Chiroptical Properties of Some Monoazapentahelicenes France Lebon, ²,‡ Giovanna Longhi, ²,‡ Fabrizio Gangemi, ² Sergio Abbate,* ,²,‡ Jan Priess, § Markus Juza, § Cristina Bazzini, | Tullio Caronna, | and Andrea Mele ⊥ Dipartimento di Scienze Biomediche e Biotecnologie, UniVersita` di Brescia, Viale Europa 11, I-25123 Brescia, Italy, INFM-Istituto Nazionale di Fisica della Materia, UdR Brescia, Via Valotti 9, I-25123 Brescia, Italy, CarboGen AG (Aarau), Schachenallee 29, CH-5001 Aarau, Switzerland, Facolta` di Ingegneria, UniVersita` di Bergamo, Via Marconi 5,I-24044 Dalmine (BG), Italy, and Dipartimento di Chimica, Materiali ed Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via Mancinelli 7, I-20131 Milano, Italy ReceiVed: September 24, 2004 Three closely related previously synthesized monoaza[5]helicenes have been resolved into their enantiomers via enantioselective HPLC using a cellulose-derivative Chiralcel OD column. Circular dichroism (CD) spectra of the enantiomerically enriched samples have been recorded and assigned. The spectra were analyzed as a function of time, and different rate constants were found in the kinetics of racemization for the three molecules. Ab initio DFT calculations for the ground electronic states were employed to determine minima and saddle point structures and to understand the racemization process. The theoretical geometries compared well with those from X-ray structures. CD spectra were calculated by TD-DFT ab initio methods, and compared with experimental data. I. Introduction Helicenes form an attractive set of molecules that, ever since they were first synthesized, 1 have stimulated the curiosity and imagination of numerous scientists, for several fundamental reasons: the synthesis, either via chemistry 1,2 or photochemis- try; 3,4 the separation of enantiomers and the determination of the absolute configuration; 5,6 the study of the thermal stability of enantiomers and their path to racemization; 2 and, most important in our opinion, the challenge posed to MO theories for determining structure and interpreting spectra. 7-11 However, recently interest in helicenes has not been merely speculative, but also applicative because these molecules have a nicely delocalized π-electron system and thus they exhibit interesting opto- and photoelectronic properties; 12,13 i.e., they are organic semiconductors. What makes these molecules special is the chirality of the π-electron system, which makes the helicene a prototypic inherently dissymmetric chromophore. 14 Undoubtedly this property also determines their unique appli- cative qualities. For all these reasons, we decided to study the chiroptical properties of a series of three previously synthesized monoaza[5]helicenes, 15 the chemical structures of which are reported below as H4, H5, and H6 (where 4, 5, and 6 designate the atomic positions occupied by the nitrogen atom). In ref 15, X-ray crystallographic studies were presented for the racemic mixtures; here, we will present the results of enantiomeric separation via HPLC. We will report also the CD and absorption spectra of the enantiomerically enriched samples, and make a rather thorough assignment of the observed CD bands, as results also from the comparison with the “parent” molecule, [5]helicene. An ab initio-DFT calculation will allow us to quantitatively understand the quite different enantiomeric stability observed for the three helicenes, H4, H5, and H6. Finally, time-dependent density functional theory (TD-DFT) calculations, which are a natural evolution of the methods first presented in ref 7, will allow a less empirical interpretation of the spectral data. II. Experimental Methods (a) Enantiomeric Resolution. A Merck HPLC column (type NW50) was packed with 200 g of Chiralcel OD, 20 μm particle size obtained from Chiral Technologies Europe (Strasbourg, France). The resulting column had a dimension of 185 × 48 mm ID. Preparative separations were performed with an isocratic eluent mixture of n-heptane/ethanol 90:10 (v:v) using a prepara- tive HPLC system provided by Knauer (Berlin, Germany), which consisted of a K-1800 pump with a 1000 mL/min pump head, a HPLC-Box and a K-2500 UV detector. The flow rate was 50 mL/min for H5 and H6 and 100 mL/min for H4, leading to run-times of 20 min per injection. The injection volume was 3 mL of approx 40 mg sample dissolved in ethanol/heptane at various compositions. Detection was performed at 254 nm. All preparative separations were performed at room temperature (approximately 22 °C). The eluent and the HPLC column were at room temperature. * Corresponding author. Dipartimento di Scienze Biomediche e Biotec- nologie, Universita` di Brescia, viale Europa 11, 25123 Brescia, Italy. Telephone: +39-0303717415. Fax: +39-0303701157. e-mail: abbate@ med.unibs.it. ² Universita` di Brescia. ‡ UdR Brescia. § CarboGen AG (Aarau). | Universita` di Bergamo ⊥ Politecnico di Milano. 11752 J. Phys. Chem. A 2004, 108, 11752-11761 10.1021/jp0456537 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/04/2004