Chemistry Reference
In-Depth Information
Contents
1 Introduction .................................................................................. 54
2 Optics for Organics .......................................................................... 55
3 Reflectance Anisotropy Spectroscopy (RAS): A Closer View ............................ 66
3.1 Experimental Apparatus . . . ........................................................... 66
3.2 Interpretation of the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4 RAS and Porphyrins ......................................................................... 75
4.1 Langmuir-Blodgett and Langmuir-Schaefer Layers . . .............................. 75
4.2 Ultrahigh-Vacuum-Deposited Layers ................................................ 91
4.3 The Application of Thin Porphyrin Layers in Gas Sensing . . . . . . . . . . . . . . . . . . . . . . . . . 103
5 Conclusion ................................................................................... 112
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
1
Introduction
The use of porphyrins as materials for significant, widespread, and even innovative
applications has produced a significant amount of results in the last few years. In
particular, the realization of thin and ultrathin molecular layers has been pursued
and achieved because of the interest for systems where the high surface-to-bulk
ratio and the low quantity of mass are basic conditions towards the assembly of new
molecular devices for organic-based electronics [
1
]. We have noticed a strong effort
to characterize the structural and electronic properties of these layers as well as to
control and to improve the methods of growth, obtaining important and sometimes
unexpected results.
It is quite surprising that within this effort, only a small number of experiments
have been reported about the investigation of the optical properties of porphyrin
layers. Porphyrins actually exhibit prominent optical properties, with very intense
transitions mainly related to the dominant absorption band, the so-called Soret
band, whose details (photon energy position, line shape, width) are strongly related
to the aggregation of molecules (in solution or in the solid state), that produce a blue
or red shift (with respect to the position of the band of the not aggregate molecules)
depending upon the coupling existing between adjacent porphyrins [
2
]. In addition,
the use of optical spectroscopies - after their sensitivity to thickness has been
demonstrated - is in principle compatible also with studying layers deposited or
immersed in liquid environment, thus exploiting the powerful opportunities offered
by chemistry in solutions.
In spite of that, a limited number of papers have been published on this particular
topic, mainly using methods borrowed from the investigation of inorganic surfaces
that naturally exhibit a very high sensitivity to low-thickness systems. In this
chapter, I will report the most significant results about porphyrin layers obtained
by a particular optical spectroscopy, namely, reflectance anisotropy spectroscopy
(RAS). I will briefly describe the technique, give some details about the experi-
mental setup, and discuss how experimental data can be interpreted. A review of the
most significant results will follow, from the very early applications on Langmuir-
