Examples: NRET

Intramolecular

The largest values of R₀ are found when the donor and acceptor are different structures, because the overlap integral is often small when the same structure is responsible for the emission and the excitation, due to the Stoke's shift. But distances between neighboring units in vinyl polymers are also small. So it is easy to find aromatic vinyl polymers in which R₀ for self-transfer is larger than the typical separation of neighboring aromatic units. Poly(vinyl naphthalene) is an example of such a polymer. Here the electronic excitation may conduct a one-dimensional random walk, by hopping from one naphthalene unit to the next. This process is called energy migration

An interesting application of energy migration is in the antenna effect studied extensively by Guillet, G Fig. 9.10. It can be illustrated by a poly(vinyl naphthalene) labelled with an anthracene group at one unit, in the middle of the chain. Forster transfer from naphthalene to naphthalene has R₀ = 0.7 nm, and the unidirectional transfer from naphthalene to anthracene has R₀ = 2.4 nm. Excitation of the naphthalene in the labelled polymer results in intense fluorescence from anthracene. The naphthalene units initially absorb the light. The excitation then experiences energy migration until it comes within R₀ of the anthracene, at which point it jumps directly to the anthracene. Thus the poly(vinyl naphthalene) acts as an antenna, harvesting the light and transmitting the energy to the anthracene, which then emits.

Intermolecular

The dynamics of the exchange of chains between micelles of diblock copolymers in a selective solvent can be studied using nonradiative singlet energy transfer. The chains are diblock copolymers of poly(ethylene oxide) and polystyrene. The former block is soluble in water or methanol, but polystyrene is insoluble in both of these solvents. When dispersed at low concentration, the diblock copolymer forms aggregates (micelles) with the polystyrene blocks in the core, away from direct contact with the solvent, and the poly(ethylene oxide) blocks are in the corona, mixed with the solvent.

Two samples of the diblock copolymer were prepared, The samples differed in the functionalization at the junction between the two blocks. In one sample, a single naphthalene was covalently attached at this site. In the other sample, a single pyrene was attached at the junction. Naphthalene-pyrene are a donor-acceptor pair that is often used in the study of polymers.

Each sample is dispersed in a solvent, in different containers. Micelles are present in each system. At zero time, the two solutions are mixed. The naphthalene units in the mixed sample are excited, and the fluorescence is monitored as a function of time, with the following results:

• In water at room temperature, fluorescence from naphthalene is observed. It remains constant over a period of a week.
• In methanol at room temperature, strong fluorescence is observed for naphthalene immediately after mixing. This fluorescence rapidly decreases in intensity, and it is replaced by fluorescence from pyrene.

In methanol, the micelles rapidly exchange chains with one another, leading to a population of micelles that contains both naphthalene and pyrene. In these micelles, the excited naphthalene units transfer their excitation to the pyrene, which is then responsible for the fluorescence. The kinetics of the exchange can be inferred from the time dependence of the change in the fluorescence. In contrast, the micelles do not exchange chains with one another in water, because water is an extremely hostile solvent for polystyrene. Reference: Wang, Y.; Kausch, C. M.; Chun, M.; Quirk, R. P.; Mattice, W. L. "Exchange of Chains between Micelles of Labeled Polystyrene-block-polyoxyethylene, as Monitored by Non-Radiative Singlet Energy Transfer" Macromolecules 1995, 28, 904-911.

Comparison of the fluorescence data and simulations leads to a kinetic description of the exchange of chains between the micelles, with identification of two active processes:

• Expulsion of a chain from a micelle, followed by its later incorporation into a different micelles. In this mechanism, the chain spends some time as a free chain, completely surrounded by solvent.
• Transitory encounters between two micelles, resulting in the exchange of several chains between them, without the necessity for any of the chains becoming a free chain during the transfer.