A photophysical study of substituted arylethynylenes

Abstract

Molecules based on the 1,4-bis(phenylethynyl)benzene (BPEB) architecture are an important class of compounds because of their luminescent properties. The rigid rod-like backbone and extended π-conjugation serves as the base upon which a wide variety of substituted species can be synthesised with a variety of photophysical properties. This thesis represents a detailed study of the photophysical properties of a series of compounds based on BPEB using a combination of steady-state and time- resolved spectroscopic methods. A comparative study of the photophysical properties of BPEB, 1,2,4,5-tetramethyl- 3,6-bis(phenylethynyl)benzene, l,2,4,5-tetramethyl-3,6-bis-(2,4,6-trimethylphenyl- ethynyl)benzene, 1,4-bis(2-(2-tórř-butylphenyl)ethynyl)benzene, 1,4-bis(2-(2-tert- butylphenyl)ethynyl)-2,3,5,6-tetramethylbenzene, 1,4-dibromo-2,5-bis(phenylethy- nyl)benzene and l,2,4,5-tetrafluoro-3,6-bis(phenylethynyl)benzene is reported. These compounds exist as a series of geometric conformers, in solution, in the ground state due the low barrier to rotation about the carbon-carbon triple bond (C≡C). The absorption and emission dipoles of BPEB are found to be almost linear and lie along the long axis of the molecule. The substituents red shift the absorption and emission spectra of BPEB. l,4-Bis(2-(2-tert-butylphenyl)ethynyl)-2,3,5,6- tetramethylbenzene, exhibits fluorescence from both high and low energy conformations at 77 K. Only l,4-dibromo-2,5-bis(phenylethynyl)benzene has a long lived phosphorescence emission detectable at 77 K. Time-resolved Raman spectroscopy reveals that the C≡C stretching vibrations of BPEB, and its substituted derivatives in the excited singlet and triplet states, which shows evidence of only minor structural changes upon excitation. The photophysical properties, and solvatochromism, of a series of compounds having donor and acceptor groups bridged by BPEB are reported. They are methyl- 4-(4-(4-dimethylaminophenylethynyl)phenylethynyl)benzoate, 4-(4-(4-dimethyl- aminophenylethynyl)phenylethynyl)benzonitrile, 4-(4-(4-dimethylammo-3,5-dimeth- ylphenylethynyl)phenylethynyl)benzonitrile, 4-(4-(4-dimethylaminophenylethynyl)- phenylethynyl)-N,N-dimethylbenzenamine, 4-(4-(4-cyanophenyIethynyl)phenylethy֊nyl)benzonitrile, 4-(4-(4-dimethylaminophenylethynyl)phenylethynyl)benzene, 4-(4- (4-cyanophenylethynyl)phenylethynyl)benzene and methyl-4-(4-phenylethynyl)- phenylethynylbenzoate. The donor-acceptor systems exhibit intramolecular charge transfer (ICT) and twisted internal charge transfer in the excited state, when in polar solvents. In non-polar solvents only the locally excited state is observed. The largest change in dipole moment in the excited state occurred when the dimethylamino group was fixed perpendicular to the adjacent phenyl ring. In protic solvents, hydrogen bonds to the amino group in the ICT state to form a hydrogen- bonded ICT state. The photophysical properties of 9,10-bis(phenylethynyl)anthracene (BPEA) and a series of nine of its donor and acceptor substituted derivatives are reported. Compounds with the strongest donor (dimethylamino) and acceptor (nitro) groups exhibit ICT. All substituted BPEA compounds had lower fluorescence quantum yields and lifetimes, and higher non-radiative decay constants than BPEA. The molecular and photophysical properties of 2,5-bis(phenylethynyl)thiophene, 1,4-bis(2-thienylethynyl)benzene and 2,5-bis(2-thienylethynyl)thiophene were investigated with specific emphasis on the bonding character of their triplet excited states. Like the parent, BPEB, these compounds exhibit evidence of triple bond character in the excited singlet and triplet states.