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Enacting Two-Electron Transfer from a TT State of Intramolecular Singlet Fission Hyungjun Kim, Bradley Keller, Rosina Ho-Wu, Neranga Abeyasinghe, Ricardo J Vázquez, Theodore Goodson III, and Paul M. Zimmerman J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01884 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018
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Enacting Two-Electron Transfer from a TT State of Intramolecular Singlet Fission Hyungjun Kim, Bradley Keller, Rosina Ho-Wu, Neranga Abeyasinghe†, Ricardo J. Vázquez, Theodore Goodson III,* Paul M. Zimmerman* Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, United States
Supporting Information Placeholder
tightly coupled " system, QOT2 is unique among intramolecular SF (iSF) chromophores because its TT state is not spontaneously separated into two free triplets in solution.2,4 This special property provides an opportunity to trap the double-triplet and achieve multi-electron extraction. In principle, two-electron transfer (2ET) from an entangled TT state may be advantageous for efficient energy harvesting, as competing relaxation channels will be outcompeted. Such 2ET has been suggested to occur in intermolecular SF (xSF) materials.6,16,17 While free triplet harvesting from iSF has been observed,18 evidence demonstrating 2ET from a TT state of iSF chromophores is lacking.
ABSTRACT: A simulation-led strategy enacts two-elec-
tron transfer between an intramolecular singlet fission chromophore (tetracyanomethylene quinoidal bithiopehene with !,!’-solubilizing groups) and multi-electron acceptor (anthraquinone). The thermodynamic plausibility of multi-electron transfer from a double-triplet state and the absorption spectra of electron transfer (ET) products were predicted using quantum chemical simulations. These predictions are consistent with experimental observations of reduced lifetimes in time-resolved fluorescence spectroscopy, changes in transmission profile, and appearance of new absorption bands in transient absorption spectroscopy, all of which support multi-ET in the QOT2/AQ mixture. The analysis suggests 2ET is favored over 1ET by a 2.5:1 ratio.
QOT2
!"#
QOT2H model S
C4H9O
CN
NC
OC4H9
NC
CN
S
NC S CN NC
AQ
S
O
CN
Generating, transferring, and storing multiple electrons from single photons could prove revolutionary for solar energy technologies. One promising process, singlet fission (SF), produces two excited electrons from a single photon.1 The multi-electron excited state in SF can be conceived as an entangled double-triplet2–8 and thus denoted as TT. The correlation between the two triplets makes TT a unique state that is not identical in character to 2!T1.9–13 Separation of the TT state is a challenging task by itself, requiring special molecular organization and delicate tuning of the intra- or intermolecular coupling.4,14 Towards the goals of achieving efficient solar energy harvesting, we propose the alternative way of extracting two excited electrons directly from a TT state in a bimolecular, 2-electron process. Tetracyanomethylene quinoidal bithiophene with !,!’solubilizing groups (QOT2)15 was herein tested to demonstrate this concept (see Figure 1). Our groups have revealed a long-lived QOT2 excited state with characteristics that closely relate to the TT state (21Ag) of SF, generated from a bright singlet state in sub-ps.2,4 As a single,
C 4H 9O
OC4H9
O
!$# 35.0
Concerted 2ET
Sequential 1ETs
Concerted 2ET 30.0
Electronic energy [kcal/mol]
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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Sequential 1ETs
25.0
22.7
21.1
QOT2+ TD/AQ-
19.8
20.0 15.0 10.0 5.0
0.0 0.0 -5.0
-3.1
-3.1
QOT2 TT/AQ QOT22+ DD/AQ2-
-10.0
!%#
QOT2
AQ
Charge
Spin
Charge
Spin 0
Before ET
QOT2 TT/AQ
0
!!,""
0
After 1ET
QOT2+ TD/AQ-
+1
!!,"
-1
"
After 2ET
QOT22+ DD/AQ2-
+2
!,"
-2
0
Figure 1. (A) Chemical structures of QOT2, QOT2H model and AQ. (B) CDFT energetics diagram of concerted 2ET and sequential 1ETs. (C) Diabatic representation of three states
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