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Quantum coherence enhances electron transfer rates to two equivalent electron acceptors Brian T Phelan, Jinyuan Zhang, Guan-Jhih Huang, Yi-Lin Wu, Mehdi Zarea, Ryan M. Young, and Michael R. Wasielewski J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06166 • Publication Date (Web): 14 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019
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Quantum coherence enhances electron transfer rates to two equivalent electron acceptors Brian T. Phelan‡, Jinyuan Zhang‡, Guan-Jhih Huang, Yi-Lin Wu, Mehdi Zarea, Ryan M. Young*, and Michael R. Wasielewski* Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, IL 60208-3113, USA Supporting Information Placeholder
1
2
OO
O
O
400
Norm. Absorbance
O
O
500
600 700800
1.0
Ex. Pulse 1 2
0.5
0.0 4.0
3.5
3.0
2.5
2.0
Energy (eV)
C
1*
D
An kCS
kCR
1*
An kCS
RP 3*
h S0
The role of quantum coherence in energy and electron transfer processes has attracted much interest in the study of both natural and artificial light-harvesting systems.1-2 Such coherences between vibronic states of the precisely arranged chlorophylls in light-harvesting antenna complexes may contribute to the near-unity quantum yields of exciton funneling to photosynthetic reaction centers (RCs).3-5 Although the observation of coherences in protein environments is surprising given the presumed numerous random fluctuations among the chromophores (the system) and between the system and the protein environment (the bath), related work suggests that the protein environment may actually shield the chromophores from random fluctuations, enabling coherent dynamics to persist for hundreds of femtoseconds.6-8 These observations have also been extended to electronic energy transfer in conjugated polymers,9 suggesting that coherence preservation via structure-correlating fluctuations may be broadly applicable. Similar effects in electron transfer (ET) reactions within RCs and organic photovoltaic (OPV) materials have also received increased interest due to the availability of multiple acceptor sites and, in OPV materials, the role of delocalization in rapidly separating charges.10-12 ET reactions have been modeled extensively using both semi-classical13 and quantum mechanical14 treatments, with the latter accounting for high frequency vibrational modes of the system. The rates of these reactions are generally described by eq 1, where kET is the ET rate
Wavelength (nm)
B
An
RP
Energy
multiple electron acceptors, quantum coherence can enhance the electron transfer (ET) rate. Here we report photo-driven ET rates in a pair of donor-acceptor (D-A) compounds that link one anthracene (An) donor to one or two equivalent 1,4benzoquinone (BQ) acceptors. Sub-picosecond ET from the lowest excited singlet state of An to two BQs is about 2.4 times faster than ET to one BQ at room temperature, but about 5 times faster at cryogenic temperatures. This factor of 2 increase results from a transition from ET to one of two acceptors at room temperature to ET to a superposition state of the two acceptors with correlated system-bath fluctuations at low temperature.
A
Energy
ABSTRACT: When a molecular electron donor interacts with
Norm. Intensity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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h
RP RP kCR
3*
An
S0
RP = An+BQ
E
Figure 1. (A) Molecules investigated and their (B) steady-state electronic absorption spectra. Diagrams showing relevant excited state processes for (C) 1 and (D) 2. (E) DFT-Optimized geometries for 1 and 2 showing donor-acceptor distances RDA (8.39 and 7.98 Å for 1 and 2, respectively) and dihedral angles d (74 and 84 for 1 and 2, respectively) See SI for computational details.
constant, VDA is the electronic coupling matrix element between the electron donor and acceptor, and is the FranckCondon-weighted density of states: 𝑘𝐸𝑇 =
2𝜋 ℏ
|𝑉𝐷𝐴|2𝜌
(1)
Additionally, numerous theoretical studies have shown that system-bath interactions in ET can impact the site energies and donor-acceptor coupling 15 as well as destroy interference between multiple pathways.16-17 Moreover, these studies indicate that quantum coherence can be preserved in the limit of weak system-bath coupling and low temperature.18 Here, we report on charge separation (CS) in a pair of donor-acceptor (D-A) compounds that link an anthracene
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An
1*
BQ
800
An
An
10 0
-10
D
6
1 / 295 K
t (ps) 1*
-1.0
0.2
0.4
4
1*
BQ
An
An+
0
1 / 90 K
-2 3.0
2.5
Energy (eV)
2.0
1*
An
500 1*
BQ
Time Delay (ps)
600
700
800 +
An
An
0
2 / 295 K -10
0.8 2
1.6 1*
An
3.2 BQ
6.4 1*
0.5
C
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E
An
2
B 20
400
1.0
1
Normalized A
1*
700
A x 103
A x 103
20
Wavelength (nm)
600
A x 103
A
500
1.5
2.0
3.30 eV, 295 K
2.5 1 2 t(1)/2 t(1)/4
0.1
0.0 -0.2
13 An+
An
0
F
1
1 2 kCS(1) x 2
3.28 eV, 90 K
Normalized A
Wavelength (nm) 400
A x 103
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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kCS(1) x 4
0.1
2 / 90 K
-2 3.0
2.5
Energy (eV)
2.0
0.0 -0.2
1
2
3
4
Time Delay (ps)
Figure 2. TA spectra of (A) 1 and (B) 2 in 1,4-dioxane at 295 K and (D) 1 and (E) 2 in glassy Me-THF at 90 K obtained with excitation at 3.49 eV (200 nJ/pulse,
