Electron Transfer in Donor-Bridge-Acceptor Systems and Derived Materials

Abstract

Some aspects of photoinduced electron transfer (ET) in (electron donor)-bridge-(electron acceptor) compounds (D-B-A) and derived materials are investigated. Aim I is to determine how and to which extent non-conjugated double bonds in an otherwise saturated hydrocarbon bridge affect the rate of forward and backward ET and other photophysical processes. The D-B-A compounds under study possess an N,N-dialkylanilino donor and a 1,1-dicyanoethene acceptor. The bridge is of the oligo(cyclohex-1,4-diylidene) type (D[3pi3]A, D[3pi3pi3]A and D[3pi3pi3pi3]A), of the oligo(cyclohex-1,4-diyl) type (D[7]A and D[11]A), or of a mixed type (D[3pi7]A and D[7pi3]A). The bridge topology is indicated between brackets (in D[3pi7]A, donor and acceptor are separated by 3 sigma-bonds, one pi-bond and 7 sigma-bonds). Efficient intramolecular triplet-triplet energy transfer from the N,N-dialkylanilino chromophore to the alkene was observed in 1-phenyl-4-cyclohexylidenepiperidine (D[3pi3]) and 4-[4-(propan-2-ylidene)cyclohexyl]-N,N-dimethylaniline (D[4pi1]) (Chapter 2). The ground state sigma-pi and pi-pi* interactions between the donor and the acceptor in 2-[4-(1-phenylpiperidin-4-ylidene)cyclohexylidene]malononitrile (D[3pi3]A) and 2-[4-(1-phenylpiperidin-4-yl)cyclohexylidene]malononitrile (D[7]A) are found to be large in the solid state due to the formation of charge-transfer complexes but small and conformation dependent in the gas phase (Chapter 3). The replacement of an exocyclic C-C single bond by a double bond increases the rate of photoinduced ET with a factor of 3.0 +/-0.8 per replaced bond (Chapter 4). For all compounds the extended, fully charge-separated conformer folds to a more compact one of which the rate of charge recombination increases with solvent polarity only for those compounds having an olefinic bond at three sigma-bonds from the acceptor (D[3pi]A, D[3pi3pi3]A and D[7pi3]A) possibly due to superexchange via a virtual charge-separated state D[Xpi·+3]A·-. Aim II is to incorporate D-B-A moieties in materials and to study the effect thereof on the ET process. The syntheses, electronic and photophysical properties of five D-B-A compounds ({4-[4-(dimethylamino)phenyl]cyclohexylidene}malononitrile (1), methyl cyano{4-[4-(dimethylamino)phenyl]cyclohexylidene}acetate (2), dimethyl {4-[4-(dimethylamino)phenyl]cyclohexylidene}malonate (3), didodecyl {4-[4-(dimethylamino)phenyl]cyclohexylidene}malonate (4) and 2,2'-({4-[4-(dicyanomethylene)cyclohexyl]phenyl}imino)diethyl diacetate (5)) that display photoinduced ET are given (Chapter 5). They contain an N,N-dialkylaniline electron donor and a 1-R2-1-R3-functionalized ethene acceptor, which are separated by a semirigid four ?-bond saturated hydrocarbon bridge. For 1 and 5, the formed charge separated state folds in apolar solvents to give a compact charge-separated conformer. In Chapter 6 the solution polymerization and photophysical properties of two high molecular weight polyesters are described. Electron donor polymer 6 (from 2,2'-(phenylimino)diethanol and adipoyl chloride) displays both excimer and local emission. Polymer 7 (from (4-{4-[bis(2-hydroxyethyl)amino]phenyl}cyclohexylidene)malononitrile and adipoyl chloride), incorporates the D-B-A moiety of 5 in the repeating unit, is insoluble in common solvents and displays charge-transfer absorption and emission in the solid state. This is explained in terms of charge transfer between different D-B-A moieties (whether inter- or intrachain). Two D-B-A compounds (bis[11-(methylsulfanyl)undecyl]{4-[4-(dimethylamino)phenyl]cyclohexylidene} malonate (8) and bis[11-(decylsulfanyl)undecyl]{4-[4-(dimethylamino)phenyl]cyclohexylidene} malonate (9)) that form self-assembled monolayers (SAMs) on a Au(111) surface by preferential (8) or possibly preferential (9) adsorption via their dialkyl sulfide moieties were synthesized (Chapter 7). On both SAMs a dense monolayer of gold nanoparticles (16 nm diameter) could be deposited. These nanoparticles in turn attach to the N,N-dimethylaniline moieties pendent in solution, forming a Au|D-B-A|Au structure.