Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface

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Ultrafast Relaxation Dynamics in Zn TetraphenylPorphyrin Surface-Mounted Metal Organic Framework Xiaoxin Li, Chenghuan Gong, Gagik G Gurzadyan, Maxim Gelin, Jinxuan Liu, and Licheng Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08696 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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The Journal of Physical Chemistry

Ultrafast Relaxation Dynamics in Zn TetraphenylPorphyrin Surface-Mounted Metal Organic Framework Xiaoxin Li,a Chenghuan Gong, a Gagik G. Gurzadyan,*a Maxim Gelin,b Jinxuan Liua and Licheng Sun*ac a

Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, China

b

c

Chemistry Department, Technische Universitat München, 85747 Garching, Germany

Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden

ABSTRACT Ordered porphyrin based metal organic frameworks (MOFs) may serve as a model for mimicking the natural photosynthesis with highly ordered chlorophylls, i.e. porphyrin like chromophores. Study of light harvesting and energy transfer as the primary event of photosynthesis is of great importance leading to improvement of photovoltaics overall performance. Detailed characterization of ultrafast dynamics of Zn-tetraphenyl-porphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various

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steady-state and time-resolved laser spectroscopic techniques, i.e. time-correlated single photon counting, fluorescence up-conversion and transient absorption pump-probe with 20 fs resolution. Obtained results in these nano-porous materials were compared with corresponding results for ZnTPP in ethanol measured under the same conditions. Dramatic quenching of both upper excited singlet state S2 and first excited state S1 was observed. Subpicosecond and picosecond lifetimes were detected in transient fluorescence and absorption. Analytical formulas are derived for the linear absorption, steady-state fluorescence, and fluorescence up-conversion signals. Theoretical description excellently reproduces experimental time and frequency resolved signals. Strong quenching of the femtosecond transients in SURMOF is explained in terms of highly efficient Förster resonance energy transfer between the neighboring porphyrin moieties which is caused by a strong spectral overlap of absorption and steady-state fluorescence spectra and quantum coherent energy transfer and redistribution. 1. INTRODUCTION Metal-organic frameworks, due to their unique optical, magnetic and electronic properties, found a wide application in various optoelectronic,1 fuel cells,2-4 gas storage,5-9 ferroelectric,10-12 nonlinear optical,13,14 photovoltaic,4,15 photocatalytic,4,7,16 sensoric2,5,16-20 devices. Photon upconversion due to triplet-triplet annihilation in SURMOF heterojunctions was demonstrated in Refs. 21, 22. Perovskite quantum dots loaded MOF thin film was air exposure insensitive and was shown to be uniform.23 Porphyrins have become prospective photosensitizers because of the vital roles of porphyrin derivatives in photosynthesis, their strong absorption in the visible region, and the ease of adjusting their chemical structures (hence their electrochemical and photochemical properties)


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for light harvesting. Adsorbing porphyrins with modified structures onto films thus provides an opportunity to improve solar cell applications. Excited state properties of tetraphenyl porphyrins in various solvents were studied by use of nanosecond flash-photolysis and time-correlated single photon counting (TCSPC) fluorescence methods (see e.g. Ref. 24) and femtosecond pump-probe25-27 and fluorescence up-conversion techniques.25, 26, 28-32 As known, porphyrins are within few molecules that exhibit in addition to S1 fluorescence also emission from the second excited singlet state S2. The quantum yield of S1 fluorescence is low: S1 = 0.02, 31- 34 whereas of S2 fluorescence an order of magnitude smaller.31 The S1 fluorescence lifetime is polarity independent, in different polar and nonpolar solvents is 1.9-2.1 ns.24 The lifetime of S2 fluorescence in various solvents is 1.4-3.4 ps for ZnTPP and MgTPP.25, 29-31, 35, 36. It should be mentioned that in metal free porphyrins S2S1 electronic relaxation is much faster, < 100 fs.26, 37, 38 Intermolecular vibrational relaxation in the excited state S1 due to cooling via interaction with the solvent for various porphyrins was in the range of pico- and tens of picoseconds.25, 38-41 Intramolecular relaxation in S2 state was measured to be 60-180 fs.29, 30, 36, 42 Ultrafast laser spectroscopy was applied in order to study excited state properties of the covalently linked porphyrin arrays compared with monomers. Strong quenching of the excited states S2 and S1 was observed as a result of intramolecular S1-S1 and S2-S2 Förster energy transfer and charge transfer both in S2 and S1.41, 43-50 Both S1 and S2 fluorescence quenching in porphyrin J-aggregates was observed:51-56 S1= 40-200 ps, S2=130 fs-1 ps.


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Electron transfer from the S2 excited state of ZnTPP to a covalently linked with other agents (e.g. pyromellitimide, naphthalene diimide) was studied.57-62 Quenching of the S2 state of ZnTPP in chlorinated solvents due to electron transfer to CH2Cl2 was observed25,35 and discussed.63 Quantum yield of intersystem crossing of metal porphyrins is high. Nanosecond flashphotolysis study shows triplet-triplet absorption in the range of 450-550 nm; the triplet state lifetime T1 = 40 s - 1.5 ms.64 The quantum yield of triplet formation in ZnTPP T = 0.88 in toluene and DMSO.64 The only system studied by use of femtosecond transient absorption was Zr-based MOF; ligand-to-cluster charge transfer was found to proceed in femtosecond time range.65 Strong quenching of the fluorescence of ZnTPP based MOF was observed,66 however the authors have used only methods of steady-state spectroscopy. Study of energy migration in porphyrin based MOFs, i.e. in highly ordered porphyrin systems with an exactly known structures, is of great importance, in particular, due to their similarity with natural photosynthesis with ordered chlorophylls (porphyrin-like chromophores). Understanding of the ultrafast processes in MOF/porphyrin may be useful for photosynthetic systems for enhancement of the light harvesting and thus improvement of solar energy conversion properties. In this work, we present detailed experimental and theoretical ultrafast spectroscopic analysis of relaxation processes in ZnTPP/SURMOF and compare the obtained results with the porphyrin monomer in solution. To the best of our knowledge, this is the first ultrafast spectroscopic study on porphyrin-MOF systems. 2. EXPERIMENTAL SECTION


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2.1 Preparation of substrates and of ZnTPP SURMOF The quartz substrates were cleaned with water, acetone and ethanol sequentially in an ultrasonic bath for about 30 min each. After that the substrates were treated with plasma (Diener, FEMTO SR CE, 0.3 mbar, 70 W) under O2 for 3 min. The plasma-treated substrates were immediately used for fabrication of MOF thin films. Porphyrin-based Zn-SURMOFs were fabricated by Liu group in Dalian University of Technology by using a well-established high throughput spray system as shown in Figure 1(a).67 In brief, 1 mM zinc acetate in ethanol (spray time: 15 s, waiting time: 30 s) and 50 μM Zn(II) 5, 15-diphenyl-10, 20-di(4-carboxyphenyl)porphyrin (Zn porphyrin, Figure 1(b)67) in ethanol (spray time: 20 s, waiting time: 30 s) were alternatively deposited onto quartz substrate. After each spray step, pure ethanol was used for rinsing to remove the unreacted molecule from surface (rinsing time: 5 s). By repetition of this process, a sample with 10 spray cycles was obtained. Figure 1(c) shows the proposed structure of ZnTPP SURMOF. The samples were already characterized in Ref. 67, however we again made AFM image which is provided in S1 together with XRD data (p.S1-2 Figs. S1, S2). The surface roughness of the sample is about 20 nm and the domain size of the sample was estimated to be in the range of several hundred nanometers. All the samples were prepared at room temperature and stored in a vacuum desiccator for further use. Note that compared to “classical” ZnTPP there are additional COOH groups attached to two benzene rings; however further in the text we use term ZnTPP for our case as well.


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Figure 1. (a) Setup employed for the fabrication of SURMOF with the spray method. (b) Molecule structure of ZnTPP. (c) Proposed structure of ZnTPP SURMOF with parallel 1-D channels, layers perpendicular to substrates with a layer distance about 0.65 nm.67 2.2 Steady-state spectroscopy The steady state absorption and fluorescence spectra were obtained by use of UV-Visible spectrophotometer (Agilent, Cary 100) and spectrofluorometer (Horiba Jobin Yvon, Flurorolog3), respectively. In both cases the wavelength resolution was 1 nm. 2.3 Time-resolved fluorescence The fluorescence lifetimes were recorded with a time resolved fluorescence spectrometer (Newport) in combination with a mode-locked Ti-sapphire laser (Mai Tai DeepSee, SpectraPhysics). Briefly, the femtosecond laser system generated light pulses at 800 nm of duration 150 fs at a repetition rate 80 MHz and average power 2.9 W. The frequency of the laser pulse was doubled with a BBO crystal and served for excitation (pump). The residual fundamental pulse that served as a gate pulse was split from the pump beam with beam splitter. The intensity of the excitation beam (λexc = 400 nm) was kept below 20 mW in order to avoid photochemical


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degradation of the sample. Diameter of the 400 nm excitation beam was measured by using the “knife edge” technique: D = 80 m (Fig. S3). Thus the fluence was only 5J/cm2. ZnTPP in ethanol was measured in a 1 mm thickness flow cuvette. Polarizations of the excitation and gate beams were at magic angle (55) in order to eliminate contribution of the rotational diffusion. The emitted fluorescence was collected by lens and focused into a BBO crystal together with the gate beam (800 nm) to create the up-converted signal at the sum-frequency generation (SFG). The resulting SFG radiation was focused onto the entrance slit of a monochromator. The fluorescence decay curve is obtained by varying the optical path of the delay stage for the gate beam. As an instrument response function (IRF） a cross-correlation signal was obtained between gate pulse at 800 nm and Raman line of water at 467 nm. Overall time resolution of the setup was 100 fs. Time-resolved photoluminescence (TRPL) measurements were carried out at room temperature by the time-correlated single photon counting (TCSPC) technique (PicoQuant PicoHarp 300). By use of deconvolution/fit program (PicoQuant FluFit) the time resolution was reached down to 10 ps. The second harmonic of a Titanium–sapphire laser (Mai Thai DeepSee) at 400 nm (150 fs, 80 MHz) was used as the excitation source. 2.4 Transient absorption femtosecond pump-probe The transient absorption (TA) spectra were measured by optical femtosecond pump-probe spectroscopy. The output of a mode-locked Ti-sapphire amplified laser system (Spitfire Ace, Spectra-Physics) with wavelength 800 nm, pulse-width 35 fs, repetition rate 1 kHz, average power 4 W was split into two beams (10:1). The strong radiation was converted into UV-VIS-IR in the range of 240-2400 nm by use of Optical Parametric Amplifier (TOPAS, Light Conversion)


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and used as a pump beam. ZnTPP in SURMOF and ethanol were excited at 400 nm. The weaker beam after passing a variable delay line (up to 6 ns) was focused in a 3 mm thickness rotated CaF2 plate to produce a white light continuum (WLC), which was used as a probe beam. WLC range between 350 and 850 nm was used for measurements. Home-built pump-probe setup was used for obtaining transient absorption spectra and kinetics. WLC was split into two beams (probe and reference) and, after propagation through the cuvette containing the sample solution, directed into two diode arrays attached to spectrograph (Jobin Yvon CP140) with detection range 250-850 nm (Entwicklungsbuero Stresing, Germany). The relative polarizations between pump and probe beam were set to the magic angle (54.7°) to avoid rotational depolarization effects. The entire setup was controlled by a PC with the help of LabView software (National Instruments). All measurements were performed at room temperature under aerated conditions. The experimental data were fitted to a multiexponential decay function convoluted with the instrument response function B(t-t0) centered at t0: n   t '   A(t )    A0   Ai exp     B(t  t '  t 0 )dt ' i 1  ti   0 

Here A(t) is the difference absorption at time t, Ai is the amplitude of the component with lifetime I and A0 is the offset due to long-living species. The instrument response function was modeled by a Gaussian with a variable Full Width Half Maximum (FWHM) of typically 50-70 fs. 3. RESULTS 3.1 ZnTPP in ethanol


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Steady-state absorption and fluorescence spectra are displayed in Figure 2. As well known, absorption spectrum of the ZnTPP in ethanol consists of intense Soret band (max = 423 nm) and two Q bands (max = 557 nm and 597 nm), which correspond to S0→S2 and S0→S1 transitions, respectively.

Figure 2. Steady-state absorption and fluorescence spectra of ZnTPP in ethanol.

The fluorescence emission spectra in Figure 2 clearly show both S2→S0 and S1→S0 fluorescence bands. S1 fluorescence maxima are at 605 nm and 657 nm with small Stokes shift 221 cm-1, and S2 fluorescence is at 435 nm with Stokes shift 652 cm-1. Fluorescence up-converted spectrum of ZnTPP in ethanol in the range of 430-660 nm is displayed in Figure 3a. In agreement with previously reported data, 25, 29-36 a short lived emission was observed in 440-500 nm range, and longer one in 600-660 nm range. Kinetic traces at various wavelengths are presented in Figure 3b and Figure S4 (supporting information). The


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profile of the S2 fluorescence of ZnTPP in ethanol with excitation at 400 nm can be fit as a single-exponential decay with a lifetime of 2.3 ps. which is in good agreement with the previously reported data: 2.35 ps29 in ethanol, 1.45 ps in benzene.48 No rise of S2 emission was found in these experiments due to limited time resolution; note that the reported rise of S2 fluorescence29 was 60-90 fs, which was assigned to the vibrational relaxation in S2. S1 fluorescence decays with 2.1 ns (measured by TCSPC, supporting information Figure S5). The reported lifetimes of S1 fluorescence are 1.7±0.2 ns in benzene,48 2.41 ns in ethanol,68 2.0 ns in benzene and 2.1 ns in methanol.68 Decay of S2 fluorescence was accomplished with the rise of S1 fluorescence (Figure 3b), as was previously observed.25,

29, 30, 36

Table S1 compiles all

measured data. From the results we can get that S2 decay correlates to the rise of S1 fluorescence which implies that the intramolecular vibrational relaxation process in S1 is too fast to be resolved.

Figure 3. (a) Fluorescence spectra map of ZnTPP in ethanol at excitation wavelength 400 nm, (b)Fluorescence up-conversion kinetics at 460 nm (S2) and 650 nm (S1) emission wavelengths.


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Transient absorption spectra of ZnTPP in ethanol at various delay times and kinetic traces at different wavelengths are presented in Figure 4. Due to the high extinction coefficient of the Soret band, the TA measurements have been carried out by exciting at 400 nm. Promptly after excitation TA shows the well-known spectral feature of ground state bleaching with minimum at 423 nm and excited state absorption with maximum at 452 nm. The nature of the excited state absorption between 400 and 500 nm was discussed in detail by Rodriguez et al.69 Since the exciting radiation used in our experiments falls on the blue edge of the Soret band, it thus corresponds to the higher vibrational levels of the electronic S2 state (or according to Yu et al25 S3 state). In all cases, the transient absorption has a long-lived (> nanosecond) time component which corresponds to triplet state absorption.64 The TA signals at 423-433 nm were well fit with a femtosecond rise time component and decay with three components. The fit parameters are given in Table S3. As seen in Figure 4 and indicated in Table S3, from 423 to 433 nm the rise time components strongly depend on the probe wavelength, increasing from 157 to 631 fs. Moreover, the short decay time components also increase from 30 to 700 fs with increasing amplitude and from 2 to 7 ps with decreasing amplitude. The transient absorption signals at longer wavelengths 438-520 nm can be fit with fast rise time component 300 fs - 1ps and long decay with > 1 µs. Strong contribution from the solvent (ethanol) as a result third-order nonlinear χ(3) process (Kerr effect and two-photon absorption) was detected, similar to observations of Ref. 70.


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Figure 4. (a) Transient absorption spectra of ZnTPP in ethanol at different delay times, under excitation at λ = 400 nm. (b) Transient absorption kinetics at different wavelengths. Solid lines are multiexponential fits (see Table S3).


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The transient absorption data were globally fit by using Glotaran (Joris-Joost Snellenburg, Holland) software. As a model was used consecutive process S0S2S1S0. Results are presented in Figure S7a-d. Three excited state absorption (ESA) spectra corresponding to S1SN, S2SM and T1TN are separated, as well as the ground state bleaching (GSB). It should be mentioned that, as observable from the Figure S7a, b, all three ESA are very similar. Triplet state ESA for ZnTPP in toluene was previously reported by Harriman et al,71 Pekkarinen et al,72 Hurley73 and Tran-Thi et al:64 absorption maximum is at 470 nm with molar extinction coefficient = 7.3x104 M-1cm-1. The maximum of triplet-triplet absorption for ZnTTP in toluene and DMSO is also at 470 nm.64 The lifetime of T1 is 1.4 ms for ZnTPP in toluene and 1.5 ms for ZnTTP in DMSO.64 The quantum yield for intersystem crossing is very high: isc= 0.88. EAS which corresponds to 2.1 ns (S1—SN) is in good agreement with the reported data for ZnTPP in toluene: 2.0 ns.64 Our results are in agreement with recently reported femtosecond transient absorption data for ZnTPP in ethanol processed by Glotaran software.48 We should also mention, that Enescu et al27 have presented transient absorption spectra obtained under the same conditions corresponding to 150, 500 and 5000 fs, which are similar to our Glotaran treatment (Figure S7a). The spectra at 150 and 500 fs in their article can be considered as ESA from S2 state. As already mentioned, all those three ESA have similar shapes in the 440-550 nm range. It should be mentioned that below 440 nm ESA is overlapping with the strong GSB (425 nm) and also the overall spectrum is distorted because of the scatter of intense excitation pulse (400 nm). 3.2 ZnTPP SURMOF


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Steady-state absorption and fluorescence spectra are displayed in Figure 5.The absorption spectrum of the ZnTPP SURMOF consists of intense Soret band with maximum at λ = 433 nm and two Q bands at λ = 555 and 596 nm. Soret band absorption is red shifted by 10 nm compared with the absorption of ZnTPP in ethanol, whereas virtually no shift of Q-bands was noticed. Similar behavior was observed with MgTPP films deposited on nanostructured silver substrates.74 The Soret band in the steady-state UV−VIS absorption spectra of MgTPP film was red shifted and broader compared to MgTPP in solution. It was explained in terms of coupling between the aromatic π-electron systems of the molecules by condensation into a solid film which results in formation of excitonic states with lower energies compared to the Soret band of the free molecule (J-aggregation).

Figure 5. Steady state absorption and fluorescence specta of ZnTPP SURMOF.

Red shift of the Soret band was also observed in ZnTPP films on quartz.75 Strong broadening and weakening of the Soret band was considered to be a feature of solid porphyrin films. ZnTPP


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films spin-coated from 5 mM chloroform solution76 show only a single, broad, structureless absorption band centered at 440 nm, similar to the absorption spectrum of our SURMOFs (Figure 5). Soret band fluorescence is also red shifted compared to ZnTPP in solution, max = 466 nm, with large Stokes shift of 1582 cm-1 (compare with 652 cm-1 in solution). Steady-state

absorption

and

emission

spectra

of

a

family

of

metalated

octakis@decoxyethyl)porphyrins (MODEP) in ordered thin films exhibit substantial shifts from their solution phase maxima.77 Steady-state absorption and fluorescence maxima of ZnTPP in various solvents and films are compiled in Table S6 (Supporting Information). Time-resolved fluorescence spectroscopy Fluorescence lifetime of ZnTPP SURMOF was measured by use of TCSPC, see Table S4 (Supporting Information). At 580-700 nm more than 99% is due to a short component of < 20 ps (time resolution of the setup) and negligible contribution (< 1%) is due to 250 ps component. Figure 6a shows 3D time-resolved fluorescence of ZnTPP SURMOF measured by use of upconversion with higher time resolution of 100 fs. Decay kinetics at various wavelengths (Figure 6b) was well fit biexponentialy. In wavelength range 440-660 nm both the short and long time components are increasing: 1 from 50 to 700 fs, 2 from 1.5 to 7 ps (Table 1). Moreover, the contribution of the short component is decreasing with increasing the wavelength (Figure S9).


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Figure 6. (a) Fluorescence map of ZnTPP SURMOF at excitation wavelength 400 nm, (b) Decay kinetics of fluorescence at various emission wavelengths. Solid lines are multiexponential fits (see Table 1).


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The maxima of fluorescence spectra at different delay times are the same; however, at shorter times (< 200 fs) the fluorescence spectrum is broader in blue edge (Figure S10). These ultrashort times can be assigned to vibrational relaxation in S2 state (see discussion below).

Table 1. Fluorescence up-conversion fit results of ZnTPP SURMOF Wavelength/nm

1/ps

A1

2/ps

A2

440

0.051

0.98

1.5

0.02

450

0.11

0.94

2.4

0.04

460

0.21

0.83

2.1

0.17

470

0.28

0.84

2.6

0.16

480

0.35

0.84

2.9

0.16

490

0.32

0.83

3.5

0.17

580

0.54

0.61

5.2

0.39

600

0.67

0.77

6.8

0.27

650

0.65

0.71

5.8

0.29

660

0.78

0.64

5.6

0.36

Femtosecond transient absorption spectroscopy Figure 7a shows TA spectra of ZnTPP SURMOF after 400 nm excitation. Similar to above TA spectra for solution (Figure 4) there is bleach at 400-450 nm due to Soret band and ESA at wavelengths above 450 nm. The dump around 552 nm is due to the overlap of ESA with bleach due to Q-band. Clear isosbestic point at 450 nm is indicative of a single photophysical process. The TA kinetics was fit with three time components (Table S5): 1  1 ps, 2  25 ps and the longtime of 3 > 100 ns. Risetime of 120 fs was detected at ESA wavelengths 465-620 nm.


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Longtime 3 is due to the triplet state formation, picosecond lifetimes match well with the above fluorescence up-conversion data. The ultrafast risetime is indicative of S2S1 internal conversion with the following ESA from S1. The nature of 2 is discussed below.

Figure 7. (a) Transient absorption spectra of ZnTPP SURMOF at various delay times. (b) Transient absorption kinetics at different probe wavelengths.


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4. DISCUSSION 4.1 Comparison with PMMA films, dimers, trimers, J-aggregates, covalently linked complexes 4.1.1 ZnTPP in ethanol: As already mentioned, porphyrins exhibit fluorescence from both S1 and S2 excited singlet states. The quantum yield of S1 fluorescence of ZnTPP in ethanol is S1 = 0.02-0.022,31-34 whereas of S2 fluorescence even smaller: S2 = 1.84x10-3.31 The quantum yield of S2 fluorescence was found to be concentration and excitation wavelength dependent: at < 400 nm the quantum yield decreases.31,33 Thus additional relaxation channel from upper excited singlet states bypassing S2 or S1 was proposed.33 Our fluorescence and transient absorption results with ZnTPP-COOH are in full agreement with previously published results on ZnTPP in ethanol. The S1 fluorescence lifetime was found to be polarity independent, in different polar and nonpolar solvents is 1.9-2.1 ns.24 The S2 fluorescence lifetime was studied by use of up-conversion method.25,29-31,35 The lifetime of S2 fluorescence in various solvents is 1.4-3.4 ps for ZnTPP and MgTPP,30 1.45 ps in benzene,25 1.9 ps in dichloromethane,25 2.35 ps in ethanol.29,36 Hot fluorescence from upper vibrational levels of S1 as a result of intramolecular vibronic redistribution was detected.36 It should be mentioned that in metal free porphyrins S2S1 electronic relaxation is much faster, > 100 ns is due to the triplet state:64 T1 = 40 s in toluene and 1.5 ms in DMSO. The nature of 2 = 25 ps, is as follows. According to previous publications, intermolecular vibrational relaxation in the excited state S1 due to cooling via interaction with the solvent was in the range of pico- and tens of picoseconds: 1.5 ps,38 10 ps,40 5-12 ps,25 18 ps,41 20 ps39 for various porphyrins. Relaxation of the vibrationally excited ground state S0 in porphyrin dimers


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was reported  10 ps.82 Therefore, our 2 we assign to vibrational relaxation in excited singlet state S1 and/or ground state S0. Note that vibrational coherence was observed by Yoon et al83 with chirped femtosecond pulses in ZnIITPP and ZnIIOEP; they observed that the S2 state dynamics are much different for those two molecules: 1.2 ps and < 20 fs respectively. This was explained by difference in curvature and displacement of the excited potential energy surfaces. Femtosecond coherent anti-Stokes Raman scattering (CARS) was applied for the investigation of the electronic ground-state vibrational dynamics of porphyrin molecules in solution: dephasing time 2 in MgTPP was 1.72.1 ps,84 i.e. shorter than our observed 2. Very little is known on porphyrin-SURMOF systems. Zr-based MOF was studied by use of femtosecond transient absorption;65 ligand-to-cluster charge transfer was found to proceed in femtosecond time range. It was demonstrated that the fluorescence resulting from one-photon (UV-light) and two-photon excitation (IR femtosecond laser) of MOF is different, enabling spatial modulation of the fluorescence property of the MOF.13 Strong quenching of the steadystate fluorescence of ZnTPP based MOF was explained to be due to efficient energy transfer.66 The authors concluded that the exciton migration can proceed over a distance of up to 45 porphyrin struts. However, they assumed that the exciton lifetime is order of nanoseconds.66 4.2 Theoretical treatment of linear absorption and time-resolved fluorescence The present description is based on the so-called doorway-window approximation,85 which assumes that the processes of the excitation of fluorescence and subsequent emission are temporally well separated. We thus consider, that the up-conversion experiment can be described as a three-step process. (i) Excitation of fluorescence. (ii) The dynamics of MOF in the excited


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electronic states, which involves population decays of S2 and Q states as well as the S2-Q energy transfer. (iii) Detection of fluorescence. 4.2.1 Parameters We assume that only two states, the state S2 ( S) from the Soret band and the sate Q1 ( Q) from the Q-band, predominantly contribute to the linear absorption spectrum and to the fluorescence signal. The energies of these states and other related model parameters are collected in Table 2. The values of these model parameters are varied to get an agreement with the experimental signals.

Table 2. Model parameters used for the simulation of absorption spectrum and fluorescence upconversion signal of ZnTPP SURMOF.

−1 Γ𝑒𝑥 = 𝜏𝑒𝑥 = 150 fs

Duration of the excitation pulse;

𝜔𝑒𝑥 = 3.1 eV

Carrier frequency of the excitation pulse;

−1 Γ𝑢𝑝 = 𝜏𝑢𝑝 = 150 fs

Duration of the up-conversion pulse;

𝜔𝑢𝑝

𝛾𝑢𝑝 = 0.4 eV

Carrier frequency of the up-conversion pulse;

An extra broadening during the detection of the up-converted fluorescence. In theory, it describes the quality of spectrometer,


24

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so that 𝛾𝑒𝑥 = 0 corresponds to ideal spectrometer;

𝐸𝑆1 = 2.86 eV

Energy of the S-state, which corresponds to the main peak in the absorption spectrum.

−1 Γ𝑆1 = 𝜏𝑆1 = 3.3 ps

Lifetime of the S-state

𝐸𝑄1 = 2.66 eV

Energy of the Q-state, which corresponds to the smaller peak in the absorption spectrum.

−1 Γ𝑄1 = 𝜏𝑄1 = ∞

Lifetime of the Q-state

𝜇𝑆1 = 2

Dipole moment of the S-transition

𝜇𝑄1 = 0.7

Dipole moment of the Q-transition

Γ = 0.15 eV

Optical dephasing rate

𝛾𝑆1 = 0.178 eV

S-state to Q-state energy exchange rate

𝛾𝑄1 = 0.022 eV

Q-state to S-state energy exchange rate


25


Page 26 of 43

The rates 𝛾𝑆1 and 𝛾𝑄1 obey the master equation −𝛾𝑆1 𝑑 𝜌𝑆1 (𝑡) ( )=( 𝛾 𝑆1 𝑑𝑡 𝜌𝑄1 (𝑡)

𝛾𝑄1 𝜌𝑆1 (𝑡) ) ( −𝛾𝑄1 𝜌𝑄1 (𝑡))

which controls the populations 𝜌𝑆1 (𝑡) and 𝜌𝑄1 (𝑡) of the states S and Q after the excitation. For simplicity, we assume that the signals can be described by the Lorentzian lineshape function

𝐴(𝜉, 𝜔) =

𝜉 𝜉2 + 𝜔2

Other lineshapes can be considered very similar. 4.2.2 Working formulas The absorption spectrum is given by the formula: 2 2 𝐼(𝜔) = 𝜇𝑆1 𝐴(Γ + Γ𝑆1 , 𝜔 − 𝐸𝑆1 ) + 𝜇𝑄1 𝐴(Γ + Γ𝑄1 , 𝜔 − 𝐸𝑄1 )

The formula reveals that the widths of the absorption peaks are determined by the electronic −1 −1 dephasing Γ as well as by the lifetimes Γ𝑆1 and Γ𝑄1 of the states S and Q. For the parameters of

Table 2, the simulated absorption spectrum of Figure 8a matches well the experimental spectrum presented in Figure 5. The description of the fluorescence up-conversion follows Ref. [86]. The fluorescence upconversion signal is calculated by the formula: (1)

𝐼(𝜔𝑢𝑝 , 𝑡) =


26

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(𝑋𝑆1 𝑒

−Δ1 𝑡

+(𝑋𝑄1 𝑒

−Δ1 𝑡

+ 𝑌𝑆1 𝑒

−Δ2 𝑡 )

+ 𝑌𝑄1 𝑒

−Δ2 𝑡

2 𝜇𝑆1 𝐴(Γ + Γ𝑆1 + 𝛾𝑢𝑝 + Γ𝑢𝑝 , 𝜔𝑢𝑝 − 𝐸𝑆1 ) Γ𝑆1 + Γ𝑢𝑝

2 𝜇𝑄1 ) 𝐴(Γ + Γ𝑄1 + 𝛾𝑢𝑝 + Γ𝑢𝑝 , 𝜔𝑢𝑝 − 𝐸𝑄1 ) Γ𝑄1 + Γ𝑢𝑝

where the decay rates are defined as

Δ1 =

−(𝛾𝑆1 + Γ𝑆1 + 𝛾𝑄1 + Γ𝑄1 ) + Δ 2

Δ2 =

−(𝛾𝑆1 + Γ𝑆1 + 𝛾𝑄1 + Γ𝑄1 ) − Δ 2

2

Δ = √(𝛾𝑆1 + Γ𝑆1 + 𝛾𝑄1 + Γ𝑄1 ) + 4𝛾𝑆1 𝛾𝑄1

Δ12 = Δ1 − Δ2 The remaining parameters read

𝑋𝑆1 =

𝑅𝑄1 𝛾𝑄1 + (Δ1 + 𝛾𝑄1 + Γ𝑄1 )𝑅𝑆1 Δ12

𝑌𝑆1 = −

𝑅𝑄1 𝛾𝑄1 + (Δ2 + 𝛾𝑄1 + Γ𝑄1 )𝑅𝑆1 Δ12


27


𝑋𝑄1 =

Page 28 of 43

𝑅𝑆1 𝛾𝑆1 + (Δ1 + 𝛾𝑆1 + Γ𝑆1 )𝑅𝑄1 Δ12

𝑌𝑄1 = −

𝑅𝑆1 𝛾𝑆1 + (Δ2 + 𝛾𝑆1 + Γ𝑆1 )𝑅𝑄1 Δ12

and

𝑅𝑆1

𝑅𝑄1

2 𝜇𝑆1 = 𝐴(Γ + Γ𝑆1 + Γ𝑒𝑥 , 𝜔ex − 𝐸𝑆1 ) Γ𝑆1 + Γ𝑢𝑝

2 𝜇𝑄1 = 𝐴(Γ + Γ𝑄1 + Γ𝑒𝑥 , 𝜔ex − 𝐸𝑄1 ) Γ𝑄1 + Γ𝑢𝑝

are the initial populations of the S and Q states after the excitation by the pump pulse. The calculation with the parameters of Table 2 yields a preliminary simulated up-conversion signal of Figure 8b. An excellent agreement between the simulated signal and its experimental counterpart of Figure 6a can be observed.

Figure 8. (a) Simulated absorption spectrum and (b) fluorescence time-resolved map of ZnTPP


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SURMOF.

Equation (1) predicts bi-exponential kinetics of the fluorescence up-conversion, in the agreement with the experimental results. In the present approach, the rates Γ𝑆1 and Γ𝑄1 are responsible for the energy transfer from the S2 and Q bands of the excited ZnTPP to all other neighboring ZnTPPs in SURMOF. The rates γ𝑆1 and γ𝑄1 describe the interband (S-Q or S2-S1) energy exchange of the excited ZnTPP in SURMOF. Γ𝑆1 and γ𝑆1 specify the S2 fluorescence lifetime, while Γ𝑄1 and γ𝑄1 determine the S1 fluorescence lifetime. The values of γ𝑆1 and γ𝑄1 obtained in the simulation are indicative of extremely fast S2-S1 internal conversion, γ−1 𝑆1 = 4 fs and γ−1 𝑄1 = 30 fs. Even though the present simulation yields too short internal conversion times and does not explicitly account for vibrational relaxation (which may be important at the early stage of internal conversion42), the conclusion about ultrafast S2-S1 internal conversion in SURMOF is corroborated by the presence of ultrafast (femtosecond) kinetics in the timeresolved fluorescence signal reported in the present work. 4.2.3 Discussion on fluorescence quenching There exist several reasons for quenching of the S2 and S1 states in SURMOF. We argue that the main one is the electronic energy transfer and redistribution owing to a relatively strong dipole-dipole coupling of ZnTTPs in SURMOF. Our arguments go like this. A sensitive measure of the efficiency of the energy transfer is the overlap integral of the absorption and emission spectra (see, e.g., Ref. 66). The evaluation of this integral for the experimental steady-state absorption and fluorescence spectra of individual ZnTTPs in ethanol (Figure 2) yields the value


29


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of 0.1 eV-1, which estimates the efficiency of excitation hops from one neighboring ZnTTP to another. These hops can hardly be a reason of the fluorescence quenching. They are responsible for energy migration, which can be quite efficient in Zn-porphyrin-based MOFs.66 On the other hand, the overlap integral of the experimental steady-state absorption and fluorescence spectra of SURMOF (Figure 7) yields 1.1 eV-1, which estimates the efficiency of excitation spreading among a group of neighboring ZnTTPs in SURMOF. The fact that the overlap integral in SURMOF is an order of magnitude higher than that for ZnTTPs in ethanol indicates the importance of collective (excitonic) effects in SURMOF. It is these effects (e.g., superradiance) which shorten the lifetimes of assemblies of coupled chromophors. Related phenomena were also detected in porphyrin J-aggregates and other multi-porphyrin arrays.

43,51,53-56

Since the

structure of SURMOF is much more regular than that of J-aggregates, we presume that not only incoherent energy transfer (Förster mechanism) is responsible for the shortening of the fluorescence lifetimes of the S2 and S1 states, but also quantum coherent energy transfer. The latter mechanism is related to the electronic coherence (off-diagonal elements) of the density matrices of individual ZnTTPs comprising the SURMOF. Since the presence of coherent effects in energy transfer has been demonstrated in a variety of natural and artificial light-harvesting systems,87 our assumption about significance of these effects in SURMOF sounds realistic. Note also that higher broadening of the steady-state absorption and emission spectra in SURMOF in comparison with their counterparts for ZnTTPs in ethanol is not a manifestation of the lifetime shortening. It is caused by a spread of S2-S0 and S1-S0 excitation energies throughout the SURMOF sample (inhomogeneous broadening). This observation also corroborates the importance of electronic coherence effects.


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Let us discuss now other possible scenarios. Quenching of the S2 and S1 states due to electron transfer (Dexter mechanism) requires the presence of electron acceptors. These species may be produced in tiny concentrations upon photoexcitation of SURMOF by an intense laser pulse. Plausible candidates to this role are porphyrin radical cations (holes) which were detected e.g. in Zn SURMOFs.67 Since these SURMOFs can be regarded as “indirect semiconductors” with a small (~ 5 meV) band-gap,67 direct recombination of the electron-hole pairs may be suppressed, maintaining thereby a certain concentration of the electron acceptors. This scenario may be partially responsible for the quenching. However, due to a low concentration of the acceptors, it cannot be considered as dominant. Vibrational relaxation is also partially responsible for the electronic energy deactivation and losses. Several other mechanisms, on the other hand, are rather unlikely. The singlet-to-triplet conversion, which is a one of the common reasons of fluorescence quenching in Zn-porphyrins, can hardly be responsible for the picosecond lifetimes of the S2 and S1 states. The exciton-exciton annihilation, which is also one of the common reasons of the fluorescence quenching in molecular aggregates, can also be excluded since the power of the excitation pulse in our time-resolved fluorescence experiments has been kept low. 5. CONCLUSIONS We have examined the relaxation processes of ZnTPP in ethanol and in SURMOF by means of pump-probe transient absorption and femtosecond time-resolved fluorescence up-conversion and time-correlated single-photon counting spectroscopy. Our results reveal ultrafast S2-S1 internal conversion followed by dramatic quenching of both S2 and S1 states of ZnTPP in SURMOF due to the efficient energy transfer between adjacent ZnTPP moieties, where both S2 and S1 excited states are involved. The strong quenching is a manifestation of the significant spectral overlap of absorption and steady-state fluorescence spectra in SURMOF, which is a


31


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signature of both incoherent energy transfer (Förster mechanism) and quantum coherent energy transfer and redistribution. Supporting Information Steady-state absorption and fluorescence maxima of various porphyrins, TA spectra of ZnTPP in solution and SURMOF, time-resolved fluorescence data, fitting results. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements GGG is pleased to acknowledge DUT start-up funding. Corresponding Authors *Gagik G. Gurzadyan: [email protected] *Licheng Sun: [email protected] REFERENCES 1. Stavila, V.; Talin, A. A.; Allendorf, M. D. MOF-based Electronic and Optoelectronic Devices. Chem. Soc. Rev. 2014, 43, 5994-6010. 2. Kitagawa, H. Metal-Organic Frameworks Transported into Fuel Cells. Nat. Chem. 2009, 1, 689-690. 3. Shimizu, G. K. H.; Taylor, J. M.; Kim, S. Proton Conduction with Metal-Organic Frameworks. Science 2013, 341, 354-355. 4. Li, S.-L.; Xu, Q. Metal-Organic Frameworks as Platforms for Clean Energy. Energy Environ. Sci. 2013, 6, 1656-1683. 5. Qiu, S.; Zhu, G. Molecular Engineering for Synthesizing Novel Structures of Metal-Organic


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Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface

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