pubs.acs.org/crystal Published on Web 03/31/2010 r 2010 American Chemical Society DOI: 10.1021/cg100021f 2010, Vol. 10 2298–2305 Solid State Conformational Preferences of a Flexible Molecular Backbone Derived from Acetone: Dependence on Electron Donating/Withdrawing Ability of Substitutions Sunil Varughese † and Sylvia M. Draper* School of Chemistry, Trinity College Dublin, College Green, D2, Ireland . † Current address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India. Received January 6, 2010; Revised Manuscript Received March 18, 2010 ABSTRACT: Conformational preferences, crystal packing, and intermolecular interactions can play a pivotal role in solid state reactions, and in particular for bis(phenyl)acetones, these factors are known to determine the rate of the photodecarbonylation process. In the present work, conformational pliability and the supramolecular synthons present in a series of variably substituted bis(phenyl)acetones have been assessed in terms of the electron donating/withdrawing ability of the substituents. While an exo-exo conformation has been observed in the case of electron donating substituents, electron withdrawing analogues preferred an endo-endo arrangement. A linear correlation has been observed between the angular shift and the Hammett constants of the functional groups. The shift in the IR spectra with respect to that of acetone has been used to ascertain the spectral and structural analogy. SEM analysis of the nano-/microparticles of the compounds reveals the direct dependence of the hydrophobic/hydrophilic character of the functional groups on the nature and morphology of the particles. Introduction Studies pertaining to conformationally flexible molecules are interesting, as they can provide insights on the intricacies involved in various complex molecules and recognition pro- cesses. 1,3 The crystallographic information obtained from simple systems can be utilized in understanding various structural motifs present in complex assemblies. For example, as universal models of protein folding and the interactions involved in the process, foldamers are well studied. 2 In such cases, various oligoimides are designed, synthesized, and characterized using X-ray diffraction, NMR, and CD analysis. Recently, Fujita and co-workers reported the conformational preferences of a series of short peptide fragments in the hydro- phobic constrained cavities of a porphyrin containing coordi- nation cage. 3a In evaluating the properties of a flexible molecule and in determining its conformational preferences, in addition to the hydrogen bonds, various factors such as electronic effect, steric factors, hydrophobicity, etc. can contribute significan- tly. 4 Molecular conformation data available from crystal structures are usually affected by crystal packing forces and cause impedance in computational modeling. Although crystal structures generally give a good indication of the conforma- tional preferences of a given molecule, intermolecular inter- actions with sufficient strength can presumably lead to a strained arrangement. 5 In 2008, Allen and co-workers stated that “...conformational diversity increases with an increasing number of different crystal environments and with an increasing number of flexible torsion angles. Overall molecules with one or more acyclic flexible torsion angle are observed to exist in more than one conformation in ca. 40% of cases.” 6 In this context, crystallographic analysis of a series of chemically related flexible molecules is engrossing and can provide insight on the conformational preferences in their crystal lattice. In the area of crystal engineering, urea has been a target molecule in achieving networks for inclusion compounds, nonlinear optical materials and modular assemblies stabilized by hydrogen bonds. 7 N,N 0 -Diaryl ureas (Scheme 1a) predo- minantly exhibit a W-conformation with the formation of an R-network, a chain of bifurcated N-H 333 O hydrogen bonds. 8 Nangia and Etter have reported strategies for effectively designing assemblies with urea, non-urea, and urea-solvent hydrogen bonding motifs by the appropriate functionaliza- tion of the aryl rings. 9 Further, diphenylcarbonates and their conformational preferences have been an interesting topic over several years. 10 Unlike the cases of urea derivatives, ab initio calculations on a series of carbonates (Scheme 1b) demonstrated that they can exist as cis-cis, cis-trans, and trans-trans conformations. 11 An isoelectronic compound, 1,3-bis(phenyl)acetone (Scheme 1c), and its derivatives have been target molecules for studying the photodecarbonylation reactions in the solid state. 12 In a series of substituted bis(phenyl)acetones, the rate of triplet R-cleavage has been reported to be in the order -OCH 3 >-CH 3 >-H. Wong and co-workers did fit the rate of decarbonylation reaction to Hammett plots. 13 It has been observed that the triplet photochemistry of the bis(phenyl) acetone is in fact controlled by a conformational “switch”. 14 Resendiz et al. reported that, in crystalline p,p 0 -disubstituted bis(phenyl)acetones, the relative quantum yields and chemical efficiencies of photodecarbonylation reactions are a function of the Hammett constants of the substituents. The authors further stated that “... reactions in crystals can be enginee- red from known molecular structure parameters”. 15 In 2007, Coppens and co-workers demonstrated that intermolecular hydrogen bonding can induce the quenching of photodecar- bonylation reactions in the monoketones. 16 Though the solid state photochemical processes of bis(phenyl)acetones are depen- dent on the conformations of the molecules, surprisingly, a systematic evaluation of their conformational preferences in their crystal structures has not been done to date. Herein, we *Telephone: þ353-1-8962026. E-mail: smdraper@tcd.ie.