Oligo(p-phenyleneethynylene) (OPE) molecular wires: synthesis and length dependence of photoinduced charge transfer in OPEs with triarylamine and diaryloxadiazole end groups

Abstract

The systematic synthesis and photophysical, electrochemical and computational studies on an extended series of triphenylamine-[C≡C-1,4-C6H2(OR)2]n-C≡C-diphenyl-1,3,4-oxadiazole dyad molecules (the OR groups are at 2,5-positions of the para-phenylene ring and R = C6H13; n = 0-5, compounds 1, 2, 3, 4 and 5, respectively) are reported. Related molecules with identical end groups, triphenylamine-C≡C-1,4-C6H2(OR)2-C≡C-triphenylamine (R = C6H13; 6) and diphenyl-1,3,4-oxadiazole-[C≡C-C6H2(OR)2]2-C≡C-diphenyl-1,3,4-oxadiazole (R = C6H13; 7) were also studied. These D-B-A 1-5, D-B-D 6 and A-B-A 7 (D = electron donor, B = bridge, A = electron acceptor) systems were synthesized using palladium-catalysed cross-coupling reactions of new p-phenyleneethynylene building blocks. Steady-state emission studies on the dyads 1-5 reveal a complicated behavior of the emission that is strongly medium dependent. In low polarity solvents the emission is characterized by a sharp high-energy peak attributed to fluorescence from a locally excited (LE) state. In more polar environments the LE state is effectively quenched by transfer into an intramolecular charge-transfer (ICT) state. The medium dependence is also observed in the quantum yields (QYs) which are high in cyclohexane and low in acetonitrile, thus also indicating charge-transfer character. Low-temperature emission spectra for 2-5 in dichloromethane and diethyl ether also reveal two distinct excited states, namely the LE state and the conventional ICT state, depending on solvent and temperature. Hybrid DFT calculations for 1-7 establish that the OPE bridge is involved in both frontier orbitals where the bridge character increases as the bridge length increases. Computed TD-DFT data on 1-5 assign the emission maxima in cyclohexane as LE transitions. Each time-resolved emission measurement on 2-7 in cyclohexane and diethyl ether reveals a wavelength dependent bi-exponential decay of the emission with a fast component in the 5-61 ps range on blue detection and a slower approximately 1 ns phase, independent of detection wavelength. The fast component is attributed to LE fluorescence and this emission component is rate limited and quenched by transfer into an ICT state. The fast LE fluorescence component varies systematically with conjugation length for the series of D-B-A dyads 2-5. An attenuation factor β of 0.15 Å(-1) was determined in accordance with an ICT superexchange mechanism.

Keywords: charge-transfer; molecular wires; oligo(phenyleneethynylene); oxadiazole; triarylamine.