CHEMPHYSCHEM 2003, 4, 67 ± 71 ¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/03/04/01 $ 20.00+.50/0 67 Correlation of Molecular Assembly and Interactions in Crystals and Langmuir±Blodgett Films of N-(2,4-dinitrophenyl)-n-octadecylamine Sonika Sharma and T.P. Radhakrishnan* [a] Correlation of molecular organization in crystals and in ultrathin films is of fundamental interest in the design of molecular materials based on thin films. We have chosen as a test case, N-(2,4- dinitrophenyl)-n-octadecylamine (DNPOA), a potential candidate for the fabrication of Langmuir ± Blodgett (LB) films for quadratic nonlinear optical applications. Like several other 4-nitroaniline derivatives, DNPOA does not form stable monolayers at the air ± water interface. This has precluded investigations of their organ- ization in LB films. We have stabilized composite Langmuir films of DNPOA with the phospholipid molecule DSPC and fabricated their LB films. Successful growth of single crystals of DNPOA allowed structure determination and detailed analysis of molecular asso- ciations in the solid state. Electronic absorption spectra of DNPOA in solution, in the solid state and in the LB film are investigated. Modeling of the various spectral signatures by semiempirical computations on molecular clusters extracted from the crystal lattice provides insight into the correlation between the molecular organization in crystals and in LB films. KEYWORDS: Langmuir±Blodgett films ¥ semiempirical calculations ¥ structure elucidation ¥ UV/Vis spectroscopy Introduction Properties of molecular materials are critically dependent on the organization of the molecular building blocks. Intermolecular interactions not only control the molecular assembly, but can exert a direct modulating influence on the molecular contribu- tions to the material attributes, which results in interesting cooperative effects. [1] Understanding the mutual disposition of the molecular components and their association is therefore of fundamental relevance in materials design. Some of the common modes of organizing molecular materials are in the form of crystals and ultrathin films such as Langmuir±Blodgett (LB) films. [2] Single-crystal X-ray diffraction provides an unam- biguous view of the organization of molecules in crystals. In the case of LB films on the other hand, though information on molecular orientation and packing can be obtained through spectroscopic, microscopic, and diffraction techniques, precise atomic positions and hence atomic/molecular level interactions are difficult to determine, except in favorable cases through specialized techniques, such as grazing incidence X-ray diffrac- tion. [3] Understanding the molecular associations and interac- tionsinthesequasi-2Dsystemswouldbeextremelyvaluablefor fine-tuningdesignstrategies.Afirststeptowardsthisgoalwould be to investigate the molecular organization in single crystals of the same amphiphiles that form the LB films, and to delineate the correlation between the 3D and quasi-2D organizational motifs through parallel investigations of the two, for example through spectroscopic studies. Such an approach is feasible if one can circumvent the difficulty of growing single crystals of the amphiphilic molecules that generically possess long hydro- carbon chains and in many cases show positional disorder and poor X-ray diffraction characteristics. We have been investigating amphiphiles based on 4-nitroani- line,theprototypical'push±pull'chromophoreofinterestinthe design of materials for quadratic nonlinear optical applica- tions. [1c, 4] These amphiphiles exhibit poor monolayer stability at the air-water interface. [5±7] We have been successful in growing single crystals in some cases, which has enabled us to gain insight into the molecular organization in the Langmuir/LB films of these compounds in the context of the crystal structure and understand the factors which affect the stability of the mono- layers. [6] Now we report a detailed investigation of N-(2,4- dinitrophenyl)-n-octadecylamine (DNPOA), which bears the well known 2D NLO-phore, the 2,4-dinitroaniline group. This system is particularly interesting in view of the extensive intermolecular interactionsitislikelytoengagein,becauseofthemultiplenitro groupsandtheaminofunctionality.Howeverthismoleculedoes not form stable monolayers at the air±water interface. [7] We have overcome this difficulty by forming a composite with the phospholipid distearoylphosphatidylcholine (DSPC). The LB film shows a similar electronic absorption spectrum to the solid sample, which suggests the presence of molecular aggregation in the film. A single-crystal X-ray study of this amphiphile, [a] Prof. T. P. Radhakrishnan, S. Sharma School of Chemistry University of Hyderabad Hyderabad - 500 046 (India) Fax: ( 91) 40 ± 301 ± 2460 E-mail : tprsc@uohyd.ernet.in Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author.