000 ps. The data matrix is analyzed by SVD decomposition into four significant components that are related to each other through the kinetic scheme based on Figure 7 with optimized rate constants presented in Table 2. The fitting result is displayed in the middle 2D graph (b), and the spectral and time components are shown in the right (c) and bottom (d) panels, respectively.

quantum yield for EET; k3 = k2(1 − 0.28)/0.28 = 2.5k2. Second, the rate constants k1 and k4 are known from timeresolved fluorescence and transient absorption measurements on the reference donor and acceptor compounds (CD-TPA and RtxMe-COPr; Supporting Information). Lastly, because we do not observe a transient spectroscopic signature of the charge-separated state, it must decay rapidly into the detected 3 *D−A species, and the rate constant of its formation then essentially becomes k3 (meaning that the fit is insensitive to the value of k6 as long as it is substantially larger than k3 + k5). This implies that other decay channels from the charge-separated state could be neglected (i.e., k7, k8, and k9 are put to zero). The nonlinear fitting procedure, optimizing k2, k5, and k10 to the measured data, leads to the rate constants collected in Table 2 and the fitted data shown in Figure 9. A different scenario is observed when RtxMe-TPA is directly excited at around the absorption maximum of the PFBP backbone (380 nm). Both time-resolved luminescence and transient absorption measurements are consistent with a twocomponent decay mechanism from D−1*A to the ground state. The luminescence spectrum of RtxMe-TPA lies at 390−480 nm (Figure 4) and resembles in shape the emission of RtxMe4560

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The Journal of Physical Chemistry C

Article

The motion of TPA units relative to the PFBP backbone was analyzed by inspecting individual TPA−PFBP radial distance time series (Figure 11). This shows that while a few TPA units

longer lived (Figures S8−S10 of Supporting Information). A possible interpretation of this difference is that segments of the conjugated polymer backbone that happen to be very close to TPA units are less likely to be excited via EET; in these cases, excitation is more likely to result in charge separation. Conversely, segments of the backbone that happen to be far from TPA units are more likely to be excited by EET. Thus, the population of D−1*A states generated by indirect excitation are likely to be more remote from TPA units; thus, they are less likely to undergo hole transfer and charge separation. Further insights into the consequences of this heterogeneity in the distance of the TPA units from the PFBP backbone were provided by molecular modeling studies. Molecular Modeling. The position and motion of electron- and energy-donating TPA units relative to the conjugated PFBP backbone was investigated in high spatial and temporal resolution by molecular dynamics (MD) simulations of a solvated model system. Two independent simulations were performed, each biased to smaller and larger PFBP-TPA distance (see Experimental Section and Supporting Information for details). Analysis of the resulting trajectories shows that the coaxial structure is remarkably heterogeneous, featuring a broad distribution of TPA−PFBP distances (Figure 10). While 93.5% of TPA units are located in a cylindrical shell

Figure 11. TPA−PFBP radial distance time series for all TPA units of our model for RtxMe-TPA extracted from the trajectory initially biased toward larger radii.

make frequent transitions to different radii, most are confined to a thin radial shell for the duration of the simulation, thus exhibiting metastability on the time scale of the simulation. This metastability can be rationalized by steric hindrance due to the fact that the system is very crowded. The average correlation time of TPA−PFBP radial distances is in excess of 50 ns (see Supporting Information); hence, on average, TPA units are static on the time scale of ET and EET processes (