recent advances in designing molecular assemblies

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Recent Advances in Designing Molecular Assemblies T he formation of molecular aggregates is an important area of research that combines a number of chemical expertise including synthesis, spectroscopy, electronic structure theory, and nally device fabrication. Indeed, the study of the photophysical properties of various molecular aggregates has received a great deal of intention over the last several decades. Both J- and H-aggregates have been examined in great detail, and the electronic structure of these systems has been documented and fairly well agreed upon. Recently, there has been renewed interest in the area of photophysical and electronic properties of molecular aggregates (and modica- tions) for a number of reasons. One such reason is that with the emergence of new experimental and theoretical methods, it is now possible to probe the excitations and experimental parameters in both J- and H-aggregates and their modications with greater detail. Also, with better theoretical methods, one can also compare more closely experimental results to theory. Another reason for renewed interest in this area has been that the arrival of new and interesting organic aggregate structures and assemblies has also opened new avenues for the study of inter- and intramolecular interactions that might be useful for optical and electronic applications. These materials are still fairly new in their characterization status, and many of the methods developed to study other aggregates are now being applied to these new and interesting materials. This brings us to the subject of the current two Perspectives in this issue. These two Perspectives are written by experts in the eld focusing on the details of the mechanism of excitations in H-aggregates and the use of the relatively new noncovalent modied graphene for intermolecular interactions with organic chromophores. The rst Perspective critically re-examines the photophysics of para-distyrylbenzevne (DSB) as an illustration of the herringbone-arranged H-aggregate. While there have been a great deal of reports regarding this conguration, there is an apparent contradiction in discussing the process of aggrega- tion-induced emission quenchingin H-aggregates with the observed highly emissive DSB crystals. The authors have set out to resolve this particular issue by looking also into the exciton states in single- and polycrystalline systems. It was found that DSB forms pronounced H-aggregates, which are highly luminescent in single crystals but become low emissive in polycrystalline NP suspensions and vapor-deposited or spin- coated lms. The authors now provide good reasons for this behavior primarily based on low trap concentrations in the single crystals, structural defects, and longer lifetimes in the excited states in the case of polycrystalline samples. There is indeed a strong eect due to the formation of polarons, triplets, and contaminations as well as other structural dislocations that aect the DSB uorescence quantum yield. The Perspective also clears up the issue regarding how one might increase the eective emission from low quantum yield polycrystalline samples with the use of proper sensitization experiments. The Perspective is a prelude to much more to come in this area and serves as a litmus test for those working in this area to be mindful of making proper assignments, valid structure- property relationships, and correct interpretations in discerning new and exciting optical and photophysical properties of conjugated materials such as DSB. The Pecond perspective involves the properties of single- and few-layer graphene. In particular, the contribution looks at the recent interest in interfacing functional molecules and materials to graphenes high surface area basal plane. Indeed, there is a need now for good of pure graphene material for this purpose, and there have been many attempts to covalently modify the surface of graphene. However, the surface of pristine graphene lacks chemical functionality to allow for covalent modication without interrupting its continuous π-orbital system. In contrast to this, the authors point out that noncovalent functionalization does not suer from these limitations. This immediately brings about potential applications for such methods and materials including molecular recognition. The authors describe a number of new strategies to interface molecular compounds and polymers as well as graphene. This process provides enhanced properties and functions in the prepared materials. For example, in the case of graphene, noncovalent function- alization strategies do not aect the transparency or conductivity of the material as is found in covalent modication. This in eect oers noncovalent functionalization as an avenue to prepare other important materials for use in electronics. The assembling of molecules on the graphene basal plane through van der Waals interactions represents an operationally simple method to introduce arbitrary functionality to the graphene surface and to inuence its charge carrier density. Graphenes atomically precise structure makes it suitable for crystallizing molecules in two dimensions and has inspired the authors eorts to control molecular orientation through multivalent binding interactions. In the Perspective, the authors point out several considerations that are important for evaluating molecular assembly on graphene. Dierences in molecular size and coverage, which can range from fractions of monolayers to multilayers, strongly inuence the accessibility of the graphene basal plane and its electronic properties. It is also important to match the stability of the adsorbed monolayers or multilayers to the desired application. According to the authors, graphenes single-atom thickness, desirable electronic proper- ties, and mechanical stability have already attracted interest for transparent electrodes and FETs and are likely to be leveraged within other device platforms. This may also lead to its use in sensing and integrated logic circuits. The process of molecular functionalization provides ultrathin layers or well-dened monolayers that have served as dopants and as nucleation sites to ensure conformal oxide coatings through atomic layer deposition. These two Perspectives provide a nice review and forward- looking view of the eld of organic aggregates and surface modication of important organic assemblies such as graphene. The use of important optical methods to probe the properties Published: August 15, 2013 Editorial pubs.acs.org/JPCL © 2013 American Chemical Society 2705 dx.doi.org/10.1021/jz401611h | J. Phys. Chem. Lett. 2013, 4, 2705-2706

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Page 1: Recent Advances in Designing Molecular Assemblies

Recent Advances in Designing Molecular Assemblies

The formation of molecular aggregates is an important areaof research that combines a number of chemical expertise

including synthesis, spectroscopy, electronic structure theory,and finally device fabrication. Indeed, the study of thephotophysical properties of various molecular aggregates hasreceived a great deal of intention over the last several decades.Both J- and H-aggregates have been examined in great detail,and the electronic structure of these systems has beendocumented and fairly well agreed upon. Recently, there hasbeen renewed interest in the area of photophysical andelectronic properties of molecular aggregates (and modifica-tions) for a number of reasons. One such reason is that with theemergence of new experimental and theoretical methods, it isnow possible to probe the excitations and experimentalparameters in both J- and H-aggregates and their modificationswith greater detail. Also, with better theoretical methods, onecan also compare more closely experimental results to theory.Another reason for renewed interest in this area has been thatthe arrival of new and interesting organic aggregate structuresand assemblies has also opened new avenues for the study ofinter- and intramolecular interactions that might be useful foroptical and electronic applications. These materials are stillfairly new in their characterization status, and many of themethods developed to study other aggregates are now beingapplied to these new and interesting materials. This brings us tothe subject of the current two Perspectives in this issue. Thesetwo Perspectives are written by experts in the field focusing onthe details of the mechanism of excitations in H-aggregates andthe use of the relatively new noncovalent modified graphene forintermolecular interactions with organic chromophores.The first Perspective critically re-examines the photophysics

of para-distyrylbenzevne (DSB) as an illustration of theherringbone-arranged H-aggregate. While there have been agreat deal of reports regarding this configuration, there is anapparent contradiction in discussing the process of “aggrega-tion-induced emission quenching” in H-aggregates with theobserved highly emissive DSB crystals. The authors have setout to resolve this particular issue by looking also into theexciton states in single- and polycrystalline systems. It wasfound that DSB forms pronounced H-aggregates, which arehighly luminescent in single crystals but become low emissivein polycrystalline NP suspensions and vapor-deposited or spin-coated films. The authors now provide good reasons for thisbehavior primarily based on low trap concentrations in thesingle crystals, structural defects, and longer lifetimes in theexcited states in the case of polycrystalline samples. There isindeed a strong effect due to the formation of polarons, triplets,and contaminations as well as other structural dislocations thataffect the DSB fluorescence quantum yield. The Perspectivealso clears up the issue regarding how one might increase theeffective emission from low quantum yield polycrystallinesamples with the use of proper sensitization experiments. ThePerspective is a prelude to much more to come in this area andserves as a litmus test for those working in this area to bemindful of making proper assignments, valid structure−

property relationships, and correct interpretations in discerningnew and exciting optical and photophysical properties ofconjugated materials such as DSB.The Pecond perspective involves the properties of single- and

few-layer graphene. In particular, the contribution looks at therecent interest in interfacing functional molecules and materialsto graphene’s high surface area basal plane. Indeed, there is aneed now for good of pure graphene material for this purpose,and there have been many attempts to covalently modify thesurface of graphene. However, the surface of pristine graphenelacks chemical functionality to allow for covalent modificationwithout interrupting its continuous π-orbital system. In contrastto this, the authors point out that noncovalent functionalizationdoes not suffer from these limitations. This immediately bringsabout potential applications for such methods and materialsincluding molecular recognition. The authors describe anumber of new strategies to interface molecular compoundsand polymers as well as graphene. This process providesenhanced properties and functions in the prepared materials.For example, in the case of graphene, noncovalent function-alization strategies do not affect the transparency orconductivity of the material as is found in covalentmodification. This in effect offers noncovalent functionalizationas an avenue to prepare other important materials for use inelectronics.The assembling of molecules on the graphene basal plane

through van der Waals interactions represents an operationallysimple method to introduce arbitrary functionality to thegraphene surface and to influence its charge carrier density.Graphene’s atomically precise structure makes it suitable forcrystallizing molecules in two dimensions and has inspired theauthor’s efforts to control molecular orientation throughmultivalent binding interactions. In the Perspective, the authorspoint out several considerations that are important forevaluating molecular assembly on graphene. Differences inmolecular size and coverage, which can range from fractions ofmonolayers to multilayers, strongly influence the accessibility ofthe graphene basal plane and its electronic properties. It is alsoimportant to match the stability of the adsorbed monolayers ormultilayers to the desired application. According to the authors,graphene’s single-atom thickness, desirable electronic proper-ties, and mechanical stability have already attracted interest fortransparent electrodes and FETs and are likely to be leveragedwithin other device platforms. This may also lead to its use insensing and integrated logic circuits. The process of molecularfunctionalization provides ultrathin layers or well-definedmonolayers that have served as dopants and as nucleationsites to ensure conformal oxide coatings through atomic layerdeposition.These two Perspectives provide a nice review and forward-

looking view of the field of organic aggregates and surfacemodification of important organic assemblies such as graphene.The use of important optical methods to probe the properties

Published: August 15, 2013

Editorial

pubs.acs.org/JPCL

© 2013 American Chemical Society 2705 dx.doi.org/10.1021/jz401611h | J. Phys. Chem. Lett. 2013, 4, 2705−2706

Page 2: Recent Advances in Designing Molecular Assemblies

of the materials as well as the use of electronic structurecalculations is now to the point that one can readily apply thesemethods to newly prepared materials and provide sufficientrational for the observed behavior. This was certainly the casein the analysis of the DSB H-aggregates as well as the formationof noncovalent modifications of graphene. With furtherinteraction in this field in these two areas, it is expected thatnew and interesting properties may be discovered and put touse in new applications.

T. Goodson, IIIThe University of Michigan

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] expressed in this Editorial are those of the author andnot necessarily the views of the ACS.

■ RELATED READINGS(1) Gierschner, J.; Luer, L.; Milian-Medina, B.; Oelkrug, D.; Egelhaaf,H.-J. Highly Emissive H-Aggregates or Aggregation-Induced EmissionQuenching? The Photophysics of All-Trans para-Distyrylbenzene. J.Phys. Chem. Lett. 2013, 4, 2686−2697.(2) Mann, J. A.; Dichtel, W. Noncovalent Functionalization ofGraphene by Molecular and Polymeric Adsorbates. J. Phys. Chem. Lett.2013, 4, 2649−2657.

The Journal of Physical Chemistry Letters Editorial

dx.doi.org/10.1021/jz401611h | J. Phys. Chem. Lett. 2013, 4, 2705−27062706