Abstract

Thin organic films are widely used in sensors, solar cells, and optical devices due to their intense absorption in the visible/near-infrared (IR) region. Shifting, quenching, or reshaping of some spectral features can be achieved by chemical functionalization of the molecules, whereas an anisotropic fingerprint due to preferential molecular alignment can be induced via a proper design and/or preparation of the substrate. Recently, we investigated the optical response of thin films of porphycene to acidification. With respect to the well-known and closely related tetraphenyl porphyrin, porphycene has the clear advantage of being optically active in the full visible range, and this makes visible by naked eye the immediate change of the film from brilliant blue-turquoise to green when exposed to HCl vapors. In this work, by exploiting a homemade reflectance anisotropy spectroscopy (RAS) apparatus, we explore possible optical anisotropies in the visible spectral range of porphycene films and relate them to the film morphology analyzed by atomic force microscopy (AFM).

Highlights

  • Organic molecules are employed in many devices such as sensors [1,2], photovoltaic systems [3–5], and electronic circuits [6,7,8]

  • Ordered arrays of tautomers can be monitored by traditional optical spectroscopies, such as reflectance anisotropy spectroscopy (RAS), opening the route toward possible optical device prototypes

  • The latter represents the angle between a RAS polarization direction (α) and the exfoliation direction used for the highly oriented pyrolytic graphite (HOPG) preparation

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Summary

Introduction

5], and electronic circuits [6,7,8] The advantage of their use over other systems is related to the presence of many synthetic tools that allows for the tuning of their physical and chemical properties [9]. Heterocyclic organic molecules offer the possibility of exploiting intramolecular mechanisms, potentially leading to compounds suitable for singlemolecule devices, as recently proposed for switches [13]. In this case, the possible interconversion of tautomers (i.e., constitutional isomers) in a free-base tetraphenyl porphyrin (H2TPP) molecule is suggested as a working principle of an electronic device. Ordered arrays of tautomers can be monitored by traditional optical spectroscopies, such as reflectance anisotropy spectroscopy (RAS), opening the route toward possible optical device prototypes

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