Article pubs.acs.org/JPCC
Ultrafast Energy Transfer in Divinylbiphenyl and Divinylstilbene Copolymers Bridged by Silylene Kuan-Lin Liu,† Sheng-Jui Lee,† and I-Chia Chen* Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China
Chao-Ping Hsu* Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China
Chih-Hsien Chen and Tien-Yau Luh Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617, Republic of China S Supporting Information *
ABSTRACT: The energy-transfer properties of a regioregular silylene-spaced alternating donor−acceptor copolymer [(donor)3-SiMe2−acceptor-SiMe2]m (denoted (D3A)m) are determined with time-resolved spectroscopy; 4,4′divinylbiphenyl serves as the donor and 4,4′-divinylstilbene as the acceptor. With excitation at wavelength 266 nm, fluorescence up-conversion curves at various detection wavelengths are measured. The observed decay of donor emission at a time constant 0.3 ps and the emission of the acceptor have a corresponding fast rise and a slower second rise at ∼2 ps. In the tail of fluorescence emission, the varied slow decay components (2−7 ps) on the donor and rise on the acceptor are observed and assigned to intrachain aggregates. With the Coulomb coupling which is beyond the multipole approximation, the Fermi golden rule rates of energy transfer are calculated between adjacent monomers and between the D2 exciton and A. In all D2A and D3A conformers calculated, we found that the coupling strengths of D−A and D−D are similar, but the coupling of exciton state D2 to the acceptor is about a factor of 1.25−1.72 weaker. According to the experimental data and the calculated results, we propose that the transfer between adjacent donors is ultrafast, so the excited donor has an energy transfer reaching the acceptor at a rate of ∼(0.6 ps)−1 and transports via either direction to extend a final rate of (0.3 ps)−1. According to the calculated ratios of coupling strength, the second rise component, 1−2 ps, is assigned to the transfer between the D2 exciton state and the acceptor.
1. INTRODUCTION
The mechanism of energy transfer in conjugated polymers is investigated because it is a key process in the working mechanism of optoelectronic devices and of many guest−host systems. Schwartz and co-workers showed that in an alkoxysubstituted poly(p-phenylenevinylene) exciton diffusion along the chain is slow for the weak dipole coupling system and in an environment where interchain interactions are inhibited.14 Beljonne et al. investigated the interchain and intrachain energy transfer of acceptor-capped polyindenofluorene polymers by means of ultrafast spectroscopy and quantum chemistry calculations.15 They obtained the transfer rates for polymers in the solution phase and in film. From the results of calculations and experimental data, they proposed that the intrachain transfer consists of two steps: hopping along the conjugated chains as the rate-limiting step and transfer to the acceptor (perylene). For films, interchain transfer is efficient. In this group, we reported the results of the photophysical
Linear polymers containing alternating silylene and πconjugated moieties have been shown to have potential as electroluminescent materials,1 electron transfer materials,2 and light-harvesting materials.3,4 This silicon-based tetrahedral spacer effectively interrupts the π-conjugation along the polymer backbone;5,6 hence, the photophysical properties of the polymer can be readily tuned by designing the chromophores with a varied length of conjugation.7 The silylene-spaced copolymers are prepared through hydrosilylation of bisalkynes with bissilyl hydrides;8,9 two or more tailored chromophores separated by a silylene group can become regioselectively placed in the polymer chain, and various copolymers with properties such as charge transfer and energy transfer might be generated.10−12 In addition, the presence of alkyl substituents on the silicon atom can increase the solubility of the polymers in organic solvents and render them more processable. With adequate chromophores, the energy transfer and photoinduced electron transfer can occur with excellent efficiency.13 © 2012 American Chemical Society
Received: August 17, 2012 Revised: October 30, 2012 Published: December 13, 2012 64
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The steady-state absorption (Hitachi U3300 spectrometer) and fluorescence (Hitachi F4500 fluorimeter) spectra were recorded with the indicated instruments. For time-resolved measurements, the excitation laser power was kept below 2 mW; absorption spectra were recorded before and after measurements to ensure no sample degradation. The absorbance of the solution for fluorescence up-conversion measurements was less than 0.5 at the excitation wavelength. 2.2. Fluorescence Up-Conversion Measurements. The laser system and setup for fluorescence up-conversion measurements are described as follows. The light source is a femtosecond mode-locked Ti:sapphire laser (Spectra-Physics, Mai Tai) pumped with a Nd:YVO4 laser (5 W cw, 532 nm, Spectra-Physics, Millennia-type). This laser outputs a pulse train (82 MHz, 800 nm) with average power ∼400 mW. The third-harmonic pulses at 266 nm were generated with two nonlinear crystals (BBO, type I) and were focused with a lens (focal length = 50 mm) onto a rotating sample cell (path length 1 mm) for excitation. The residual fundamental pulse served as an optical gate. Detection of the fluorescence at the magic angle was achieved on rotating the excitation polarization with respect to the polarization of gate pulses. The fluorescence was collected and focused onto a sum-frequency generation crystal (BBO, type I) with off-axis parabolic mirrors (focal lengths 5 and 7 cm at 90°). After traversing a variable delay stage, the gate pulse was noncollinearly (∼12°) focused with a lens (focal length 10 cm) onto a sum-frequency generation crystal (BBO, type I, θ = 38°). The sum-frequency signal was collected with a lens and separated from other light by means of an iris and a band-pass filter and then entered a double monochromator. The signal was detected with a photomultiplier tube in a photon-counting mode. Various emission wavelengths were selected on rotating the SFG crystal and tuning the monochromator, whereas the measurement at a wavelength range of 400 ± 15 nm was restricted because of strong scattering of the residual second harmonic pulse (400 nm). The instrument response function was obtained on measuring the ultrarapid S2 fluorescence of trans-azobenzene at 420 nm;18 this fluorescence decay appeared with a symmetric Gaussian shape for which the full width at half-maximum (fwhm) was ∼230 fs. 2.3. Picosecond Time-Resolved Fluorescence Measurements. These measurements were performed with timecorrelated single-photon counting (TCSPC). The light source was the same laser system as for the measurement of fluorescence up-conversion. A fraction of the generated third harmonic (266 nm) served as an excitation source. A sample was contained in a cuvette (path length 1 mm); the concentration was maintained the same as for the fluorescence up-conversion experiments. The fluorescence was filtered with a bandpass filter and detected with a multichannel plate photomultiplier (MCP-PMT). A vertical polarizer was placed before the MCP-PMT, and the polarization of the pump beam was rotated to achieve the magic-angle condition. The instrument response function was 30 ps at fwhm.
properties of a silylene-spaced alternating donor−acceptor copolymer (donor-SiMe2−acceptor-SiMe2)m (abbreviated as (DA)m), which exhibits efficient light-harvesting capability.16 The donor is 4,4′-divinylbiphenyl, and the acceptor is 4,4′divinylstilbene (Scheme 1). An ultrafast energy transfer rate Scheme 1. Molecular Structures of Silylene-Spaced Copolymer (DA)m, (D3A)m,a and Monomers D and A
a
Donor D = biphenyl and acceptor A = stilbene.
(0.6 ps)−1 between the donor and acceptor moieties is observed from their time-resolved fluorescence. The energy transfer mechanism is accounted for by a theoretical approach, the fragment excitation difference (FED) method to characterize various coupling terms for energy transfer.16 The results of calculations show that the dipole coupling mechanism17 dominates the energy transfer process, along with a nonnegligible proportion of contribution from the high-multipole terms. This is usually true because the shapes of the donor and acceptor cannot be ignored compared with the intermolecular distance; a point-dipole model like the Förster mechanism is no longer adequate. In the present work, we report the energy-transfer dynamics of a silylene-spaced alternating copolymer [(donor-SiMe2)3− acceptor-SiMe2]m (abbreviated as (D3A)m, m ∼ 12) by means of ultrafast spectroscopy and theoretical methods. As shown in Scheme 1, (D3A)m has the same donor and acceptor as (DA)m. In (D3A)m three consecutive donor moieties are linked with one acceptor; a high proportion of donor moieties in a copolymer can effectively increase the proportion of excitation on the donor moiety. In addition, excitation interaction between consecutive donor moieties is investigated by comparing the rate of energy transfer for these two polymers (DA)m and (D3A)m. We employ theoretical calculations to interpret the dynamics.
2. EXPERIMENTS 2.1. Samples. The preparation of the silylene spaced copolymer (D3A)m is described elsewhere.7 The purity is ≥95% as characterized by NMR spectra. The mass and polydispersity index of (D3A)m are 12.1 kg/mol and 3.1, respectively, obtained from gel-permeation chromatography with polystyrene as standard. For time-resolved measurements, typical concentrations of (D3A)m in each solvent were ∼1 × 10−5 M. Solvents tetrahydrofuran, p-dioxane, and dichloromethane were spectroscopic grade and obtained commercially (Aldrich); all these solvents were used as received.
3. THEORY Quantum chemistry calculations were performed with the QChem program package.19 The optimized geometries of the ground state are obtained at the B3LYP/6-31G(d) level.20,21 Prior to optimizing the whole molecule, we optimized each moiety, divinylbiphenyl, divinyldimethylsilane, and divinylstilbene, separately. In the ground state, the divinylbiphenyl molecule has trans- and cis-conformers.22 Divinyldimethylsilane 65
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has the four conformations trans−anti−anti, cis−anti−anti, anti−syn, and syn−syn through rotation of two vinyl groups as shown in Figure 1. Divinylstilbene has the three conformations
Figure 2. Absorption (solid line) and fluorescence spectra (dashed line) of copolymer (D3A)m in p-dioxane. Normalized fluorescence excitation curve of emission wavelength at 400 nm (dotted line) is shown for comparison.
copolymer (D3A)m in CHCl3. In principle, the absorption profile of (D3A)m can be accounted for by adding the absorption spectra of monomer D and A with a ratio 3:1. The absorption maximum (320 nm) and the shoulders (363 and 378 nm) of (D3A)m are slightly red-shifted by ∼5 nm, compared with that of monomer D and A, indicating some electronic interaction between chromophores within the copolymer. The fluorescence spectrum of (D3A)m resembles that of monomer A independent of the excitation wavelength, indicating an efficient energy transfer process. The energy transfer efficiency can be roughly estimated through comparison of the absorption and fluorescence excitation spectra, both normalized at 375 nm where only the acceptor absorbs. The relative intensity of the donor moiety at 320 nm yields an energy transfer efficiency of ∼72%, which is near that observed in (DA)m, ∼80%. In p-dioxane, THF, and CH2Cl2, the steadystate spectra of (D3A)m all display similar features with spectral shifts less than 5 nm. The peak positions of those spectra are listed in Table 1.
Figure 1. Distyrylsilane in four conformations; Ar1 and Ar2 denote distinct aromatic groups.
trans−trans, trans−cis, and cis−cis via rotating the σ bond between the phenyl and the vinylsilane group.22 After optimization of each moiety, we selected the individual conformations of donor (divinylbiphenyl) and acceptor (divinylstilbene) moieties, and they are combined with a divinyldimethylsilane to form the initial geometry and then to optimize the whole molecule. In total, 400 conformations of D2A were obtained and then used to calculate the D−A distances, Coulomb integral, and the rates of energy transfer. For molecule D3A, there exists a large number of combinations of conformations. We calculated 4000 conformations and obtained 3984 optimized structures. They are used to estimate the donor−acceptor distances and the energy transfer rates, from the first (D1), the second (D2), and the exciton with D1 and D2 populated, to the acceptor (A). Wong et al. have pointed out that the electronic coupling is underestimated if only the dipole−dipole interaction is included in the π-conjugated system.23 Hence, the full Coulomb integral Vcoul is used to calculate the electronic coupling.22 1 Vcoul = dr dr′ρD*(r) ρ (r′) (1) |r − r′| A
Table 1. Maxima in Steady-State Absorption Amax and in Emission Spectra Flmax of Copolymer (D3A)m in Various Solvents Amax/Flmax (nm)
∫ ∫
where ρ denotes the transition density of the donor or acceptor space, separately calculated for each fragment ρ(r) =
∑ aia(n)ψi*(r)ψa(r) ia
compd
p-dioxane
(D3A)m
319/393, 414 315/355, 372
D A
(2)
The truncated molecule with separated donor and acceptor was used to calculate the Coulomb integrals. Time-Dependent Density Functional Theory (TDDFT) was used for calculating the transition densities and the energies of vertical transitions.
THF 318/393, 413
CH2Cl2
CHCl3
321/394, 416
319/394, 417 314/357, 374 355/392, 411
n-hexane
311/351, 368
A small extent of aggregates is formed within (D3A)m, which is observed from an absorption tail in a region 400−450 nm and a fluorescence tail at wavelengths greater than 460 nm as compared with that of A. The relative contributions of these minor features remain constant over a wide range of concentrations (1 × 10−5−3 × 10−7 M); we assign this effect to the intrachain aggregation. Similar phenomena for a variety of silylene-spaced copolymers have been reported by Luh and co-workers;8,9 their results indicate that, when the chromophores contain no bulky substituents, intrachain aggregates form easily due to a flexible polymer backbone, which are
4. RESULTS AND DISCUSSION 4.1. Electronic Absorption and Emission Spectra. The absorption and fluorescence spectra of donor D and acceptor A display intense π−π* absorption bands at 314 and 355 nm, respectively.16,22 The properties of these spectra are described previously.15 Figure 2 shows the steady-state spectra of 66
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manifested as an absorption tail and fluorescence at long wavelengths.8,22 4.2. Time-Correlated Single-Photon-Counting Measurements. The technique TCSPC is used to determine the lifetime and the corresponding range of emission of each species (donor, acceptor, and intrachain aggregates) in (D3A)m. All the fluorescence decay curves at each wavelength in various solvents are well fitted with a single exponential decay function, and the fitted time constants are summarized in Table 2. Three
chromophores within (D3A)m are isolated, the proportion of the excitation energy (266 nm) absorbed by donor and acceptor moieties, respectively, can be estimated from their numbers and absorption coefficients. About 90% and 10% of the excitation energy was absorbed by donor and acceptor moieties, respectively. The fluorescence decay curves were fitted with Gaussian-convoluted multiexponential functions; the Gaussian function corresponds to the response function of the system, of which fwhm is 230 fs. A long-lived component in each decay curve was fixed to the values obtained from TCSPC measurements. The best-fitted results of (D3A)m in various solvents are summarized in Table 3. Figure 4 shows the
Table 2. Time Constants (ns) Obtained from TCSPC Measurements on (D3A)m in Various Solvents and Monitored Wavelength Rangesa
a
340−380 nm
410−440 nm
solvent
D
A
p-dioxane THF CH2Cl2 CHCl3
0.95 0.89 0.85 0.83
1.02 0.94 0.90 0.90
460 nm
480 nm
Table 3. Time Constants (ps) Obtained from Time-Resolved Fluorescence Measurements of (D3A)m in Various Solvents and the Monitored Wavelength Rangea
500 nm
A + aggregates 1.10 1.00 0.93 0.93
1.16 1.13 0.97 0.97
1.26 1.27 1.03 1.05
345−375 nm
The time constants listed are within ±0.025 ns. pdioxane THF CH2Cl2 CHCl3
representative fluorescence decay curves of (D3A)m are shown in Figure 3. The fluorescence from donor and acceptor moieties
420 or 440 nmb
460 nm
480 nm
500 nm
A + aggregates
D
A
τd1/τd2
τr1/τr2
0.29/4.6
0.34/1.6
0.27/2.0
0.29/5.3
0.29/6.5
0.34/7.4 0.29/5.0 0.33/6.0
0.25/1.7 0.23/1.8 0.34/2.5
0.23/2.7 0.44/3.6 0.41/8.1
0.37/3.9 0.34/2.3 0.46/4.7
0.41/7.3 0.58/5.6 0.19/3.8
τr1/τr2
a τd and τr are time constants for decay and rise components, respectively. b420 nm for p-dioxane and THF, and 440 nm for CH2Cl2 and CHCl3.
Figure 3. Typical fluorescence decay curves of (D3A)m in p-dioxane at 340, 440, and 500 nm. The open circles and solid lines are experimental data and best-fitted curves, respectively.
is observed at around 340−380 and 410−460 nm, respectively. The lifetime (∼0.9 ns at 340−380 nm) of the residual fluorescence from the donor moiety is close to that of the donor monomer. Beyond 460 nm, the emission from intramolecular aggregates dominates, and the fluorescence lifetime slightly increases with detection wavelength, which is attributed to the formation of various aggregates. Figure 1 depicts the four varied arrangements of the chromophores at a single silylene spacer, i.e., anti−anti, anti−syn, and syn−syn conformations, which can lead to various types of intrachain aggregates in (D3A)m. More closely packed aggregates might emit at greater wavelengths with a slightly increased lifetime. The fluorescence lifetimes of (D3A)m are slightly shorter in polar solvents, implying a nonpolar character of the excited state. 4.3. Fluorescence Up-Conversion Measurements. We employed the fluorescence up-conversion technique to investigate (D3A)m over a wide range of detection wavelengths. Because the results of steady-state measurements show that the
Figure 4. Six femtosecond time-resolved fluorescence decay curves of (D3A)m in THF monitored at wavelengths as indicated. Data at each detection wavelength were collected for several seconds and normalized to counts per second for comparison. The open circles and solid lines are experimental data and the best-fitted curves, respectively; the residuals are shown in gray solid lines. 67
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fluorescence decays at six characteristic wavelengths of (D3A)m in THF. Upon excitation, the features of the time-resolved fluorescence vary significantly with emission wavelength. The fluorescence from the donor moiety at 345−375 nm displays an instrument-limited rise followed by a rapid (0.3 ps) and a slow decay (7.4 ps). The intense fluorescence from the acceptor moiety at 420−440 nm shows a rapid (0.3 ps) and a slow rise (1.7 ps). Within 460−500 nm, the fluorescence from the acceptor moiety and intramolecular aggregates appeared simultaneously, and a rapid rise (0.4 ps) followed by a slow rise (2.7−7.3 ps), which increases with detection wavelength, is observed. The features of the time-resolved fluorescence of (D3A)m in various solvents used are similar (see Supporting Information), showing no dependence on solvent environment. The spectral evolution of the experimental curves within several picoseconds is summarized as follows. (i) The fluorescence from the donor moiety at 345−375 nm shows an instrumentlimited rise. (ii) The donor fluorescence then decays rapidly along with a rise in the fluorescence within 420−500 nm that predominantly originates from the acceptor moiety; the energy transfer process is unambiguously observed in the first few picoseconds. (iii) The donor fluorescence further exhibits a slow decay, which accompanies a slow rise in the fluorescence from aggregates at 460−500 nm; a small fractional of excitation energy of the donor moiety is quenched by the intramolecular aggregates. The time-resolved experiments of the donor monomer (D) via 266 nm excitation are reported elsewhere.22 The results showed that D initially populates the Sn state, followed by an ultrafast internal conversion (∼0.1 ps) to the S1 state. The major deactivation in the S1 state is a geometric relaxation (twisted to planar) with a time constant (13−65 ps) varying with the solvent viscosity. Because of weak interchromophoric interactions, the donor moiety in (D3A)m is assumed to populate the same Sn state with 266 nm excitation. The instrument-limited rise (
