Persistent Charge-Separated States in Self-Assembled Twisted

Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Article

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

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. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 36

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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

1


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

Page 2 of 36

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


2

Page 3 of 36

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


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.


3


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

Page 4 of 36

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

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


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).


23


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

1

Page 24 of 36

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


24

Page 25 of 36

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


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.


25


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

Page 26 of 36

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


26

Page 27 of 36

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


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,


27


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

Page 28 of 36

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


28

Page 29 of 36

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


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)

Wang, M.; Wudl, F. Top-Down Meets Bottom-up: Organized Donor-Acceptor Heterojunctions for Organic Solar Cells. J. Mater. Chem. 2012, 22, 24297-24314.

(2)

Barter, L. M. C.; Durrant, J. R.; Klug, D. R. A Quantitative Structure–Function Relationship for the Photosystem II Reaction Center: Supermolecular Behavior in Natural Photosynthesis. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 946-951.

(3)

Gust, D.; Moore, T. A.; Moore, A. L. Molecular Mimicry of Photosynthetic Energy and Electron Transfer. Acc. Chem. Res. 1993, 26, 198-205.

(4)

Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512-516.

(5)

Wasielewski, M. R. Photoinduced Electron Transfer in Supramolecular Systems for Artificial Photosynthesis. Chem. Rev. 1992, 92, 435-461.

(6)

Scott Lokey, R.; Iverson, B. L. Synthetic Molecules That Fold into a Pleated Secondary Structure in Solution. Nature 1995, 375, 303-305.


29


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

Page 30 of 36

(7)

Gust, D.; Moore, T. A. Mimicking Photosynthesis. Science 1989, 244, 35-41.

(8)

Haberhauer, G.; Gleiter, R.; Burkhart, C. Planarized Intramolecular Charge Transfer: A Concept for Fluorophores with both Large Stokes Shifts and High Fluorescence Quantum Yields. Chem. Eur. J. 2016, 22, 971-978.

(9)

Gust, D.; Moore, T. A.; Moore, A. L. Mimicking Photosynthetic Solar Energy Transduction. Acc. Chem. Res. 2001, 34, 40-48.

(10) Burrezo, P. M.; Zhu, X.; Zhu, S.-F.; Yan, Q.; López Navarrete, J. T.; Tsuji, H.; Nakamura, E.; Casado, J. Planarization, Fusion, and Strain of Carbon-Bridged Phenylenevinylene Oligomers Enhance π-Electron and Charge Conjugation: A Dissectional Vibrational Raman Study. J. Am. Chem. Soc. 2015, 137, 3834-3843. (11) Bhosale, R.; Misek, J.; Sakai, N.; Matile, S. Supramolecular n/p-Heterojunction Photosystems with Oriented Multicolored Antiparallel Redox Gradients (OMARG-SHJs). Chem. Soc. Rev. 2010, 39, 138-149. (12) Amerongen, H. V.; Valkunas, L.; Grondelle, R. V., Photosynthetic Excitons. World Scientific: London, 2000. (13) Gust, D.; Moore, T. A.; Moore, A. L.; Macpherson, A. N.; Lopez, A.; DeGraziano, J. M.; Gouni, I.; Bittersmann, E.; Seely, G. R. Photoinduced Electron and Energy Transfer in Molecular Pentads. J. Am. Chem. Soc. 1993, 115, 11141-11152. (14) Scarongella, M.; Paraecattil, A. A.; Buchaca-Domingo, E.; Douglas, J. D.; Beaupre, S.; McCarthy-Ward, T.; Heeney, M.; Moser, J. E.; Leclerc, M.; Frechet, J. M. J.; et al. The Influence of Microstructure on Charge Separation Dynamics in Organic Bulk Heterojunction Materials for Solar Cell Applications. J. Mater. Chem. A 2014, 2, 62186230. (15) Meyer, G. J., Molecular Level Artificial Photosynthetic Materials. Ed.; Wiley: New York, 1997. (16) Schwartz, P.-O.; Biniek, L.; Zaborova, E.; Heinrich, B.; Brinkmann, M.; Leclerc, N.; Méry, S. Perylenediimide-Based Donor–Acceptor Dyads and Triads: Impact of Molecular Architecture on Self-Assembling Properties. J. Am. Chem. Soc. 2014, 136, 5981-5992. (17) Dance, Z. E. X.; Mickley, S. M.; Wilson, T. M.; Ricks, A. B.; Scott, A. M.; Ratner, M. A.; Wasielewski, M. R. Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine−Anthracene Molecules with Perpendicular π Systems. J. Phys. Chem. A 2008, 112, 4194-4201. (18) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. Electron-Transfer State of 9-mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem. Soc. 2004, 126, 1600-1601. (19) Ley, D.; Guzman, C. X.; Adolfsson, K. H.; Scott, A. M.; Braunschweig, A. B. Cooperatively Assembling Donor–Acceptor Superstructures Direct Energy into an Emergent Charge Separated State. J. Am. Chem. Soc. 2014, 136, 7809-7812.


30

Page 31 of 36

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


(20) Chu, C.-C.; Raffy, G.; Ray, D.; Guerzo, A. D.; Kauffmann, B.; Wantz, G.; Hirsch, L.; Bassani, D. M. Self-Assembly of Supramolecular Fullerene Ribbons via HydrogenBonding Interactions and Their Impact on Fullerene Electronic Interactions and Charge Carrier Mobility. J. Am. Chem. Soc. 2010, 132, 12717-12723. (21) Rudolf, M.; Kirner, S. V.; Guldi, D. M. A Multicomponent Molecular Approach to Artificial Photosynthesis - The Role of Fullerenes and Endohedral Metallofullerenes. Chem. Soc. Rev. 2016, 45, 612-630. (22) Dössel, L. F.; Kamm, V.; Howard, I. A.; Laquai, F.; Pisula, W.; Feng, X.; Li, C.; Takase, M.; Kudernac, T.; De Feyter, S.; et al. Synthesis and Controlled Self-Assembly of Covalently Linked Hexa-peri-hexabenzocoronene/Perylene Diimide Dyads as Models to Study Fundamental Energy and Electron Transfer Processes. J. Am. Chem. Soc. 2012, 134, 5876-5886. (23) D'Souza, F.; Gadde, S.; Schumacher, A. L.; Zandler, M. E.; Sandanayaka, A. S. D.; Araki, Y.; Ito, O. Supramolecular Triads of Free-Base Porphyrin, Fullerene, and Ferric Porphyrins via the “Covalent-Coordinate” Binding Approach: Formation, Sequential Electron Transfer, and Charge Stabilization. J. Phys. Chem. C 2007, 111, 11123-11130. (24) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; et al. Photodriven Charge Separation in a Carotenoporphyrin-Quinone Triad. Nature 1984, 307, 630-632. (25) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.; Gust, D. Photoinduced Charge Separation and Charge Recombination to a Triplet State in a Carotene−Porphyrin−Fullerene Triad. J. Am. Chem. Soc. 1997, 119, 1400-1405. (26) Sandroni, M.; Maufroy, A.; Rebarz, M.; Pellegrin, Y.; Blart, E.; Ruckebusch, C.; Poizat, O.; Sliwa, M.; Odobel, F. Design of Efficient Photoinduced Charge Separation in Donor– Copper(I)–Acceptor Triad. J. Phys. Chem. C 2014, 118, 28388-28400. (27) Kahnt, A.; Kärnbratt, J.; Esdaile, L. J.; Hutin, M.; Sawada, K.; Anderson, H. L.; Albinsson, B. Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer–Fullerene Donor–Acceptor Systems. J. Am. Chem. Soc. 2011, 133, 9863-9871. (28) Diesenhofer, J.; Norris, J. R., The Photosynthetic Reaction Centre. Eds.; Academic Press: San Diego, 1993. (29) Jimenez, A. J.; Calderon, R. M. K.; Rodriguez-Morgade, M. S.; Guldi, D. M.; Torres, T. Synthesis, Characterization and Photophysical Properties of a Melamine-Mediated Hydrogen-Bound Phthalocyanine-Perylenediimide Assembly. Chem. Sci. 2013, 4, 10641074. (30) Polkehn, M.; Tamura, H.; Eisenbrandt, P.; Haacke, S.; Méry, S.; Burghardt, I. Molecular Packing Determines Charge Separation in a Liquid Crystalline Bisthiophene–Perylene Diimide Donor–Acceptor Material. J. Phys. Chem. Lett. 2016, 7, 1327-1334. (31) Roland, T.; Leonard, J.; Hernandez Ramirez, G.; Mery, S.; Yurchenko, O.; Ludwigs, S.; Haacke, S. Sub-100 fs Charge Transfer in a Novel Donor-Acceptor-Donor Triad Organized in a Smectic Film. Phys. Chem. Chem. Phys. 2012, 14, 273-279.


31


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

Page 32 of 36

(32) Kar, H.; Gehrig, D. W.; Allampally, N. K.; Fernandez, G.; Laquai, F.; Ghosh, S. Cooperative Supramolecular Polymerization of an Amine-Substituted NaphthaleneDiimide and its Impact on Excited State Photophysical Properties. Chem. Sci. 2016, 7, 1115-1120. (33) Guzman, C. X.; Calderon, R. M. K.; Li, Z.; Yamazaki, S.; Peurifoy, S. R.; Guo, C.; Davidowski, S. K.; Mazza, M. M. A.; Han, X.; Holland, G.; et al. Extended Charge Carrier Lifetimes in Hierarchical Donor–Acceptor Supramolecular Polymer Films. J. Phy. Chem. C. 2015, 119, 19584-19589. (34) Schenning, A. P. H. J.; v. Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.; Würthner, F.; Meijer, E. W. Photoinduced Electron Transfer in Hydrogen-Bonded Oligo(p-phenylene vinylene)−Perylene Bisimide Chiral Assemblies. J. Am. Chem. Soc. 2002, 124, 1025210253. (35) Supur, M.; Sung, Y. M.; Kim, D.; Fukuzumi, S. Enhancement of Photodriven Charge Separation by Conformational and Intermolecular Adaptations of an Anthracene– Perylenediimide–Anthracene Triad to an Aqueous Environment. J. Phys. Chem. C 2013, 117, 12438-12445. (36) Hartnett, P. E.; Dyar, S. M.; Margulies, E. A.; Shoer, L. E.; Cook, A. W.; Eaton, S. W.; Marks, T. J.; Wasielewski, M. R. Long-Lived Charge Carrier Generation in Ordered Films of a Covalent Perylenediimide-Diketopyrrolopyrrole-Perylenediimide Molecule. Chem. Sci. 2015, 6, 402-411. (37) Bolag, A.; Sakai, N.; Matile, S. Dipolar Photosystems: Engineering Oriented Push–Pull Components into Double- and Triple-Channel Surface Architectures. Chem. Eur. J. 2016, 22, 9006-9014. (38) Cheriya, R. T.; Mallia, A. R.; Hariharan, M. Light Harvesting Vesicular Donor-Acceptor Scaffold Limits the Rate of Charge Recombination in the Presence of an Electron Donor. Energy Environ. Sci. 2014, 7, 1661-1669 (39) Mallia, A. R.; Salini, P. S.; Hariharan, M. Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination. J. Am. Chem. Soc. 2015, 137, 15604-15607. (40) Cheriya, R. T.; Joy, J.; Alex, A. P.; Shaji, A.; Hariharan, M. Energy Transfer in NearOrthogonally Arranged Chromophores Separated through a Single Bond. J. Phys. Chem. C 2012, 116, 12489-12498. (41) Fukuzumi, S.; Ohkubo, K.; E, W.; Ou, Z.; Shao, J.; Kadish, K. M.; Hutchison, J. A.; Ghiggino, K. P.; Sintic, P. J.; Crossley, M. J. Metal-Centered Photoinduced Electron Transfer Reduction of a Gold(III) Porphyrin Cation Linked with a Zinc Porphyrin to Produce a Long-Lived Charge-Separated State in Nonpolar Solvents. J. Am. Chem. Soc. 2003, 125, 14984-14985. (42) Patel, D. G.; Paquette, M. M.; Kopelman, R. A.; Kaminsky, W.; Ferguson, M. J.; Frank, N. L. A Solution- and Solid-State Investigation of Medium Effects on Charge Separation in Metastable Photomerocyanines. J. Am. Chem. Soc. 2010, 132, 12568-12586.


32

Page 33 of 36

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


(43) Harriman, A.; Rostron, J. P.; Cesario, M.; Ulrich, G.; Ziessel, R. Electron Transfer in SelfAssembled Orthogonal Structures. J. Phys. Chem. A 2006, 110, 7994-8002. (44) Lauer, A.; Dobryakov, A. L.; Kovalenko, S. A.; Fidder, H.; Heyne, K. Dual Photochemistry of Anthracene-9,10-endoperoxide Studied by Femtosecond Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 8723-8732. (45) Voloshchuk, R.; Gryko, D. T.; Chotkowski, M.; Ciuciu, A. I.; Flamigni, L. Photoinduced Electron Transfer in an Amine–Corrole–Perylene Bisimide Assembly: Charge Separation over Terminal Components Favoured by Solvent Polarity. Chem. Eur. J. 2012, 18, 1484514859. (46) Wu, Y.-L.; Brown, K. E.; Wasielewski, M. R. Extending Photoinduced Charge Separation Lifetimes by Using Supramolecular Design: Guanine–Perylenediimide G-Quadruplex. J. Am. Chem. Soc. 2013, 135, 13322-13325. (47) Deng, P.; Liu, L.; Ren, S.; Li, H.; Zhang, Q. N-Acylation: An Effective Method for Reducing the LUMO Energy Levels of Conjugated Polymers Containing Five-Membered Lactam Units. Chem. Commun. 2012, 48, 6960-6962. (48) Suneesh, C. V.; Gopidas, K. R. Long-Lived Photoinduced Charge Separation in Flexible 9,10-Bis(phenylethynyl)anthracene−Phenothiazine Dyads. J. Phys. Chem. C 2009, 113, 1606-1614. (49) Kaiser, C.; Schmiedel, A.; Holzapfel, M.; Lambert, C. Long-Lived Singlet and Triplet Charge Separated States in Small Cyclophane-Bridged Triarylamine–Naphthalene Diimide Dyads. J. Phys. Chem. C 2012, 116, 15265-15280. (50) Attempts to crystallise the dyad (AN) without bromine atom was unsuccessful. (51) Rajagopal, S. K.; Philip, A. M.; Nagarajan, K.; Hariharan, M. Progressive Acylation of Pyrene Engineers Solid State Packing and Colour via C-H•••H-C, C-H•••O and - Interactions. Chem. Commun. 2014, 50, 8644-8647. (52) Rajaram, S.; Shivanna, R.; Kandappa, S. K.; Narayan, K. S. Nonplanar Perylene Diimides as Potential Alternatives to Fullerenes in Organic Solar Cells. J. Phys. Chem. Lett. 2012, 3, 2405-2408. (53) Das, M.; Ghosh, B. N.; Bauza, A.; Rissanen, K.; Frontera, A.; Chattopadhyay, S. Observation of Novel Oxygen•••Oxygen Interaction in Supramolecular Assembly of Cobalt(III) Schiff Base Complexes: A Combined Experimental and Computational Study. RSC Adv. 2015, 5, 73028-73039. (54) Spackman, M. A.; Jayatilaka, D. Hirshfeld Surface Analysis. CrystEngComm 2009, 11, 1932. (55) Loots, L.; Barbour, L. J. A Simple and Robust Method for the Identification of - Packing Motifs of Aromatic Compounds. CrystEngComm 2012, 14, 300-304. (56) Inari, T.; Yamano, M.; Hirano, A.; Sugawa, K.; Otsuki, J. Photophysical and Electrochemical Properties of Thienylnaphthalimide Dyes with Excellent Photostability. J. Phys. Chem. A 2014, 118, 5178-5188.


33


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

Page 34 of 36

(57) Aumiler, D.; Wang, S.; Chen, X.; Xia, A. Excited State Localization and Delocalization of Internal Charge Transfer in Branched Push−Pull Chromophores Studied by SingleMolecule Spectroscopy. J. Am. Chem. Soc. 2009, 131, 5742-5743. (58) Takahashi, S.; Nozaki, K.; Kozaki, M.; Suzuki, S.; Keyaki, K.; Ichimura, A.; Matsushita, T.; Okada, K. Photoinduced Electron Transfer of N-[(3- and 4-Diarylamino)phenyl]-1,8Naphthalimide Dyads: Orbital-Orthogonal Approach in a Short-linked D−A System. J. Phys. Chem. A 2008, 112, 2533-2542. (59) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. Linkage and Solvent Dependence of Photoinduced Electron Transfer in Zincporphyrin-C60 Dyads. J. Am. Chem. Soc. 1996, 118, 11771-11782. (60) Lippert, E. Dipolmoment und Elektronenstruktur von angeregten Molekülen. Z. Naturforsch. A 1955, 10, 541-545. (61) Koch, M.; Letrun, R.; Vauthey, E. Exciplex Formation in Bimolecular Photoinduced Electron-Transfer Investigated by Ultrafast Time-Resolved Infrared Spectroscopy. J. Am. Chem. Soc. 2014, 136, 4066-4074. (62) Cheriya, R. T.; Joy, J.; Alex, A. P.; Shaji, A.; Hariharan, M. Energy Transfer in NearOrthogonally Arranged Chromophores Separated through a Single Bond. J. Phys. Chem. C 2012, 116, 12489-12498. (63) Nagarajan, K.; Mallia, A. R.; Reddy, V. S.; Hariharan, M. Access to Triplet Excited State in Core-Twisted Perylenediimide. J. Phys. Chem. C 2016, 120, 8443-8450. (64) Cho, D. W.; Fujitsuka, M.; Sugimoto, A.; Yoon, U. C.; Mariano, P. S.; Majima, T. Photoinduced Electron Transfer Processes in 1,8-Naphthalimide-Linker-Phenothiazine Dyads. J. Phys. Chem. B 2006, 110, 11062-11068. (65) Terry, G. C.; Uffindell, V. E.; Willets, F. W. Triplet State of Triphenylamine. Nature 1969, 223, 1050-1051. (66) Cho, D. W.; Fujitsuka, M.; Sugimoto, A.; Yoon, U. C.; Majima, T. Regulation of Photodynamic Interactions in 1,8-naphthalimide-Linker-Phenothiazine Dyads by Cyclodextrins. Phys. Chem. Chem. Phys. 2014, 16, 5779-5784. (67) van Dijk, S. I.; Groen, C. P.; Hartl, F.; Brouwer, A. M.; Verhoeven, J. W. Long-Lived Triplet State Charge Separation in Novel Piperidine-Bridged Donor−Acceptor Systems. J. Am. Chem. Soc. 1996, 118, 8425-8432. (68) Dimitrov, S. D.; Wheeler, S.; Niedzialek, D.; Schroeder, B. C.; Utzat, H.; Frost, J. M.; Yao, J.; Gillett, A.; Tuladhar, P. S.; McCulloch, I.; et al. Polaron Pair Mediated Triplet Generation in Polymer/Fullerene Blends. Nat. Commun. 2015, 6, 1-8. (69) Sreenath, K.; Suneesh, C. V.; Ratheesh Kumar, V. K.; Gopidas, K. R. Cu(II)-Mediated Generation of Triarylamine Radical Cations and Their Dimerization. An Easy Route to Tetraarylbenzidines. J. Org. Chem. 2008, 73, 3245-3251. (70) Stamires, D. N.; Turkevich, J. Electron Paramagnetic Resonance in Some Molecular Charge Transfer Complexes. J. Am. Chem. Soc. 1963, 85, 2557-2561.


34

Page 35 of 36

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


(71) Zhang, Y.; de La Harpe, K.; Beckstead, A. A.; Improta, R.; Kohler, B. UV-Induced Proton Transfer between DNA Strands. J. Am. Chem. Soc. 2015, 137, 7059-7062. (72) Kamamoto, Y.; Nitta, Y.; Kubo, K.; Mizuta, T.; Kume, S. Selection of Two Optional Covalent Bonds by Electric Stimuli: Dual Catalytic Switching of Redox-Active Copper. Chem. Commun. 2016, 52, 10486-10489. (73) Wen, X.-L.; Zhang, J.; Liu, Z.-L.; Han, Z.-X.; Rieker, A. Micellar Effects on the Oxidative Electrochemistry of Lipophilic Vitamin C Derivatives. J. Chem. Soc., Perkin Trans. 2 1998, 905-910. (74) Vogt, R. A.; Gray, T. G.; Crespo-Hernández, C. E. Subpicosecond Intersystem Crossing in Mono- and Di(organophosphine)gold(I) Naphthalene Derivatives in Solution. J. Am. Chem. Soc. 2012, 134, 14808-14817. (75) Filatov, M. A.; Etzold, F.; Gehrig, D.; Laquai, F.; Busko, D.; Landfester, K.; Baluschev, S. Interplay Between Singlet and Triplet Excited States in a Conformationally Locked DonorAcceptor Dyad. Dalton Trans. 2015, 44, 19207-19217. (76) Fraser, D. D.; Bolton, J. R. Intramolecular Photochemical Electron Transfer. 8. Decay of the Triplet State in a Porphyrin-Quinone Molecule. J. Phys. Chem. 1994, 98, 1626-1633. (77) Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S.; Hurley, J. K.; Wasielewski, M. R. Intramolecular Photochemical Electron Transfer. 4. Singlet and Triplet Mechanisms of Electron Transfer in a Covalently Linked Porphyrin-AmideQuinone Molecule. J. Am. Chem. Soc. 1988, 110, 1733-1740. (78) Delaney, J. K.; Mauzerall, D. C.; Lindsey, J. S. Electron Tunneling in a Cofacial Zinc Porphyrin-Quinone Cage Molecule: Novel Temperature and Solvent Dependence. J. Am. Chem. Soc. 1990, 112, 957-963. (79) Hirota, J.; Okura, I. Photoinduced Intramolecular Electron Transfer in Bisviologen-Linked Porphyrin in Acetonitrile. J. Phys. Chem. 1993, 97, 6867-6870. (80) Bullock, J. E.; Vagnini, M. T.; Ramanan, C.; Co, D. T.; Wilson, T. M.; Dicke, J. W.; Marks, T. J.; Wasielewski, M. R. Photophysics and Redox Properties of Rylene Imide and Diimide Dyes Alkylated Ortho to the Imide Groups. J. Phys. Chem. B 2010, 114, 17941802. (81) Toby, S.; Toby, F. S. Singlet and Triplet Emission from Difluoromethylene in the Reaction of Ozone with Tetrafluorethene. J. Phys. Chem. 1980, 84, 206-207. (82) Hui Bon Hoa, G.; Kossanyi, J.; Demeter, A.; Biczok, L.; Berces, T. Pressure Dependence of the Dual Luminescence of Twisting Molecules. The Case of Substituted 2,3naphthalimides. Photochem. Photobiol. Sci. 2004, 3, 473-482. (83) Demeter, A.; Berces, T.; Biczok, L.; Wintgens, V.; Valat, P.; Kossanyi, J. Spectroscopic Properties of Aromatic Dicarboximides. Part 2.-Substituent Effect on the Photophysical Properties of N-Phenyl-1,2-Naphthalimide. J. Chem. Soc., Faraday Trans. 1994, 90, 26352641.


35


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

Page 36 of 36

(84) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.; Svec, W. A.; Wasielewski, M. R. Mimicry of the Radical Pair and Triplet States in Photosynthetic Reaction Centers with a Synthetic Model. J. Am. Chem. Soc. 1995, 117, 8055-8056. (85) Berman, A.; Izraeli, E. S.; Levanon, H.; Wang, B.; Sessler, J. L. Photoinduced Intraensemble Electron Transfer in a Base-Paired Porphyrin-Quinone System. TimeResolved EPR Spectroscopy. J. Am. Chem. Soc. 1995, 117, 8252-8257. (86) Chanon, M.; Fox, M. A., Photoinduced Electron Transfer. Part A: Conceptual Basis. Elsevier: 1988, p 173. (87) Evans, D. R.; Kwak, H. S.; Giesen, D. J.; Goldberg, A.; Halls, M. D.; Oh-e, M. Estimation of Charge Carrier Mobility in Amorphous Organic Materials Using Percolation Corrected Random-Walk Model. Org. Electron. 2016, 29, 50-56. (88) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X.; et al. Quantitative Intramolecular Singlet Fission in Bipentacenes. J. Am. Chem. Soc. 2015, 137, 8965-8972. (89) Ratio of rate of charge recombination in the aggregated to the monomeric state is defined as, =[cra/crm]. TOC GRAPHIC

e-

e-

e-

e-

D

D

A

D e-

cr  110 fs

e-

A

vs. D

e-

A

D

D

e-

D

D A

e-

A

D

D

cr  11 s


36

Persistent Charge-Separated States in Self-Assembled Twisted

Recommend Documents