Thickness Dependence of the Optical Anisotropy for Porphyrin Octaester Langmuir-Schaefer Films † C. Goletti,* ,‡ R. Paolesse, § E. Dalcanale, | T. Berzina, ⊥ C. Di Natale, # G. Bussetti, ‡ P. Chiaradia, ‡ A. Froiio, § L. Cristofolini, ⊥ M. Costa, | and A. D’Amico # Dipartimento di Fisica and Unita ` INFM, Universita ` di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy, Dipartimento di Scienze e Tecnologie Chimiche and Unita ` INFM, Universita ` di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy, Dipartimento di Chimica Organica e Industriale and Unita ` INSTM, Universita ` di Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy, Dipartimento di Fisica and Unita ` INFM, Universita ` di Parma, Parco Area delle Scienze 7/A, 43100 Parma, Italy, and Dipartimento di Ingegneria Elettronica, Universita ` di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy Received March 20, 2002. In Final Form: June 20, 2002 We have studied the optical anisotropy of porphyrin layers with different thicknesses deposited onto gold substrates by the Langmuir-Schaefer technique. In coincidence with the Soret band of the molecule, the optical spectral line shape as determined by reflectance anisotropy spectroscopy exhibits a characteristic, large structure, which changes from a “peaklike” to a “derivative-like” shape at a well-defined thickness (8-10 monolayers). We interpret this result in the framework of electronic effects due to structural changes in the layers, yielding solid-state effects originating from the coupling of the π orbitals of the porphyrin macrocycles. Introduction Molecular materials have aroused considerable interest in recent years due to their large potential impact on molecular nanotechnologies. 1-4 However, their use in solar cells, optoelectronics, transistors, and so forth has not materialized because of intrinsic limits such as low carrier mobilities or instability and degradation of performance due to exposure to ambient conditions. Quite recently, some of these problems have been overcome and efficient p-n junctions have been built using an organic structure. 5 Characterization of the electronic states in the fabri- cated molecular structures is essential; recently, reflec- tance anisotropy spectroscopy (RAS) has been applied to organic layers, showing that the spectra are reliably connected to the electronic properties of the molecule and to the morphological characteristics of the layer. 6-10 We believe that RAS will have in organic films deposition an impact similar to the one already encountered in inorganic growth. 11 In a previous work, by using RAS some of us have studied thick Langmuir-Blodgett (LB) layers of a free- base porphyrin (H 2 THOPP). 10 The optical spectra were dominated by a characteristic oscillation in the Soret band region, reminiscent of the derivative of the layer dielectric function. We proposed an explanation involving the existence of strain due to the deposition process, which would produce a slight variation with polarization in the transition energy and in the broadening factor of the Soret optical transition characteristic of the porphyrin layer. In this paper, we present new results obtained with metalloporphyrin octaesters, having a true D 4h symmetry (H 2 THOPP is D 2h ), deposited onto a metal substrate using the Langmuir-Schaefer technique, which ensures a highly ordered deposition. The RAS line shape measurements were performed for every step of variable thickness. We also characterized the samples by atomic force microscopy (AFM); this gave us an independent determination of the sample thickness at each coverage stage. The main result we report is the abrupt change in the RAS line shape at 8-10 monolayer coverage: the line shape, which at lower coverage is essentially proportional to the Soret band absorption, becomes derivative-like. Experimental Section Film Deposition. The porphyrin complexes used in this work (NiC10OAP and PdC10OAP) have been synthesized according to * To whom correspondence should be addressed. Phone: 39.06.72594288. Fax: 39.06.2023507. E-mail: goletti@Roma2.infn.it. † In memory of Vladimir I. Troitsky. ‡ Dipartimento di Fisica and Unita ` INFM, Universita ` di Roma “Tor Vergata”. § Dipartimento di Scienze e Tecnologie Chimiche and Unita ` INFM, Universita ` di Roma “Tor Vergata”. | Dipartimento di Chimica Organica e Industriale and Unita ` INSTM, Universita ` di Parma. ⊥ Dipartimento di Fisica and Unita ` INFM, Universita ` di Parma. # Dipartimento di Ingegneria Elettronica, Universita ` di Roma “Tor Vergata”. (1) Mitzi, D. B.; Chondroudis, K.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 29 and references therein. (2) Shaw, J. M.; Seidler, P. F. IBM J. Res. Dev. 2001, 45, 3 and references therein. (3) Alivisatos, P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E. Adv. Mater. 1998, 10, 1297. (4) Kagan, C.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945. (5) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. Liu, J.; Shi, Y.; Yang, Y. Appl. Phys. Lett. 2001, 79, 578 and references therein. (6) Frederick, B. G.; Power, J. R.; Cole, R. J.; Perry, C. C.; Chen, Q.; Haq, S.; Bertrams, Th.; Richardson, N. V.; Weightman, P. Phys. Rev. Lett. 1998, 80, 4470. (7) Miller, E. K.; Hingerl, K.; Brabec, C. J.; Heeger, A. J.; Sariciftici, N. S. J. Chem. Phys. 2000, 113, 789. (8) Di Natale, C.; Goletti, C.; Drago, M.; Chiaradia, P.; Paolesse, R.; Della Sala, F.; Lugli, P.; D’Amico, A. Appl. Phys. Lett. 2000, 77, 3164. (9) Kampen, T. U.; Rossow, U.; Schumann, M.; Park, S.; Zahn, D. R. T. J. Vac. Sci. Technol., B 2000, 18, 2077. (10) Goletti, C.; Paolesse, R.; Di Natale, C.; Bussetti, G. L.; Chiaradia, P.; Froiio, A.; Valli, L.; D’Amico, A. Surf. Sci. 2001, 501, 31. (11) Richter, W. Philos. Trans. R. Soc. London, Ser. A 1993, 344, 453. 6881 Langmuir 2002, 18, 6881-6886 10.1021/la025756l CCC: $22.00 © 2002 American Chemical Society Published on Web 08/01/2002