Molecular Doping, Transport and Desorption in Organic Semiconductor Films—Insights from Studies of Fluorescent Chemical Sensors

The inclusion of extrinsic dopant molecules can significantly influence the photophysical and electronic properties of organic semiconductor thin films and devices. For example, dopant molecules at low to medium densities can enhance charge transport in OLEDs, or provide recombination sites in the emissive layer. Dopants can act as charge traps or exciton quenchers, facilitating the study of transport phenomena in organic thin films. In thin film chemical sensors extrinsic analyte molecules can dope the film and modify the light emission process, for example through fluorescence quenching. The interactions and transport of dopant molecules in solution-processible optoelectronic materials are therefore important across a range of applications.

Fluorescence-quenching polymer sensors offer an attractive approach for very sensitive detection of explosives or environmental pollutants [1]. Their molecular interactions also offer a route to study the doping of extrinsic molecules through a polymer film. For example, molecules of explosives (such as TNT) present in the head space vapor surrounding a fluorescent polymer sensor can be absorbed into the polymer film and then alter the optical absorption or fluorescence. Non-covalent interactions between the fluorescent molecules and absorbed dopant can strongly affect the accumulation and transport of these dopant molecules through the film [2], which may be monitored by the change in fluorescence. Such molecular interactions can be used to engineer the detection speed and sensitivity, or provide a signature for selective analyte detection on desorption [3].

Using these fluorescence-quenching chemical sensors as a model system, we can study the introduction/removal of small molecules to/from thin solution-processed organic films. By combining spectroscopic measurements and modelling, we investigate molecular doping of thin films from vapor and solution phases, transport of the sorbed dopant molecules through the film, and their retention and thermal desorption.
We will discuss molecular doping and transport in solution-processed films of several exemplar molecular systems (1) a family of oligo- and polyfluorenes with different backbone and sidechain lengths, (2) poly(phenylene-vinylene) derivatives, and (3) polymers of intrinsic microporosity [4] which have rigid, highly-contorted backbones that render them solution processable and provide a nanometer-scale microporous structure in the solid-state. We find that side-chain structure and length in fluorene molecules significantly influence the fluorescence quenching by dinitrotoluene (DNT). Using a Stern-Volmer analysis of the time-dependent fluorescence response we can identify different regimes of molecular diffusion arising from dopant-host interactions. We also characterise the binding energy and photophysics of DNT and TNT in microporous polymer films. Through a comparison of spectroscopic measurements and density functional theory calculations, we can gain insight to molecular binding of dopants in the polymer PIM-1, and the resulting changes to the electronic properties of the doped film. We will also report measurements of dopant desorption at ambient and elevated temperatures. Through combinations of photophysical measurements and computational modelling, we can gain insight to the processes and dynamics involved when introducing extrinsic molecular dopants into thin polymer films, and their distribution and stability.

[1] S. W. Thomas, G. D. Joly, T. M. Swager, Chem. Rev. 107, 1339 (2007).
[2] P. E. Shaw, P. L. Burn, Phys. Chem. Chem. Phys. 19, 29714 (2017).
[3] E.B. Ogugu, R.N. Gillanders, S. Mohammed, G.A. Turnbull, Phys. Chem. Chem. Phys. 25, 29548 (2023).
[4] N.B. McKeown, Polymer 202, 122736 (2020).