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Fluorescence imaging of single molecules at room temperature is a powerful technique for studying complex condensed phase systems and revealing structure and dynamics hidden by ensemble measurements. Successful single-molecule spectroscopic experiments rely upon strong emitters that can be detected at the level of individual copies above the relevant background signals. This Account discusses a class of nonlinear optical chromophores that not only are well-suited for single-molecule imaging but also offer additional beneficial properties such as a significant ground-state dipole moment, moderate hyperpolarizability, and sensitivity to local environment. An overview of the photophysical properties of several members of this class of molecules as well as a mechanism to help understand the environmental sensitivity is presented. Some preliminary applications of the chromophores as single-molecule reporters in cellular and polymer systems are discussed, along with detection of the emitters by two-photon fluorescence.

We examine the effects of pressure on polymer blend miscibility for two polyolefin blends as well as for blends of polystyrene (PS) and polybutadiene (PB). Each of the blends studied exhibits an upper critical solution temperature (UCST), and for the PS/PB blend we report experimental results on the pressure dependence of the UCST for different molecular weight combinations. We make use of the Born−Green−Yvon integral equation theory to exploit the connection between the pressure dependence of the UCST and the sign of the volume change on mixing, leading us to predict that through judicious choice of polymer molecular weights the response of the UCST to pressure may be controlled.

A series of linear polymers labeled with one perylene diimide per polymer chain were synthesized using living free-radical polymerizations based on nitroxides. Depending on the choice of the initiator, the perylene diimide was either at the end or the middle of the polymer chain. The polymers were synthesized as homopolymers or block copolymers from butyl acrylate, styrene, isoprene, and butadiene. Poly(butadiene) labeled with a perylene diimide dye in the center of the polymer was studied using single-molecule imaging in a poly(methyl methacrylate) host. Most of the dyes remained stationary during the experiments, but approximately 5% of the dyes reoriented. The fraction of dyes reorienting was observed to be independent of pumping intensity. This system is an excellent test bed for studying the local environment of the dye and the heterogeneity of the host polymer.