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Exciplex Stabilization in Asymmetric Acene Dimers Rachel Ann Krueger, and Guillaume Blanquart J

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Exciplex Stabilization in Asymmetric Acene Dimers Rachel Ann Krueger, and Guillaume Blanquart J Subscriber access provided by Caltech Library A: Spectroscopy, Molecular Structure, and Quantum Chemistry Exciplex Stabilization in Asymmetric Acene Dimers Rachel Ann Krueger, and Guillaume Blanquart J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 19 The Journal of Physical Chemistry 1 2 3 4 Exciplex Stabilization in Asymmetric Acene 5 6 Dimers 7 8 y ∗,z 9 Rachel A. Krueger and Guillaume Blanquart 10 11 Department of Chemistry, California Institute of Technology, Pasadena, California 91125, 12 United States, and Department of Mechanical Engineering, California Institute of 13 Technology, Pasadena, California 91125, United States. Phone: 1-626-395-4306. Email: 14 [email protected] 15 16 17 18 19 Abstract state is derived. Moderate exciplex stabiliza- 20 tion observed for the benzene-naphthalene and 21 Excimers play an important role in photochem- 22 naphthalene-anthracene exciplexes results from 23 ical processes ranging from singlet fission to a mixture of charge transfer and exciton delo- 24 DNA damage, and the characteristic red shift calization. 25 in fluorescence spectra associated with excimer 26 formation can provide information about ag- 27 gregate formation and the orientation of chro- 1. Introduction 28 mophores. When a mixture of chromophores is 29 Characterization of exciplexes represents an im- 30 present, exciplex formation may lead to spec- 31 tral characteristics distinct from those of ei- portant step in understanding the dynamic 32 ther monomer or the corresponding excimers. photophysical and photochemical processes of 33 To predict the effects of aggregation in a sys- multi-chromophore systems. Exciplex forma- 34 tion occurs when two or more molecules are 35 tem containing a mixture of small acenes, bind- involved in a photoabsorption event, often but 36 ing energies and minimum-energy geometries not necessarily beginning when the absorption 37 have been calculated for three mixed S1 exci- 38 plexes. Benchmark CASSCF/NEVPT2 mul- of one molecule perturbs the electronic state of 39 tireference binding energies of 18.2 kJ/mol, a neighbor. The resulting structure is called 40 an excimer|an excited dimer|in the case that 41 27.7 kJ/mol, and 49.3 kJ/mol are reported for 42 the benzene-naphthalene, benzene-anthracene, the molecules involved are identical, or as an 43 and naphthalene-anthracene exciplexes, respec- exciplex|an excited complex|in other cases. 44 tively. TDDFT calculations have been per- The wave function for an arbitrary exciplex con- 45 formed using a range of exchange-correlation taining molecules P and Q may be described as 46 a combination of Frenkel excitations, in which 47 functionals, showing that many functionals per- 48 form inconsistently, and the error in binding electrons are excited from donor orbitals into 49 energy often depends on the character of the acceptor orbitals located on the same molecule, ∗ ∗ 50 monomer excitation from which the exciplex P Q and PQ , and charge transfer states, in 51 which the donor and acceptor orbitals lie on 52 ∗To whom correspondence should be addressed different molecules, P ·−Q·+ and P ·+Q·−.1,2 Ex- 53 y Department of Chemistry, California Institute of ciplex formation often results in the stabiliza- 54 Technology, Pasadena, California 91125, United States 55 zDepartment of Mechanical Engineering, California tion of complexes at geometries that would be 56 Institute of Technology, Pasadena, California 91125, unfavorable or repulsive in the ground state.1,3 57 United States. Phone: 1-626-395-4306. Email: Recent interest in exciplex formation has 58 [email protected] been driven by efforts to determine singlet fis- 59 60 ACS Paragon Plus Environment 1 The Journal of Physical Chemistry Page 2 of 19 sion mechanisms, in which exciplexes repre- portant exciplexes represents a way forward, 1 2 sent intermediate or trap states in the conver- provided that computationally tractable meth- 3 sion of a singlet exciton into two lower-energy ods with acceptable accuracy are available. 4 triplet excitons.4{7 Long-lived charge-transfer- To date, most ab initio studies have focused 5 dominated exciplexes have been observed in on the smallest acene excimers. Binding en- 6 photoexcited DNA and RNA strands.8 ergies and singlet excitation energies for the 7 8 In laser-induced fluorescence (LIF) experi- benzene S1 excimer have been characterized us- 18,19 20 9 ments, excimer emission produces broad, struc- ing CASPT2, coupled cluster methods, 10 tureless peaks that are red-shifted with respect and equation-of-motion coupled cluster meth- 11 to the corresponding emissions from the parent ods.21 Binding energies have been evaluated 12 molecules due to excimer stabilization.9 In ex- at the global minimum potential energy con- 13 14 periments involving a single chromophore, ex- figuration for the excimer, corresponding to 15 cimer fluorescence is readily identified. When a an eclipsed configuration, with one monomer 16 wide range of chromophores are present, com- translated along the intermolecular coordinate 17 plexes containing two different chromophores with respect to the other. The results range 18 are likely to be more prevalent than dimers of from 33-48 kJ/mol19,20 when corrected for basis 19 10 20 a single chromophore, and it it not a pri- set superposition error (BSSE) by the counter- 22 21 ori apparent whether the emission from these poise (CP) method. Even the lowest binding 22 complexes will be distinguishable from the par- energy obtained is more than twice as large as 23 ent monomer emissions. This question is of the CP-corrected benchmark CCSD(T) binding 24 paramount importance for combustion systems, energies obtained for the ground state benzene 25 23 26 where LIF represents a promising technique for dimer in a parallel-displaced configuration. 11 27 characterizing soot precursors, which might Naphthalene and larger acenes are charac- 28 consist of PAH van der Waals dimers12 or even terized by two close-together, low-lying singlet 29 pairs of PAHs connected by aliphatic linkers13 π ! π∗ excited states, polarized along the 30 capable of forming intramolecular exciplexes. two axes of the molecules. The B state, la- 31 2u 32 Accurately assigning fluorescence peaks, beled La, is described almost completely by a 33 though, will require a broad database of ex- HOMO!LUMO transition, while the B3u Lb 34 ciplex binding energies, the major contribu- state is the result of a mixed HOMO-1!LUMO 35 tor to the characteristic redshift.9,10 Exciplex and HOMO!LUMO+1 transition. In the 36 binding energy is defined here as the potential valence-bond theory framework, the La state is 37 24 38 energy difference between the minimum energy ionic and the Lb state covalent. 39 exciplex configuration and the two separated Time-dependent density functional theory 40 monomers, one of them excited. (TDDFT) calculations performed using hybrid 41 For ground state complexes, a roughly lin- exchange-correlation functionals tend to under- 42 ear scaling relationship between PAH molecu- estimate the energy of the La state, and this er- 43 25,26 44 lar weight and noncovalent dimer binding en- ror increases with increasing acene size. Al- 45 ergy has emerged for several homodimers,14,15 though the ionic description of the state hints at 16 46 the benzene-naphthalene heterodimer, and charge separation, the La state cannot be clas- 47 the naphthalene-anthracene heterodimer.17 For sified as a \true" charge transfer state,27,28 indi- 48 PAH exciplexes, though, the effects of monomer cating that the error is not directly attributable 49 50 substitution remain unclear|no binding ener- to the well-known failure of hybrid TDDFT 51 gies have yet been reported for mixed exciplexes to capture charge transfer behavior. Use of 52 containing small, unsubstituted PAHs. double-hybrid functionals improves La energies 53 Because of the large number of exciplexes that substantially.25,29 54 55 may be formed from even a moderate number The energy of the Lb state, on the other hand, 56 of small PAHs, characterizing each one exper- tends to be overestimated by TDDFT calcula- 57 imentally would represent a monumental task. tions using hybrid, double-hybrid, and range- 58 Obtaining accurate theoretical estimates of im- separated functionals.
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