Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 50−61
Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface-Mounted Metal Organic Framework Xiaoxin Li,† Chenghuan Gong,† Gagik G. Gurzadyan,*,† Maxim F. Gelin,‡ Jinxuan Liu,† and Licheng Sun*,†,§ †
Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, China ‡ 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 S Supporting Information *
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 zinc tetraphenylporphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various steady-state and time-resolved laser spectroscopic techniques, i.e., timecorrelated single photon counting, fluorescence up-conversion and transient absorption pump−probe with 20 fs resolution. Obtained results in these nanoporous 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 and 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, © 2017 American Chemical Society
and the ease of adjusting their chemical structures (hence their electrochemical and photochemical properties) 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 Received: August 31, 2017 Revised: December 10, 2017 Published: December 11, 2017 50
DOI: 10.1021/acs.jpcc.7b08696 J. Phys. Chem. C 2018, 122, 50−61
Article
The Journal of Physical Chemistry C
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
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: ϕS1 = 0.02,31−34 whereas S2 fluorescence is 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, 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 to 1 ps and long decay with >1 μs. Strong contribution from the solvent (ethanol) as a result of third-order nonlinear χ(3) process (Kerr effect and twophoton absorption) was detected, similar to observations of ref 70. The transient absorption data were globally fit by using Glotaran (Joris-Joost Snellenburg, Holland) software. As a
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).
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 Figure S7a,b, all three ESA spectra are very similar. 53
DOI: 10.1021/acs.jpcc.7b08696 J. Phys. Chem. C 2018, 122, 50−61
Article
The Journal of Physical Chemistry C
The 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 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, λmax = 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
