Reversible monomer-excimer kinetics in solution

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Thermoresponsive Micelles of Phenanthrene-alpha-end-labeled Poly(N-decylacrylamide-b-N,N-diethylacrylamide) in Water

Block copolymers of poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA-b-PDEA), with differentPDEAblock lengths and a constantPDcAblock labeled with a phenanthrene fluorescent dye at the PDcA R-chain-end were prepared by RAFT polymerization. These copolymers form star-like micelles in water, (critical micelle concentration below 0.1 g/L, determined using coumarine 153) with a PDcA insoluble core surrounded by a PDEA corona showing thermoresponsive properties. The kinetics of Forster resonance energy transfer (FRET) between the chain-end phenanthrene groups and anthracene loaded into the hydrophobic core of the micelles in water, was analyzed using a new model for energy transfer in spherical nanodomains. This model takes into account the Poisson distribution of the acceptors in the micelle population and the existence of two phenanthrene states with different fluorescence lifetimes. The analysis yields the radius of the micelle core, Rc=2.7±0.1 nm, with no need for deuteration of the core block. The result is compared with the value obtained by extrapolation of the light scattering data using the star micelle model, Rc(DLS)=3.0 nm. Themodel for star-likemicelles also yields a solvent-corona interaction parameter that changes with temperature due to the thermoresponsive nature of PDEA

RAFT polymerization and self-assembly of thermoresponsive poly(N-decylacrylamide-b-N,N-diethylacrylamide) block copolymers bearing a phenanthrene fluorescent alpha-end group

Phenanthrene -end-labeled poly(N-decylacrylamide)n-b-poly(N,N-diethylacrylamide)m (PDcAn-b-PDEAm) block copolymers consisting in a highly hydrophobic block (n=11) and a thermoresponsive block with variable length (79 ≤m≤ 468) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. A new phenanthrene-labeled chain transfer agent (CTA) was synthesized and used to control the RAFT polymerization of a hydrophobic acrylamide derivative, N-decylacrylamide (DcA). This first block was further used as macroCTA to polymerize N,N-diethylacrylamide (DEA) in order to prepare diblock copolymers with the same hydrophobic block of PDcA (number average molecular weight: Mn = 2720 g mol-1, polydispersity index: Mw/Mn = 1.13) and various PDEA blocks of several lengths (Mn = 10000 to 60000 g mol-1) with a very high blocking efficiency. The resulting copolymers self-assemble in water forming thermoresponsive micelles. The critical micelle concentration (CMC) was determined using the Förster resonance energy transfer (FRET) between phenanthrene linked at the end of the PDcA block and anthracene added to the solution at a low concentration (10-5 M), based on the fact that energy transfer only occurs when phenanthrene and anthracene are located in the core of the micelle. The CMC (~ 2 M) was obtained at the polymer concentration where the anthracene fluorescence intensity starts to increase. The size of the polymer micelles decreases with temperature increase around the lower critical solution temperature of PDEA in water (LCST~32 ºC) owing to the thermoresponsiveness of the PDEA shell

: Schizophrenic Behavior of a Thermoresponsive Double Hydrophilic Diblock Copolymer at the Air-Water Interface

The thermoresponsive behavior of the rhodamine B end-labeled double hydrophilic block copolymer (DHBC) poly(N,N-dimethylacrylamide)-b-poly(N,N-diethylacrylamide) (RhB-PDMA207-b-PDEA177) and the 1:1 segmental mixture of PDEA and rhodamine B end-labeled PDMA homopolymers was studied over the range of 10-40°C at the air-water interface. The increase in collapse surface pressure (second plateau regime) of the DHBC with temperature confirms the thermoresponsiveness of PDEA at the interface. The sum of the π-A isotherms of the two single homopolymers weighted by composition closely follows the π-A isotherm of the DHBC, suggesting that the behavior of each block of the DHBC is not influenced by the presence of the other block. Langmuir-Blodgett monolayers of DHBC deposited on glass substrates were analyzed by laser scanning confocal fluorescence microscopy (LSCFM), showing schizophrenic behavior: at low temperature, the RhB-PDMA block dominates the inside of bright (core) microdomains, switching to the outside (shell) at temperatures above the lower critical solution temperature (LCST) of PDEA. This core-shell inversion triggered by the temperature increase was not detected in the homopolymer mixture. The present results suggest that both the covalent bond between the two blocks of the DHBC and the tendency of rhodamine B to aggregate play a role in the formation of the bright cores at low temperature whereas PDEA thermoaggregation is responsible for the formation of the dark cores above the LCST of PDEA

Mechanistic principles and applications of resonance energy transfer

Resonance energy transfer is the primary mechanism for the migration of electronic excitation in the condensed phase. Well-known in the particular context of molecular photochemistry, it is a phenomenon whose much wider prevalence in both natural and synthetic materials has only slowly been appreciated, and for which the fundamental theory and understanding have witnessed major advances in recent years. With the growing to maturity of a robust theoretical foundation, the latest developments have led to a more complete and thorough identification of key principles. The present review first describes the context and general features of energy transfer, then focusing on its electrodynamic, optical, and photophysical characteristics. The particular role the mechanism plays in photosynthetic materials and synthetic analogue polymers is then discussed, followed by a summary of its primarily biological structure determination applications. Lastly, several possible methods are described, by the means of which all-optical switching might be effected through the control and application of resonance energy transfer in suitably fabricated nanostructures.Key words: FRET, Förster energy transfer, photophysics, fluorescence, laser