Chapter 20
Applications of Luminescence Spectroscopy in Polymer Science Section Overview
Downloaded by UNIV OF PITTSBURGH on June 10, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch020
Ian Soutar School of Physics and Chemistry, Lancaster University, Lancaster LA1 4YA, United Kingdom
Luminescence spectroscopy is an extremely powerful and versatile means of interrogation of polymer systems. Photophysical techniques may be used to study energy transfer and trapping phenomena: in this case, the chromophores involved are generally intrinsic to the macromolecule and present in relatively high local concentrations. Alternatively, luminescence methods may be applied in explorations of the physical behavior and characteristics of macromolecular assemblies: in this instance the "reporter" is usually a luminescent species which is introduced to the system under investigation either as a molecular probe or as a covalently bound label. In such probe and label studies, the guest sensor is incorporated at extremely low levels of concentration(typically,10 -lO M)in the hope that minimal perturbation of the system will result. It is the high degrees of sensitivity afforded by luminescence spectroscopy which allows luminescent sensors to be added at such low concentrations. The sensitivity of the technique is such that, even in labelling -5
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experiments, polymer concentrations can often be accessed which are below the limits of resolution of most other spectroscopic and scattering approaches. Luminescence techniques have enjoyed increasing application in polymer science in recent years. Several reviews of the topic are available (see, for example, references 1-5) and only the briefest of overviews is offered below. Luminescence Processes The term "luminescence" refers to the emission of radiation from an excited state and covers two phenomena; fluorescence and phosphorescence. The unimolecular processes leading to these radiative deactivation steps are conveniently introduced by reference to a "Jablonski Diagram" of the type shown in Figure 1. The lowest energy state of the vast majority of organic molecules is a singlet state (i.e. one in which electrons with opposite spin angular momenta are paired in molecular orbitals). This state is depicted as S in Figure 1. Absorption, occurring with retention of spin, promotes an electron from the highest occupied molecular orbital into one of higher energy, creating a molecule in a higher excited state S . 0
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0097-^156/95/0598-0356$12.00/0 © 1995 American Chemical Society In Multidimensional Spectroscopy of Polymers; Urban, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Luminescence Spectroscopy in Polymer Science
Downloaded by UNIV OF PITTSBURGH on June 10, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch020
20· SOUTAR
Figure 1
State diagrm depicting the unimolecular processes of A - absoφtion; F - fluorescence; IC - internal conversion; ISC - intersystem crossing; Ρ - phosphorescence and VR - vibrational relaxation.
In Multidimensional Spectroscopy of Polymers; Urban, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS
Vibrational levels of the excited state are also populated and a range of wavelengths are absorbed in the excitation process as revealed in the absorption spectrum. In condensed media, the processes of internal conversion (a radiationless, isoenergetic transition from a vibrational level of an upper excited state to a vibrational level of a lower state) and vibrational relaxation (the intermolecular exchange of energy whereby a vibrationally "hot" species achieves thermal equilibrium with its surroundings) rapidly (
