Editorial pubs.acs.org/JPCL
Recent Advances in Designing Molecular Assemblies
T
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 graphene’s 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 modification without interrupting its continuous π-orbital system. In contrast to this, the authors point out that noncovalent functionalization does not suffer 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 functionalization strategies do not affect the transparency or conductivity of the material as is found in covalent modification. This in effect offers 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 influence its charge carrier density. Graphene’s atomically precise structure makes it suitable for crystallizing molecules in two dimensions and has inspired the author’s efforts 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. Differences in molecular size and coverage, which can range from fractions of monolayers to multilayers, strongly influence 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, graphene’s single-atom thickness, desirable electronic properties, 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-defined 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 forwardlooking view of the field of organic aggregates and surface modification of important organic assemblies such as graphene. The use of important optical methods to probe the properties
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 finally 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 modifications) 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 modifications 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 field focusing on the details of the mechanism of excitations in H-aggregates and the use of the relatively new noncovalent modified graphene for intermolecular interactions with organic chromophores. The first 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 configuration, there is an apparent contradiction in discussing the process of “aggregation-induced emission quenching” in 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 spincoated films. 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 effect due to the formation of polarons, triplets, and contaminations as well as other structural dislocations that affect the DSB fluorescence quantum yield. The Perspective also clears up the issue regarding how one might increase the effective 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− © 2013 American Chemical Society
Published: August 15, 2013 2705
dx.doi.org/10.1021/jz401611h | J. Phys. Chem. Lett. 2013, 4, 2705−2706
The Journal of Physical Chemistry Letters
Editorial
of the materials as well as the use of electronic structure calculations is now to the point that one can readily apply these methods to newly prepared materials and provide sufficient rational for the observed behavior. This was certainly the case in the analysis of the DSB H-aggregates as well as the formation of noncovalent modifications of graphene. With further interaction in this field in these two areas, it is expected that new and interesting properties may be discovered and put to use in new applications.
T. Goodson, III
■
The University of Michigan
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Notes
Views expressed in this Editorial are those of the author and not necessarily the views of the ACS.
■
RELATED READINGS
(1) Gierschner, J.; Lüer, L.; Milián-Medina, B.; Oelkrug, D.; Egelhaaf, H.-J. Highly Emissive H-Aggregates or Aggregation-Induced Emission Quenching? The Photophysics of All-Trans para-Distyrylbenzene. J. Phys. Chem. Lett. 2013, 4, 2686−2697. (2) Mann, J. A.; Dichtel, W. Noncovalent Functionalization of Graphene by Molecular and Polymeric Adsorbates. J. Phys. Chem. Lett. 2013, 4, 2649−2657.
2706
dx.doi.org/10.1021/jz401611h | J. Phys. Chem. Lett. 2013, 4, 2705−2706
