Abstract
The photophysics of water and propan-1-ol suspensions of poly [N-9”-heptadecanyl-2,7-carbazole-alt−5,5-(4,7-di-2-thienyl-2′,1′,3′- benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles has been studied by ultrafast spectroscopy. High molar mass polymer (HMM > 20 kg/mol) forms nanoparticles with around 50 nm diameter via mini-emulsion post-polymerization, while low molar mass (LMM < 5 kg/mol) polymer prepared by dispersion polymerization results in particles with a diameter of almost one order of magnitude larger (450 ± 50 nm). In this study, the presence of excited-states and charge separated species was identified through UV pump and visible/near-infrared probe femtosecond transient absorption spectroscopy. A different behavior for the HMM nanoparticles has been identified compared to the LMM mesoparticles. The nanoparticles exhibit typical features of an energetically disordered conjugated polymer with a broad density of states, allowing for delayed spectral relaxation of excited states, while the mesoparticles show a J-aggregate-like behavior where interchain interactions are less efficient. Stimulated emission in the red-near infrared region has been found in the mesoparticles which indicates that they present a more energetically ordered system.
Highlights
Semiconducting polymer particles and nanoparticles with optical properties are of considerable importance for applications in a diverse range of technologies including light-emitting diodes (Wong, 2017), solar cells (Zhou et al, 2014; Jana et al, 2017), biosensing (Wu and Chiu, 2013; Chan and Wu, 2015; Xu et al, 2015), and cancer phototherapy (Li et al, 2018; Meng et al, 2018)
We have found that the high molar mass (HMM) nanoparticles exhibit typical features of an energetically disordered conjugated polymer with a broad density of states, allowing for delayed spectral relaxation of excited states, while the low molar mass (LMM) mesoparticles show a J-aggregate-like behavior where interchain interactions are less efficient
The absorption spectrum consists of several broad bands, which are ascribed to the lowest π-π∗ interband transition, as is commonly observed in donor-acceptor copolymers of similar chemical structure (Westerling et al, 2007)
Summary
Semiconducting polymer particles and nanoparticles with optical properties are of considerable importance for applications in a diverse range of technologies including light-emitting diodes (Wong, 2017), solar cells (Zhou et al, 2014; Jana et al, 2017), biosensing (Wu and Chiu, 2013; Chan and Wu, 2015; Xu et al, 2015), and cancer phototherapy (Li et al, 2018; Meng et al, 2018). By way of contrast decreasing nanoparticle size has led to a red shift in emission spectra of a poly(p-phenylene vinylene) (PPV) derivative (Sun et al, 2014), an observation that is not supported by experiments in which a different PPV derivative were allowed to aggregate in solution (Grey et al, 2006). Aggregated particles are expected to behave differently to individual particles and one can suppose that chain conformation and packing within the particles play an important role in their optoelectronic behavior. This conclusion was reached by considering size-dependent brightness, which increased with increasing particle size less than would be expected by considering the number of fluorophores present in the volume (Sun et al, 2014). Experiments reporting the crystalline behavior of poly(3-hexylthiophene) nanoparticles showed that chain ordering controlled the optical behavior of these polymers (Labastide et al, 2011; Nagarjuna et al, 2012)
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