Photophysics of polymers excited state relaxation

Readers interested in polymer science should be familiar with the photochemistry of polymers. Photochemistry plays a role in polymerisation reactions, in the degradation of the backbone chain, or in cross-Hnking of different chains, all of which can be initiated by light Further, photochemistry has been used in a variety of different ways to change and probe polymer structure and intermolecular interactions. However, there are a number of primary physical processes, which take place between the photon impinging on the polymer and the commencement of chemical reaction. It is these primary physical processes that are the subject of this chapter. 

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Photophysics of polymers (excited state relaxation) 171 > A AAAAAAAABAA... 

A similar unique effect of PMA on the photophysics of RuCbpy) is observed at pH 5, for example both the lifetime and luminescence intensity of RuCbpyjj show maxima at pH of about 5. The luminescence of the probe also exhibits a blue spectral shift at this particular pH compared to other pH. The change in the photophysical properties are due to binding of RuCbpyjj " " into a partially coiled or swollen polymer PMA at pH 5. The binding is electrostatic in nature and the ligands of the organometallio complex probe are quite restricted in a hydrophobic environment, so that unlike more mobile systems such as water or a stretched polymer, complete relaxation of the excited state is not achieved. Hence, the lifetime and the yield of luminescence increase accordingly and the emission spectra show a blue shift.(42)... 

It should be noted that no emission from the zwitterionic form of the proton-transferred tautomer was observed from any of the benzotriazoles studied in the present work. This implies that non-radiative relaxation processes from the excited state of this species are very efficient in all of the solvent and polymer environments studied. Thus no information is available on the effect of the medium polarity on the room-temperature photophysics of the zwitterionic form using fluorescence techniques. 

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

Polymer photophysics is determined by a series of alternating odd (B ) and even (Ag) parity excited states that correspond to one-photon and two-photon allowed transitions, respectively [23]. Optical excitation into either of these states is followed by subpicosecond nonradiative relaxation to the lowest excited state [90]. This relaxation is due to either vibrational cooling within vibronic sidebands of the same electronic state, or phonon-assisted transitions between two different electronic states. In molecular spectroscopy [146], the latter process is termed internal conversion. Internal conversion is usually the fastest relaxation channel that provides efficient nonradiative transfer from a higher excited state into the lowest excited state of the same spin multiplicity. As a result, the vast majority of molecular systems follow Vavilov-Kasha s rule, stating that FT typically occurs from the lowest excited electronic state and its quantum yield is independent of the excitation wavelength [91]. 

The fluorescence properties of these probes permits us to study the rotational relaxation in various polymers and even during polymerization reactions and thereby obtain information on the microscopic rigidity of the media. In the following discussion a description of the photophysical properties of the dyes 1-3 will be given, with particular emphasis on the excited-state conformational relaxation in various media. This will be followed by a discussion related to the application of these probes to study polymerization reactions, the effect of polymer molecular structure on free-volume, the dependence of polymer chain relaxation on molecular weight, and the effect of temperature on polymer conformation and free-volume. 

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