The effects of supramolecular assembly on exciton decay rates in organic semiconductors

Abstract

We present time-resolved photoluminescence measurements on two series of oligop-phenylenevinylene ͑OPV͒ materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs ͑MOPVs͒ form chiral, helical stacks while bifunctionalized OPVs ͑BOPVs͒ form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of -conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

Mesoscopic order and the dimensionality of long-range resonance energy transfer in supramolecular semiconductors

“…4 displays the PL decay kinetics at these detection photon energies of various blend solutions with mole fraction of acceptor ranging from 0% to 10%. BOPV3 displays non-exponential decay kinetics over all timescales investigated here, 30 while MOPV3 displays exponential decay kinetics after ∼ 2 ns (the time window that was used for the fit procedure). In order to extract R and ∆ from the data of the three blends, we first fitted the MOPV3 and BOPV3 decays from the 0% mole fraction solutions with a stretched-exponential function (I(t) = a exp(−kt −h )).…”

“…Note that h = 0.6 can be related to diffusion-assisted exciton-quenching at defects on a three-dimensional lattice. 27,30 We then applied a global fit to PL decays at all different MOPV4 and BOPV4 mole fraction with…”

“…In previous work, we explored the extrinsic consequences of intermolecular coupling, namely diffusion-assisted exciton transfer and quenching and exciton bimolecular annihilation at high exciton densities. 26,27,28,29,30,31,32,33 We have demonstrated that RET between MOPV derivatives of different length (and exciton energies) is greatly enhanced by supramolecular assembly. At low MOPV4 mole fraction ( 2%), isolated MOPV4 chromophores are incorporated into MOPV3 helical assemblies as long as the solution is thermally cycled to dissolve and then re-assemble the stacks.…”

“…We applied time-correlated single photon counting (TCSPC) to measure excited-state lifetimes and photoluminescence (PL) spectra as described elsewhere. 30 The excitation source was a pulsed diode laser (PicoQuant LDH400, 20 MHz, 70 ps FWHM, 407 nm (3.05 eV)). The luminescence was detected with a microchannel plate photomultiplier (Hamamatsu) coupled to a spectrometer and TCSPC electronics (Edinburgh Instruments Lifespec-ps and VTC900 PCI card).…”

Monte Carlo Simulation of Exciton Bimolecular Annihilation Dynamics in Supramolecular Semiconductor Architectures

“…Note that even in this weak limit, the interchromophore couplings modify the excitonic levels and therefore the intrinsic radiative and nonradiative deactivation rates. 31 In order to explore the applicability of the line-dipole approximation in more closely packed organic semiconductors, we have applied ultrafast spectroscopy on isolated supramolecular nanoassemblies in solution, built by self-assembly of oligo-p-phenylenevinylene derivative monofunctionalised with ureido-s-triazine (MOPV; see Figure 1 for the molecular structure). 32,33 These molecules dimerize by quadruple hydrogen bonding in nonpolar solution.…”