CHAIN STRUCTURE AND CONFIGURATION
2.10 PHOTOPHYSICS OF POLYMERS
Photophysics is the science of the absorption, transfer, localization, and emis-sion of electromagnetic energy, with no chemical reactions occurring. By con-trast, photochemistry deals with those processes by which light interacts with matter so as to induce chemical reactions (38). The portion of the electro-magnetic spectrum of interest to photophysics includes both the ultraviolet and the visible wavelength ranges. In many of the experiments performed, light is absorbed in the ultraviolet range, and a fluorescence is measured in the visible range. Often two molecular moieties must be in proper juxtaposition for the phenomenon of interest to be measured.
The first step, of course, is the absorption of electromagnetic energy, trans-forming it into excited molecular states,
(2.40) where A is the molecule to be excited, A* represents the excited state, and hv represents the electromagnetic energy absorbed.
Next most important is energy migration, either along the chain or among the chains. This allows the energy to reach the sites of interest. Such energy migration mimics that observed in the ordered chlorophyll regions of green plant chloroplasts, that is, the antenna chlorophyll pigments (38–40). These light-gathering antennas are composed of chlorophylls, carotenoids, and special pigment-containing proteins. These large organic molecules, some of them natural polymers, harvest light energy by absorbing a photon of light and
A+hv=A* Ds =( )s -( )0 =a sx
storing the absorbed energy temporarily in the form of an electron in an excited singlet energy state. The energy migrates throughout the system of antennas within about 100 ps, being transmitted to the reaction center protein (40). Hence, in polymer photophysics, this phenomenon is termed the
“antenna effect.”
2.10.1 Quenching Phenomena
In situations where bimolecular encounters dominate, typical for polymers, such encounters may lead to an electronic relaxation of the system, termed quenching. In general, such collisions may be written
(2.41) where the excited molecule A* encounters another molecule B. Most often, the bimolecular interaction is between an excited molecule in the singlet state and a quencher molecule in the ground state.
The possible bimolecular quenching process includes (41) (a) chemical reaction, (b) enhancement of nonradiative decay, (c) electronic energy trans-fer, or (d) complex formation.
Chemical reactions involve cross-linking, degradation, and rearrangement.
Electronic energy transfer involves exothermic processes, where part of the energy is absorbed as heat, and part is emitted via fluorescence or phospho-rescence from the donor molecule.
Polarized energy is absorbed in fluorescence depolarization. This phenom-enon is also known as luminescence anisotropy (39). If the chain portions are moving at about the same rate as the reemission, the energy is partly depo-larized. The extent of depolarization is related to the various motions and their relative rates. Important are the inherent degree of anisotropy of the fluorescent chromophore, the degree of energy migration, and rotation of the chromophore during its excited state lifetime. From steady-state and transient emission anisotropic measurements, rotational relaxation times can be deduced.
Complex formation between two species is very important in photophysics.
Two terms need definition—exciplex and excimer. An exciplex is an excited state complex between two different kinds of molecules, one being initially excited and the other in the ground state. A complex between an excited molecule and a ground-state molecule of the same species is called an excimer, being derived from the phrase excited dimer (38). Excimer formation is emphasized in this discussion. In excimer formation, excited-state complexes are usually formed between two aromatic structures. Re-sonance interactions lead to a weak intermolecular force, which binds the two species together, involving p bonds. Such excimers exhibit strong fluo-rescence characterized by being red-shifting with respect to the uncomplexed fluorescence.
A*+B=A+B*
2.10 PHOTOPHYSICS OF POLYMERS 59
Features of excimer fluorescence important in polymer characterization include intensity of radiation, intensity changes, decay rates, extent of fre-quency shifting of the fluorescence, and depolarization effects.
2.10.2 Excimer Formation
The formation of an excimer from an excited-state moiety A* and a ground-state moiety A may be illustrated as
(2.42) The excimer, (AA)*, decomposes due to a variety of interactions, the most important one being the emission of fluorescence:
(2.43) where the emitted frequency, vE, is lower than the input frequency, the remain-ing energy beremain-ing required to separate the two moieties and/or heat genera-tion, and h is Planck’s constant.
The stability of excimers can be examined with the aid of Figure 2.13 (41).
Highly stable excimers lie in the bottom of the energy well. Figure 2.14 illus-trates the relationships among the initial light frequency, single mer emission, and excimer emission. The fluoresced light is more red-shifted in the more stable excimers, because it takes more energy to break them apart. There are three parameters determining excimer stability. With reference to the relative positions of the two aromatic groups in Figure 2.15 (38), these are the angle that the two planes make with each other, their distance apart, and their lateral displacement.
AA A
( ) =* 2 +hvE
A*+A= (AA)*
Figure 2.13 An energy-well diagram for excimer formation, illustrating the effects as a func-tion of the distance, rMM, between the two moieties (41).
2.10 PHOTOPHYSICS OF POLYMERS 61
Figure 2.14 Schematic description of the excitation and fluorescence phenomena.
Figure 2.15 Geometry of the naphthalene excimer (38).
2.10.3 Experimental Studies
2.10.3.1 Microstructure of Polystyrene One of the first polymers studied was polystyrene (Figure 2.16) (42). The single mer emission is at about 290 nm, with the band at 335 nm being attributed to excimer emission. In dilute solu-tions, excimer formation is largely intramolecular rather than intermolecular, the excimers arising from adjacent phenyl groups. This is because the chains are far separated from one another. In the bulk state, excimers may involve neighboring chains as well. Excimer formation in atactic polystyrene requires the tt conformation of the meso-isomer, or the tg-or g-t isomers of the racemic isomer.
David et al. (43) found that the ratio of excimer emission to single mer emis-sion, ID/IM, increased from 10 to 100 times with increasing degree of tacticity;
see Table 2.9. Since isotactic polystyrene exists in a 3-1 helix in the crystalline state with ring spacings of the order of 3 Å, this arrangement provides more excimer sites than in the atactic configuration (43).
There is generally an increase in the ratio ID/IMas the molecular weight of the polymer increases. This has been taken as a confirmation of the energy migration along polymer chains.
2.10.3.2 Excimer Stability While the adjacent phenyl rings in polystyrene form relatively stable excimers, putting substituent groups on the phenyls may alter the bonding energy. Chakraborty et al. (44) substituted bulky t-butyl groups in the para position,
(2.44)
The t-butyl group forces the phenyl groups away from their ideal excimer posi-tions, resulting in a blue shift of the fluorescence to 320 nm. Thus excimers in
CH3 CH3 CH3 C
CH2 C
H
CH3 CH3 CH3 C
CH2 C H
Table 2.9 Ratio of excimer to normal fluorescence intensity (ID/IM) in polystyrene at 77°C (43)
Polystyrene Type ID/IM
Atactic, unoriented 1.43
Atactic, oriented (F = 0.07) 1.67
Atactic, oriented (F = 0.1) 2.08
Isotactic (25% crystalline) 10
Isotactic (35% crystalline) 100
Figure 2.16 Room-temperature absorption and fluorescence spectra of atactic polystyrene in 1,2-dichloroethane (1 ¥ 10-3M). The three fluorescence curves denote nitrogen solution (dotted line), aerated solution (broken line), and oxygenated solution (solid line) (42).
poly(p-t-butylstyrene) are less stable than those in unsubstituted polystyrene.
Analysis of the resulting blue shift, together with molecular modeling, led to a comparison of the relative phenyl positions, shown in Table 2.10.