Synthesis of Functionalized RAFT Agents for Light Harvesting Macromolecules

Abstract

With the rapid development of nanotechnology, methodologies that will enable the cost-effective, controlled assembly of nanostructures in a routine manner are in high demand. Photoactive polymers are promising candidates to fulfill the materials requirements for energy storage and conversion devices, molecular sensors, and photonic materials. 1 Of particular interest is the utilization of efficient energy transfer processes in a polymer chain containing sequences of donor chromophores that absorb the incident light and the subsequent trapping of the energy in suitably placed acceptor species. 2 Such "antenna" polymers simulate the efficient light harvesting process of pigment arrays in the photosynthetic apparatus of green plants and can act to effectively increase the light absorption cross section of components in photomolecular devices.The synthesis of photoactive polymers with welldefined architectures has long been of great interest to photochemists but is difficult to achieve by conventional free radical polymerization methods. Living radical polymerization has made great progress in recent years and emerged as one of the most effective synthetic routes to make well-defined polymers. 3 Among them, reversible addition-fragmentation chain transfer (RAFT) polymerization appears particularly useful and, in principle, could be applied to all classical radical polymerization systems.The RAFT process involves performing a conventional free radical polymerization in the presence of certain thiocarbonylthio compounds SdC(Z)-SR that act as highly efficient reversible addition-fragmentation chain transfer agents and provide the polymerization with living characteristics. Following RAFT polymerization, nearly all polymer chains will have the thiocarbonylthio and R as end groups. RAFT polymerization thus provides a means of introducing specifically placed photoactive moieties into polymer chains using appropriately functionalized RAFT agents.We have previously incorporated chromophores into RAFT agents by the coupling reaction of 4-cyano-4-((thiobenzoyl)sulfanyl)pentanoic acid (RAFT-acid) with the required hydroxy-functionalized chromophores. 4 However, the stability of the resulting ester linkage can limit the applications in a number of circumstances, such as in the presence of acid or base. Although the ester linkage could be replaced by a more stable amide linkage, this general method is limited by the low yield of the coupling reaction between RAFT-acid and aminefunctionalized chromophores due to the competing aminolysis reaction of the thiocarbonylthio group by the amine.We describe here a novel alternative method to introduce suitable chromophores into a RAFT agent. The overall process and mechanism are outlined in Scheme 1. This procedure involves performing a RAFTlike polymerization reaction, except the molar ratio of monomer to RAFT agent is kept at unity. A small amount (1-2% molar equivalent) of free radical initiator is used to initiate the reaction. In our work 2-cyanoprop-2-yl dithiobenzoate 5 and 2,2′-az...

Living Radical Polymerization by the RAFT Process - A Second Update

“…[425] The success of this process relies on the addition of R to the monomer being substantially faster than subsequent monomer additions and requires a very active RAFT agent such that the number of monomer additions per activation cycle is greater than 1. It requires selective initialization (see section on Mechanism of RAFT above).…”

SFRP Synthesis of Acenaphthylene Oligomers and Block Copolymers. Potential Light Harvesting Structures

“…Oligomer Analysis by 1 H NMR Spectrometry − The 1 H NMR spectra (400 MHz, Figure 2S in Supplementary Information) show characteristic peaks in the aromatic region 6.9-8.2 ppm corresponding to H 1 -H 12 for the azobenzene and phenyl moieties, and H 27-33 for the AcNp repeat units; aliphatic protons H [14][15][16][17][18][19][20][21][22][23][24] are found at 0.6-2.0 ppm. As the maximum number of repeating acenaphthylene monomer units n increases, broadening of signals in both the aromatic and the aliphatic region increases and is in accordance with formation of higher molecular weight oligomers, as shown through MALDI-MS and GPC results.…”

“…This is illustrated in Scheme 1: Donor D groups absorb light energy which is then transferred from the excited D (D*) to the acceptors A; D groups in the case of polymers and particularly peripherally-substituted starburst or dendrimeric polymers, have been likened to "antennae." 12 Various strategies have been used to approach controlled radical polymerization to give well-defined structure, [13][14][15][16] including Atom Transfer Radical Polymerization (ATRP), 17 Radical Addition Fragmentation (RAFT), [18][19][20] and Nitroxide Mediated Polymerization (NMP), 13 also termed Stable Free Radical Polymerization (SFRP).…”

The Radical Chemistry of Thiocarbonylthio Compounds: An Overview

“…Finally, an instance of addition of a dithioester, 103, to a coumarin-substituted styrene is also provided in Scheme 5.27. The resulting adduct 104 was used in the synthesis of light-harvesting macromolecules [33].…”