Article pubs.acs.org/JPCA
Charge and Energy Transfer Dynamics in Dimethylsilylene-Spaced Aminostyrene Stilbene Monomer Using Time-Resolved Techniques Hsuan-Hsiao Yao,† Meng-Ru Chung,† Chiling Huang,† Sandra Meng-Hsuan Lin,† Chih-Hsien Chen,‡ Tien-Yau Luh,§ and I-Chia Chen*,† †
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China Department of Chemical Engineering, Feng Chia University, No. 100, Wenhwa Road, Seatwen, Taichung, Taiwan 40724, Republic of China § Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617, Republic of China ‡
S Supporting Information *
ABSTRACT: We used transient absorption and time-correlated single photon counting (TCSPC) techniques to investigate the charge transfer reaction in monosilylene-spaced aminostyrene stilbene monomer. With 266 nm excitation, both stilbene (sti) and aminostyrene (ast) moieties were excited. In nonpolar solvents, the transient absorption band centered at 600 nm appeared promptly and is assigned to the excited state of sti*; this state relaxes at time constant 1.2−1.4 ps and is explained to proceed energy transfer to ast 1ππ*. The second transient band at 460 nm is assigned to absorption of ast 2ππ*; this state accessed from direct excitation has a lifetime 65 ps. This agrees with the observation of 85−89 ps emission decay from the TCSPC measurements. In polar solvent, an excited absorption band centered at 530 nm appeared with a rise time constant 0.2−0.6 ps. This band is assigned to the charge transfer state. This charge transfer process occurs as the acceptor fluorophore (sti) is excited and the electron moves from the occupied π orbital of donor ast to sti* forming ast+sti−. This rise time corresponds to the combined processes of charge and energy transfers. The second rise in this charge-transfer state at time constant 0.74−1.5 ps is observed and assigned to occur from electron hopping from ast 2π* orbital to sti π*. The third time constant 18−31 ps is observed and is attributed to conversion of anti to syn form in the charge-transfer state because the syn form is more polar and further stabilized in polar environment. A rapid charge transfer process in monosilylene-spaced system although two Si−C single bonds are used as spacer is possibly because of the short distance of the ast and the sti conjugated systems, resulting in π orbital overlap between donor and acceptor.
I. INTRODUCTION Silylene-spaced copolymers with varied aryl groups regioselectively incorporated into polymer chains have been synthesized.1−5 Two examples of these kinds of copolymers are displayed in Scheme 1. The silylene serves as a tetrahedral insulator, hence, electron delocalization through the silicon spacer between adjacent conjugated chromophores is prohibited. However, when the adjacent chromophores are nonequivalent in these copolymers, Forster resonance energy transfer (FRET), photoinduced electron transfer (PET) or light harvesting via sequential photophysical processes may take place. The energy transfer mechanisms of the silylene-spaced alternating donor−acceptor copolymers (donor−SiMe2−acceptor−SiMe2)m (abbreviated as (DA)m, structure shown in Scheme 1) and [(donor−SiMe2)3−acceptor−SiMe2]m (D3A)m were reported.6−8 In these studies, the donor is 4,4′divinylbiphenyl and the acceptor is 4,4′-divinylstilbene. An ultrafast energy transfer rate (0.6 ps)−1 was assigned to conversion between the neighboring donor and acceptor moieties. Theoretical calculations based on fragment excitation difference (FED) method were employed to interpret the © XXXX American Chemical Society
experimental data. The calculated results concluded that the dipole coupling mechanism dominated the energy transfer along with a non-negligible proportion of contribution from the high-multipole terms. Because of small intermolecular distances 12−14 Å between the donor and acceptor, the point-dipole model like Forster mechanism is no longer adequate. Dexter transfer mechanism also accounts for a small portion of the energy transfer process.7 In these silylene-spaced copolymers anti−anti, anti−syn, and syn−syn conformers around the silicon atom exist. To decrease the complexity of conformational changes, one of the two vinyl moieties was replaced by an aryl group so that only two conformers anti and syn forms exist, as shown in Scheme 1 for the compound 4-((E)-2-(dimethyl(4-((E)-styryl)phenyl)silyl)vinyl)-N,N-dimethyl aniline, 1.3 Chen et al. synthesized a series of monosilylene-spaced copolymers with 4-aminostyrene moiety as the donor and varied acceptor chromophores with different reduction potentials.3 The charge transfer states were Received: July 24, 2017 Revised: August 31, 2017 Published: September 5, 2017 A
DOI: 10.1021/acs.jpca.7b07282 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Scheme 1. Chemical Structures of 1 Anti and Syn Forms, (DA)m, D = Biphenyl, and A = Stilbene, Aminostyrene 2, and Stilbene 3
2.2. Measurements. Transient absorption spectra were measured using femtosecond transient absorption spectrometer (ExciPro, CDP). The details of the setup are described elsewhere.10 The samples were excited at wavelength 266 nm; this light source was generated by tripling the output from a femtosecond laser (Hurricane, Spectra Physics, 800 nm, 1 kHz, 1 mJ/pulse, and 100 fs). The probe beam white light continuum was generated by focusing the fundamental output 800 nm on a 3-mm-thick water cell (range 440−670 nm). Both the pump and the probe beams were focused on a rotational cell and each measurement was performed within 1.5 h. The polarization of the excitation pulses relative to the probe was set to the magic angle (54.7°) using a half waveplate. The pump beam was chopped at 500 Hz repetition rate, and probe at 1 kHz. The transient absorption signal was collected by fibers, dispersed by monochromator, and detected by photodiode arrays. The cross correlation of the instrumental response function has a fwhm ∼360 fs for water cell. All measurements were performed at 22 °C. Picosecond time-resolved fluorescence measurements were performed with TCSPC. The light source was a mode-locked femtosecond laser system (MaiTai, Spectra Physics). The generated third harmonic (266 nm) served as excitation source. The fluorescence was filtered with a bandpass filter (±10 nm) and detected with a multichannel plate photomultiplier (Hamamatsu). A vertical polarizer was placed before the PMT and the polarization of the pump beam was rotated to achieve the magic-angle condition. The instrument response function was 30 ps at full-width at half-maximum (fwhm). 2.3. Theoretical Calculations. All calculations were performed using Gaussian 09 software package.11 DFT and time-dependent DFT (TD-DFT) method PBE0 and basis set 6-311G(d,p) were used in the calculations. The other method B3LYP was also used and the obtained optimized structures, vertical transitions, and molecular orbitals involved are similar. The geometries of the ground state and the first excited state were optimized without any symmetry constraint. The solvent effect was incorporated according to the polarizable continuum model (PCM); this model considers solvent molecule with spherical structure. In the calculation using PCM, we only performed for solvent acetonitrile (ACN). Vibrational
identified in those copolymers based on their dual emission character and the time-resolved data. They reported that the rate constant of charge transfer in the copolymer from the dimethylaminostyrene to the stilbene moiety is the fastest nearly (10 ps)−1.3 Although in this system, two Si−C single bonds are between the donor and the acceptor, the charge transfer process remains quite efficient. The silylene-spaced molecules exhibit efficient energy and charge-transfer dynamics and should serve as a prototypical DA unit for studying the detailed mechanism of energy dissipation. During the polymer folding, aggregation may occur and interaction within the aggregated chromophores may need to be considered.1−5,7−9 This generates some complications in studying the relaxation dynamics in this donor−acceptor system. At the present work, we employed femtosecond transient absorption and time correlated single-photon counting (TCSPC) techniques to study the dynamics of 1 in solution. 1 is a monomer in this monosilylene-spaced copolymer system, so that the charge transfer between nonadjacent chromophores does not exist. Instead, in stilbene moiety of the monomer trans−cis isomerization can occur upon photoexcitation because of less structural constraint. We identify the charge transfer species in the transient measurements and study the interplay of these processes. On the basis of the dynamics of excited states of 1, further understanding the copolymer system may become more feasible. Identification of charge transfer state is also presented using theoretical calculations based on density functional theory (DFT).
2. EXPERIMENTS 2.1. Samples. Molecules 1−3 were synthesized according to the method described previously.3 For TCSPC measurements, typical concentrations were ∼10−5 M and for transient absorption the absorbance of the sample solution was less than 0.5 at the excitation wavelength 266 nm. The steady-state absorption and fluorescence spectra were recorded with the Hitachi U3900 and F-7000 spectrometers, respectively. For TCSPC measurements, the excitation laser power was kept
