Persistent Charge-Separated States in Self-Assembled Twisted

Jan 18, 2017 - ... Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Sreekaryam, Thiruvananthapuram, Kerala 695016, ...
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Persistent Charge Separated States in Self-Assembled Twisted Non-Symmetric Donor-Acceptor Triads Ajith R. Mallia, Abbey M. Philip, Vinayak Bhat, and Mahesh Hariharan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11153 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry C 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.

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Persistent Assembled

Charge

Separated

Twisted

States

Non-Symmetric

in

SelfDonor-

Acceptor Triads Ajith R. Mallia, Abbey M. Philip, Vinayak Bhat and Mahesh Hariharan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Sreekaryam, Thiruvananthapuram, Kerala, INDIA

ABSTRACT. Organic materials which can self-assemble into higher order superstructures have extensive applications in artificial light harvesting systems, solution processable bulkheterojunction solar cells and photo-functional devices owing to their unique charge transport properties. In this report, we demonstrate a self-assembled non-symmetric donor-acceptor triad, TAN composed of triphenylamine (T), anthracene (A) and naphthalimide (N) units, for achieving long-lived charge separation via aggregation. Steric hindrance imposed by diisopropyl groups of naphthalimide and the propeller shaped triphenylamine unit obstruct planarization in TAN. Quantum theory of atoms in molecules analyses demonstrated the presence of synergistic C-H•••, C-H•••H-C, - and C−O•••O−C interactions between the adjacent TAN units in the crystalline state, leading to significant electronic coupling (Hab). The nonplanar geometry of TAN triad in the monoclinic space group dictates a unique antiparallel arrangement between the adjacent TAN units along b-axis. Solvent polarity dependent Lippert-Mataga and spin density

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distribution analyses of TAN established the presence of charge transfer interactions in the molecule. Solvent polarity dependent nanosecond and femtosecond transient absorption measurements of TAN revealed ca. 108-fold enhancement in the lifetime of the charge separated 𝑎 𝑎 state in the aggregated TAN in CHCl3 (𝜏𝑐𝑟  11 s) and THF (𝜏𝑐𝑟  11.20 s) when compared 𝑚 to that in monomeric TAN (𝜏𝑐𝑟 < 110 fs) in CH3CN. Extension in lifetime of the charge

separated state in self-assembled TAN could be due to synergetic effect induced by delocalization of charge carriers across alternate slipped antiparallel (Hab=13 meV) and antiparallel dimers (Hab=10 meV) of TAN and the presence of triplet charge separated states.

INTRODUCTION Self-assembled π-conjugated donor-acceptor molecular architectures have gained widespread consideration owing to their distinct charge transport properties.1 Stimulated by peculiar chromophoric arrangements in natural photosynthetic systems (PS),2 enormous designs have been demonstrated to generate electron donor-acceptor (D-A) architectures3-8 for efficient energy conversion.9 Optimization9-10 of stoichiometry and distance between the complementary redox gradients11 is vital for persistent charge separated (CS) states in monomeric D-A systems.12-13 In D-A aggregates, precise organization14 of modular components is quintessential in regulating the rate of charge recombination. To install oriented “transport highways” for efficient exciton migration, in D-A dyads,15-18 emergence-upon-assembly approach16, 19-27 have been employed by several groups.28 Supramolecular29 J-aggregates of bisthiophene(BT)-perylenediimide(PDI)bisthiophene,16,

30-31

melamine(M)-naphthalenediimide(NDI)-melamine,32

arrangement of diketopyrrolopyrrole(DPP)-PDI-diketopyrrolopyrrole,19, oligo(p-phenylenevinylene)[OPV]−PDI−oligo(p-phenylenevinylene),34

33

double

helical

hydrogen-bonded columnar

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anthracene(An)–PDI–anthracene,35 slip-stacked PDI-DPP-DPP36 triads and oriented push-pull triple-channel surface architectures37 have been demonstrated for extending the lifetimes of photogenerated excitons in the aggregated state. To date, aggregated non-symmetric D-D-A triads for extending the lifetime of the CS states remain elusive. In our earlier contributions, we have successfully demonstrated attenuation in the rate of charge recombination in i) vesicular scaffold38 and ii) nonparallel stacks39 of D-A bichromophores.40 Realization of long-lived CS states41-42 in a self-assembled naphthalenenaphthalimide (NIN) dyad, compared to the monomeric NIN, could be due to the delocalization of excitons through individual D and A stacks oriented in different spatial planes. In contrast, a homologous dyad containing anthracene and naphthalimide (AN) showed ultrafast charge recombination (< 110 fs) in monomeric/aggregated state. Contrary to NIN, absence of donor-ondonor and/or acceptor-on-acceptor arrangement in the AN dyad could be responsible for shortlived CS states in the aggregate state. Lack of long-lived CS states in aggregated AN dyad prompted us to design and synthesize a non-symmetrical triphenylamine-anthracenenaphthalimide (TAN) D-D-A triad. We herein report an example of aggregated non-symmetric D-D-A triad TAN for prolonging the survival time of CS states upon photoexcitation. EXPERIMENTAL DETAILS Synthesis. The syntheses and characterization of the anthracene-naphthalimide (AN) and triphenylamine-anthracene-naphthalimide (TAN) triad are presented in the Supporting Information (SI). Yellow single crystals of ANBr and TAN were grown by slow evaporation from a solution in dichloromethane:hexane (1:3) mixture. Crystal structure data of the derivative ANBr (1479113) and TAN (1479058) is tabulated in Table S1, SI.

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Steady-state spectroscopy. Photophysical measurements of the derivatives were carried out in a cuvette of 3 mm path length unless otherwise mentioned. Absorption and emission spectra were recorded on Shimadzu UV-3600 UV-VIS-NIR and Horiba Jobin Yvon Fluorolog spectrometers respectively. To avoid the distortion arising from the inner filter effect the concentration dependent steady-state measurements were carried out in a cuvette of 2 mm pathlength. Solution state40 relative quantum yield measurements were performed using [Ru(bpy)3]Cl2 hydrate in water as the reference (Reported quantum yield f =0.045) exciting at 450 nm. Lifetime measurements were carried out in an IBH picosecond time correlated single photon counting (TCSPC) system.12,

38

Pulse width of the excitation (exc=375 nm) source is determined to be

3LE >> 3( D•+ A•− ), there is a large driving force for the charge separation to occur from the (a)

(b)

[

T

A

N

1

*

[

]

1 

[

T

A

 N

[

T

A

N

A

]* 1 

[

]

Geminate charge recombination

hn

T

3

N

*

]

T

 N

A

hn

[

T

A

3

[

T

A

N

]

Geminate charge recombination

Energy

1

Energy

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T

A

3

N

 T

*

]

 A

N

]

N

Figure 7: Qualitative Jablonski diagram depicting the photoexcited processes upon photoexcitation of either A or N in TAN in (a) non-polar solvent and (b) polar solvent (in nonpolar solvents the triplet CS states do not get populate whereas in polar solvents the triplet CS states get populated).

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LE state to 3( D•+ A•− ) across short spacers, following rapid electron transfer.67 Photoexcitation (exc = 355 and 400 nm) of triad TAN results in generation of local

excited states of either A or N; (T-A-N)  (T-1A-N)* or (T-A-1N)*; the state (T-1A-N)* through rapid energy transfer populates (T-A-1N)*. Naphthalimides are known to undergo efficient ISC with short singlet state lifetimes (f) and low fluorescence quantum yields,80-83 hence (T-A-3N)* state gets populated via ISC from (T-A-1N)* state (Figure 7). The short centroid-to-centroid distance between A and N units (5.40 Å, Figure S18, SI) in TAN allows sufficient exchange interaction and spin correlations to give 1( T •+ AN•− ) and 3( T •+ AN•− ) wherein the latter state is primarily populated via a T-A-3N state.84-85 Geminate charge recombination67 of 1( T •+ AN•− ) could also result in the population of (T-A-3N)*. The prerequisite for the formation of 3

( T •+ AN•− ) is that the ISC pathway (T-A-1N  T-A-3N) should be considerably faster relative

to the charge separation leading to the formation of singlet charge separated states T-A-1N  1

( T •+ AN•− ).67 The driving force for charge separation in the singlet manifold (∆𝐺𝑠𝑜 ) in nonpolar

solvents is smaller which promote faster singlet charge separation T-A-1N  1(T+-A-N−) relative to ISC (T-A-1N  T-A-3N).86 Polar solvents such as CHCl3 and THF display larger ∆𝐺𝑠𝑜 which slows down the T-A-1N  1( T •+ AN•− ) process, allowing ISC (T-A-1N  T-A-3N), which further transforms to form 3( T •+ AN•− ) through T-A-3N  3( T •+ AN•− ).86 nTA measurements of TAN in oxygen saturated solvents exhibited reduction in lifetime of CS states by two orders of magnitude due to triplet-triplet energy transfer, suggesting the long-lived species being a triplet CS state (Figure S34, SI).49

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To examine the enhancement in the lifetime of the CS states upon self-assembly in TAN, we have performed theoretical calculations and estimated (see experimental section) the electronic coupling in antiparallel and slipped antiparallel dimers of TAN as obtained from the crystal structure (Figures 1c,e, 8a-c). The antiparallel and slipped antiparallel dimer of TAN displayed charge transfer coupling (Hab) constants of 10 meV and 13 meV respectively. The larger electronic coupling observed for the slipped antiparallel dimer relative to the antiparallel dimer could be due to the lesser separation between the N units leading to greater overlap of electronic wave functions. The electron hopping rate constant for antiparallel and slipped

(a)

e-

e-

or

e-

e-

ecr  110 fs (monomer, ACN)

(c)

(b) e-

e-

e-

e-

e-

Antiparallel dimer (Hab=10 meV)

Slipped antiparallel dimer (Hab=13 meV) (d)

e-

e-

(e)

e-

e-

e-

e-

ee-

cr = 1.51 ns

cr  11 s

Figure 8: Scheme (a,d,e) representing heterogeneity in structure responsible for differences in charge recombination rates observed in TAN. (b-c) illustrates electronic coupling in slipped antiparallel and antiparallel dimers of TAN generated via C-O•••O-C and C-H•••interactions respectively.

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antiparallel dimer was enumerated to be 2.03 x 1011 s-1 and 1.21 x 1011 s-1 respectively, whereas the hole hopping rate constant for slipped anti parallel and antiparallel dimer was estimated to be 1.91 x 107 s-1 and 3.02 x 1010 s-1 respectively. The charge carrier mobility predictions employing the method established by Evans et al.87 based on Marcus theory yielded a value of 1.79 x 10-2 cm2 V-1 s-1 and 1.91 x 10-3 cm2 V-1 s-1 for electron and hole mobility respectively, revealing the ntype nature of TAN. Persistence of CS states, by  108-fold in the aggregated state of TAN in CHCl3 and THF comparison to the monomeric/aggregated AN and monomeric TAN could be attributed to the in delocalization of charge carriers across the TAN aggregates. Formation of long-lived triplet CS states in the aggregated state upon photoexcitation could also assist in prolonging the survival time of the CS states. Upon photoexcitation of A, intramolecular88 electron transfer from 1A* to N unit, in monomeric/aggregated AN and monomeric TAN could generate (GET = -0.48 eV) CS states, namely A•+ and N•− . The generated CS states, A•+ and N•− undergo ultrafast geminate charge recombination ( 110 fs, Figure 8a). Subsequently, charge shift involving hole transfer from A•+ to T could result in hole residing on T and electron localized on N in monomeric TAN. 𝑚 𝑎 Enhancement in the survival time of CS states from 𝜏𝑐𝑟  110 fs in monomeric TAN to 𝜏𝑐𝑟 =

10.86 s and 11.20 s in the aggregated state of TAN could be due to synergetic effects induced by the delocalization of charge carriers across smaller/larger aggregates (Figures 8b-e) and the presence of triplet charge separated states. Observation of 3N*/3T* in the aggregated state of TAN from fTA measurements, upon excitation of A further corroborates formation of triplet CS states via geminate charge recombination.18 𝑎 Small increment in the charge separation lifetime (𝜏𝑐𝑟 ) of ca. 200 and 20-/22-fold were

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observed in the aggregated An-PDI-An35 and liquid crystalline BT-PDI-BT respectively relative to the monomeric state. Enhancement (Table S9-10, SI) in the charge recombination lifetime 𝑎 (𝜏𝑐𝑟 ) of CS states in the aggregated state relative to the monomeric state is defined by a factor

 For J-aggregates of M-NDI-M (≈500000), solvent vapor annealed thin films of PDI-DPPPDI (≈ 11765), helical DPP-PDI-DPP (≈ 1000 and >150), and liquid crystalline BT-PDI-BT (≈ 18) remarkable enhancement in is achieved by various groups. We have demonstrated a significant enhancement ca. 108-fold in through assembling simple modular components via bottom-up approach. Observed long-lived CS states in the aggregated vs. monomeric TAN could be ascribed to the significant electronic coupling in the alternate slipped antiparallel and antiparallel dimers of TAN in the aggregated state. The retardation of charge recombination due to hopping of mobile charge carriers through aggregate domains (Figure 8) could also enhance the survival time of CS states consistent with the reports by Mery, Burghardt and co-workers (Table S10-11, SI) on liquid crystalline BT-PDI-BT triad.16, 30-31 The ongoing efforts in our lab focus on fabricating photovoltaic devices incorporating these functional molecular architectures. CONCLUSIONS In summary, a twisted non-symmetrical triphenylamine-anthracene-naphthalimide (TAN) D-D-A triad for prolonging the charge separated states upon aggregation is reported. TAN triad undergoes self-assembly in toluene, CHCl3 and THF owing to weak co-operative interactions. Solvent polarity dependent absorption and emission measurements demonstrated the presence of ground state CT interactions. Photophysical and electrochemical measurements establish delocalization/hopping of charge carriers through smaller and larger aggregate domains of TAN in the self-assembled state relative to the monomeric state. Following photoexcitation of A/N,

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the triad TAN in the aggregated state exhibit significant enhancement in the charge separated 𝑎 𝑚 lifetime (𝜏𝑐𝑟 ) of ca. 11s compared to the (𝜏𝑐𝑟 ≤ 110 fs) monomer in ACN, as monitored using

nTA and fTA spectroscopic techniques. A 108−fold enhancement in the lifetime of CS states in the aggregated state when compared to the monomeric TAN could be due to the sequential intraand inter-molecular electron transfer. The 3(D.+A.-) state with microsecond lifetime acts as a buffer for the CS state before recombining to the ground state, resulting in persistent CS states. Significant electronic coupling between the constituent units in D-A pair possessing larger exchange coupling between singlet and triplet charge separated states could endorse population of long-lived CS states. Delocalization of charge carriers across the aggregate domains in conjunction with solvent polarity driven population of 3(D.+-A.-) state could aid in prolonging the survival time of CS states. Emergence-upon-assembly approach, thus explored, could be considered as a unique strategy for the construction of ordered and oriented multicomponent architectures to develop organic photovoltaics and photofunctional materials. ASSOCIATED CONTENT Supporting Information includes experimental methods, synthesis and characterization of ANBr and TAN; CCDC numbers for ANBr and TAN are 1479113 and 1479058 respectively. Details of QTAIM, crystal structure and Hirshfeld analyses (Figures S1-S6), morphological analyses (Figures S7-S10), solvent polarity and concentration de-pendent photophysical measurements (Figures S12-16) and transient absorption spectroscopic measurements with corresponding decay curves (Figures S18-S27). “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION

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Corresponding Author * (M.H.) E-mail: [email protected]. Telephone: 0471-2599413. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT M. H. acknowledges Nanobiotechnology Task Force, DBT, Govt. of India for the support of this work, BT/PR5761/NNT/106/599/2012. A. R. M. thank Council of Scientific & Industrial Research (CSIR) for fellowship. V. B. thank INSPIRE Fellowship for the financial assistance. Authors thank Amrita Centre for Nanoscience and Molecular Medicine (ACNSMM), Cochin for TEM measurements, Sophisticated Analytical Instrument Facility (SAIF), IIT-Bombay for EPR measurements, Dr. Sunil Varughese, NIIST-Thiruvananthapuram, Mr. Alex P. Andrews for Xray crystal structure analysis and Prof. Jyotishman Dasgupta, TIFR-Mumbai for helpful suggestions. REFERENCES (1)

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