Size-Dependent Photophysical Behavior of Low Bandgap Semiconducting Polymer Particles

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.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

“…The absorption spectrum of the continuous polymer film (red line) exhibits two broad transitions with peaks at 390 (3.19 eV) and at 550 nm (2.26 eV). In agreement with earlier work [ 24 , 27 , 28 ] the two bands are identified as the π–π*-transition of the first and second excited singlet states (S 1 and S 2 ). The emission spectrum shows a broad and unstructured emission between 620 (2 eV) and 920 nm (1.35 eV) (red line), peaking at 703 nm (1.77 eV).…”

“…The left panel of Figure 2 presents the pump-probe spectra at different probe delays for the continuous polymer film after excitation at 400 nm ( Figure 2 a) and at 610 nm ( Figure 2 c). After excitation at 400 nm the pump-probe spectrum shows the presence of an initial bleaching band centred around 550 nm (low energy peak in the absorption spectrum) and a photoinduced absorption band between 650 nm and 780 nm attributed to charged states [ 24 ]. In the first ~10 ps a red shift of the bleaching is present ( Figure 2 a, red line) indicating an energy migration towards the lower energetic sites of the absorption spectrum.…”

“…The nanoparticle film is first considered. After excitation at 400 nm, the pump-probe spectrum shows the presence of an initial bleaching band centred at around 570 nm (low energy peak in the absorption spectrum) and a photoinduced absorption band at 700–800 nm attributed elsewhere to charged states [ 24 ]. The spectrum is similar to the one of the polymer film even if the dynamics show an instantaneous charge formation (blue line in Figure 4 a) indicating a more efficient process.…”

“…Nevertheless, it could be reasonably argued that nanoparticle films convey some advantages because confinement inherent in the particles allows some element of dilution by enhancing intrachain properties due to the extended conjugation length with respect to interchain interactions. However, transient absorption spectroscopy revealed no evidence of stimulated emission in an aqueous nanoparticle suspension of poly[ N -9″-heptadecanyl-2,7-carbazole- alt −5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) [ 24 ]. In contrast, a mesoparticle suspension of PCDTBT in propanol did show stimulated emission in the infrared region of the spectrum.…”

Synthesis, characterization, and biological applications of semiconducting polythiophene‐based nanoparticles

“…It is interesting to note that differently from the emission the absorption spectra of PT‐NPs are dependent on their size. More in general, the smaller NPs show blue‐shifted structureless spectra while the bigger ones exhibit well‐structured and strong red‐shifted spectra 50,51,85,86 …”