Article pubs.acs.org/JPCA
Excitonic Coupling and Femtosecond Relaxation of Zinc Porphyrin Oligomers Linked with Triazole Bridge: Dynamics and Modeling Alexey Bukreev,† Konstantin Mikhailov,*,† Ivan Shelaev,† Fedor Gostev,† Yuliya Polevaya,‡ Vladimir Tyurin,‡ Irina Beletskaya,‡ Stanislav Umansky,† and Victor Nadtochenko†,¶,§,∥ †
Semenov Institute of Chemical Physics RAS, Moscow 117977, Russia Frumkin Institute of Physical Chemistry and Electrochemistry, Moscow 119071, Russia ¶ Moscow Institute of Physics and Technology State University, Dolgoprudny, Moscow region 141700, Russia § Moscow State University, Faculty of Chemistry, Moscow 119991, Russia ∥ Institute of Problems of Chemical Physics RAS, Chernogolovka, Moscow region 142432, Russia ‡
S Supporting Information *
ABSTRACT: The synthesis of new zinc porphyrin oligomers linked by a triazole bridge was carried out via “click” reaction. A split in the porphyrin oligomer B-band was observed. It was considered as evidence of exciton−excitonic coupling. The relaxation of excited states in Q-band porphyrin oligomers was studied by the femtosecond laser spectroscopy technique with a 20 fs pump pulse. The transient oscillations of two B-band excitonic peaks have a π-radian shift. For explanation of the coherent oscillation, a theoretical model was developed. The model considered the combination of the exciton−excitonic coupling between porphyrin rings in dimer and weak exciton−vibronic coupling in one porphyrin ring. By varying the values of the structural parameters of porphyrins (the strength values of this couplings and measure of symmetry breaking), we obtained correspondence between the experimental data (phase shift and amplitudes of the spectrum oscillations) and the predictions of the model developed here.
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study,14 understanding the theoretical basis of the spectra of different metalloporphyrins has been investigated using different methods and models. Many studies were devoted to the prediction of geometry and electronic absorption spectra of porphyrins and their derivatives.14−18 Methods of nonlinear spectroscopy19,20 were applied for investigation of the processes of energy transfer, for example, pump−probe, measuring absorption spectrum of excited molecules and its variation in time. Relaxation and energy transfer processes in porphyrin complexes were observed at a picosecond scale.5−8 At femtosecond scale, one can observe coherent oscillations in the absorption spectrum.21−24 Combination of nuclear vibrations and dipole−dipole (exciton) interactions between porphyrin rings in oligomers can lead to such oscillations of spectrum.22,23 Some researchers suggested a relationship between energy transfer and vibrational coupling.22 Spectra of Q and B bands depending on the symmetry of the porphyrin ring were analyzed, and a vibrational wave packets without an antiphase component were observed.21 Kim and others studied systems consisting of a single25 and a couple26 of porphyrin subunits with the exciton interaction. Complex
INTRODUCTION The relationship between the vibrational wave packet and the molecular exciton is drawing great attention nowadays. The root of this problem lies in energy transport in photosynthetic light harvesting complexes. Porphyrins are used in optoelectronics,1 sensors,2 and medicine.3 Problems of energy and charge transfer in such complexes are of great interest,4,5 as well as in various oligomers6,7 and large complexes.8 The importance of photosynthesis has driven researchers to mimic its fundamental features in simplified structures. For example, the discrete cyclic porphyrin arrays have been considered as artificial lightharvesting antenna as was reported by Aratani et al.9 Various large porphyrin complexes could give really interesting results if the mechanistic aspects behind them can be clearly revealed. Photochemical and photophysical properties of porphyrins are highly dependent on their electronic structures determined by the high symmetry of the porphyrin ring. In our work, we use the Gouterman four-level model of the porphyrin.10−12 Transitions from two highest occupied orbitals to two lowest free orbitals form two pairs of electronic excited states called Q (lowest excited state) and B (next lowest excited state). Based on this model and taking into account the interaction of the nuclear vibrations and electronic structure, the visible region of the porphyrin spectrum was largely studied.13 In a subsequent © 2016 American Chemical Society
Received: November 10, 2015 Revised: March 2, 2016 Published: March 3, 2016 1961
DOI: 10.1021/acs.jpca.5b11025 J. Phys. Chem. A 2016, 120, 1961−1970
Article
The Journal of Physical Chemistry A Scheme 1. Structure of Porphyrin Oligomers
mechanistic understanding of coherent oscillation occurrence. It is also of great interest to understand their role in the transport of energy in the process of photosynthesis. Potential application allows the creation of devices converting solar energy into electricity in a green way.
processes occurring under photoexcitation including antiphase oscillations were observed for the Fenna−Matthews−Olson complex consisting of a large number of bacteriochlorin subunits, which have conjugated structure with four pyrrole rings.24 Analysis of these processes is complicated due to the complex structure of natural photosynthetic systems. In order to find a way to analyze that phenomena it is good to study an exciton coupling of conjugated porphyrin oligomers27 and antiphase oscillations.22 In our work, we use new synthesized porphyrin oligomers connected with a triazole bridge. Rare examples of such oligoporphyrins have been synthesized before.28−30 Our oligoporphyrins have a simple design as opposed to earlier studied analogs. We dealt with five oligoporphyrins (1−5), which can be divided in two groups: rigid triazole-bridged meso-tetraaryl porphyrins31,32 (1, 2, and 3), and more flexible cyclic diamine bridged meso-aryl-β-octaalkyl porphyrins33 (4 and 5). Porphyrins 1, 4, and 5 were dimers, 2 was a linear trimer,31 and 3 was a “star” trimer.32 'Our main porphyrin, 1, was 1,4-bis((zinc 10,20-ditrimethylphenyl-15-(4-tert-butylphenyl)porphyrin-5-yl)-phenyl)-1H1,2,3-triazole.31 Its structure is shown in Scheme 1. For more detailed structure of all porphyrins, see Supporting Information (SI) 1 and Table 1. In the first part of our work, we demonstrate the vibrational wave packet and a relationship between this packet and electronic structure of the model molecule. As a model we use new simple porphyrin oligomers linked with a triazole bridge. In the second part, we introduce a theoretical model explaining the differential spectrum and other observed phenomena. Pump pulse excited Q state of porphyrin and oscillations of absorption intensity occurred in B band bleach. These oscillations allow us to assume the formation of a vibrational wave packet. Exciton coupling (dipole−dipole interaction) between Q states of the porphyrin dimer was very weak. Oscillator strength was significantly less than coupling between B states. Thus, we neglected it and considered only vibrational dynamics in a delay between pulses. Comparison of theoretical model and experimental data led to estimate some parameters of porphyrin, namely, parameters of exciton−vibronic coupling and measure of asymmetry of the porphyrin ring in the oligomer. Finally, the study of a small number of subunits systems, such as zinc porphyrin dimer and trimer, is important for deeper
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EXPERIMENTAL SECTION Synthesis. 1H NMR spectra (400 and 600 MHz) were recorded with Bruker AM-400 and Avance 600 (IPCE RAS) spectrometers. Chemical shifts are reported in δ (ppm), referred to 1H (of residual proton δ 7.28) signal of CDCl3. MALDI-TOF mass spectra were recorded on an Ultraflex mass spectrometer, Bruker Daltonics (positive-ion mode, voltage 20 mV); 1,8,9-trihydroxyanthracene matrix was used for dimer and trimers. All other chemicals used for the synthesis were reagent grade unless otherwise specified. Oligoporphyrins31,32,33 were synthesized using the “click” reaction method.34,35 Bis-porphyrin. Monomeric porphyrins zinc(II) 5,15-dimesityl-10-(4-tert-butylphenyl)-20-(4-ethynylphenyl) porphyrin (55 mg, 0.064 mmol) and zinc(II) 5,15-dimesityl-10-(4-tertbutylphenyl)-20-(4-azidophenyl)porphyrin (54 mg, 0.064 mmol) were dissolved in a solvent mixture of THF (5 mL) and MeCN (1 mL), and then diisopropylethylamine (0.005 mL) and CuI (1.5 mg) were added. The reaction mixture was stirred at 60 °C for 3 h. After removal of the solvent in vacuo, a colored solid was obtained. Product was isolated by column chromatography: unreacted starting materials were eluted with first fraction in CH2Cl2/hexane (1:1), and then the porphyrin trimer product was eluted as the second fraction with 1% MeOH in CH2Cl2. Evaporation of the second fraction yielded 65 mg (60%) of product. Porphyrin 1 (dimer): 1H NMR (400 MHz, CDCl3) δ = 1.64 (s, 18 H), 1.88 (s, 24 H), 2.67 (s, 12 H), 7.33 (s, 6 H), 7.78 (d, 3 J = 6.8 Hz, 2 H), 8.19 (d, 3J = 8.1 Hz, 4 H), 8.32 (d, 3J = 8.5 Hz, 2H), 8.38−8.44 (s, broad, 4 H), 8.53 (d, 3J = 8.5 Hz, 2 H), 8.74 (s, 1 H, triazole), 8.80−9.03 (m, 16 H, β-H); λmax (ε × 10−5, M−1 cm−1) = 426 (3.5), 558 (0.32), 598 (0.23); MS(MALDI-TOF) 1702.15 [C110H95N11Zn2]+, 1674.93 [M+ − N2]. Trimer of Zinc-Porphyrin. Monomeric porphyrins zinc(II) 5,15-dimesityl-10,20-bis(4-azidophenyl)porphyrin (49.7 mg, 0.06 mmol) and zinc(II) 5,15-dimesityl-10-(4-tert-butylphenyl)-20-(4-ethynylphenyl)porphyrin (101 mg, 0.12 mmol) were dissolved in a solvent mixture of THF (8 mL) and MeCN (1.6 mL), and then diisopropylethylamine (0.008 mL) and CuI (2.5 1962
DOI: 10.1021/acs.jpca.5b11025 J. Phys. Chem. A 2016, 120, 1961−1970
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The Journal of Physical Chemistry A mg) were added. The reaction mixture was stirred at 60 °C for 3 h. After removal of the solvent in vacuo, a colored solid was obtained. Product was isolated by column chromatography: unreacted starting materials were eluted with first fraction in CH2Cl2/hexane 1:1, then the porphyrin trimer product was eluted as the second fraction with 1% MeOH in CH2Cl2. Evaporation of the second fraction yielded 110.8 mg (73%) of product. Porphyrin 2 (trimer): 1H NMR (400 MHz, CDCl3) δ = 1.64 (s, 18H), 1.88 (s, 24H), 1.91 (s, 12H), 2.67 (s, 18H), 7.33 (s, broad, 12H), 7.78 (d, broad, 4H), 8.19 (d, broad, 4H), 8.32 (d, broad, 4H), 8.38−8.44 (broad, 8H), 8.53 (d, broad, 4H), 8.73 (s, triazole, 2H), 8.79−9.04 (m, 24H, β-H); λmax (ε × 10−5, M−1 cm−1) = 425 (5.8), 431 (5.9), 558 (0.49), 599 (0.29); MS(MALDI-TOF) 2473.42 [M+ − 2N2]. Transient Absorption Measurement. Transient absorption spectra were measured using a femtosecond pump− supercontinuum probe setup. The output of a “Tsunami” Ti:sapphire oscillator (800 nm, 80 MHz, 80 fs, Spectra-Physics, USA) was amplified by a “Spitfire” regenerative amplifier system (Spectra- Physics) up to 1 mJ/pulse at repetition rate of 1 kHz. The amplified pulses were split into two beams. One half of the energy was directed into a noncollinearly phasematched optical parametric amplifier. Its output centered at 600 nm (or 610 nm) was subsequently compressed by a pair of quartz prisms. The gauss pulse of 20 fs was used as a pump. The other half of the energy was focused into a quartz cell with optical length of 3 mm with H2O to generate supercontinuum probe pulses. The pump and probe pulses were time-delayed with respect to each other by means of a computer-controlled delay stage. They were then attenuated, recombined, and focused into the sample cell. The flow sample cell was 0.45 mm optical length. The cell windows were made from quartz of 0.1 mm thickness. The pump pulse energy was 100 nJ. The pump light spot had a diameter of 300 μm. The pump pulse repetition rate was 60 Hz. The electro-optical gate “Spitfire” regenerative amplifier system provided the conversion of 1 kHz to 60 Hz pulse repetition rate. The relative polarizations of pump and probe beams were adjusted to 54.7° (magic angle). After the sample, the supercontinuum was dispersed by a polychromator (Acton SP-300) and detected by a CCD camera (Roper Scientific SPEC-10). Absorption difference spectra, ΔA(t,λ), were recorded over the range 400−740 nm. The measured spectra were corrected for group delay dispersion of the supercontinuum using the procedure described previously.36 The experimental data were treated using our own software programmed using MatLab. The experiments were carried out at 21 °C. The circulation rate in the flow cell was fast enough to avoid multiple excitation of the same sample volume.
Figure 1. Absorption spectra of porphyrin oligomers in B-band area.
Figure 2. Absorption spectrum of 1 and its second derivative.
Table 1. Porphyrins and Their Excitonic Peaks in B-Band wavelength, nm
porphyrin
description
1
tetraphenylporphyrin Zn dimer, linked with the triazole bridge linear tetraphenylporphyrin Zn trimer, linked with the triazole bridge star-shaped tetraphenylporphyrin Zn trimer, linked with the triazole bridge octaalkylporphyrin Zn dimer, linked with diaza-18crown-6 bridge octaalkylporphyrin Zn−Sn heterodimer, linked with ethylenediamine bridge
2 3 4 5
423; 429 423; 431 423; 426.5; 428.5 412 411
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RESULTS AND DISCUSSIONS Absorption Spectra. Figure 1 shows the absorption spectra of the porphyrin oligomers in the B-band area. Using the second derivative, we can obtain the position of excitonic bands (Figure 2). We use the second derivative due to high resolving power. However, the Gaussian decomposition may be founded in SI 2. Excitonic peaks of other porphyrins are in Table 1. Transition Absorption Spectroscopy. Femtosecond dynamics was obtained by the single value decomposition (SVD) method from raw data to minimize background noise. Obtained dynamics have been analyzed by fast Fourier transform (Figure 3).
Figure 3. Femtosecond spectrum of 1. The inset contains the femtosecond dynamics of exciton bands.
Further kinetic curves were processed using the SVD method. Different frequencies depending on the porphyrin and the solvent were obtained and shown in Figure 4 and Table 2. A frequency equal to 390 cm−1 appeared in experiments with 1, 2, and 3, except for the pure solvent case. The oscillation frequency (390 cm−1) and the decay time (1.2 ps) suggested the vibrational nature of the phenomenon. A similar frequency was observed in the experiments carried out 1963
DOI: 10.1021/acs.jpca.5b11025 J. Phys. Chem. A 2016, 120, 1961−1970
Article
The Journal of Physical Chemistry A
magnitude. The energies of these states are EQ and EB (Table 3). Dipole moments of transitions lie in the molecular plane. Table 3. Values of Energies, Corresponding Wavelength (or Δ Wavelength in B-Band), and Period or Characteristic Time for Oligomer 1 parameter
energy, cm−1
λ or Δλ, nm
τ, fs
B-state energy, EB Q-state energy, EQ Dipole−Dipole Interaction, Vd Vibrational Frequency, Ω Relaxation Rate, γ
23 640 ∼16 500 −330 390 220
429 ∼600 6 7 4
85 150
Figure 4. Fourier spectrum of oscillation in 1 differential absoption spectrum (obtained by SVD method).
The absorption spectrum in the visible range of the porphyrin contains two bands: intensive B band at 425 nm and much less intensive Q-band at approximately 600 nm. The B-band contains two indistinguishable peaks corresponding to Bx and Bz transitions. The Q-band has well-defined vibrational structure, and one visible peak corresponds to the Qx and Qz 0−0 transitions, and the second peak to the Qx and Qz 0−1 transitions. There is vibrational mode, which is excited by electron transition (both Q and B) and is observed in differential absorption spectra. Its frequency is Ω = 390 cm−1. Mainly four nitrogen and four meso-carbon atoms participate in this vibration. The mode corresponds to one-dimensional representation, b1g; this means that the coordinate of oscillation changes sign when the porphyrin ring rotates by 90°. Calculations showed that both ground and excited states have similar vibration modes of the same symmetry and frequency difference
