Caracterização fotofísica dos derivados de Carbazol e Tetrazina para sensoriamento de radiação ultravioleta

Abstract

This work investigates the photophysical properties and charge transport mechanisms in donor–acceptor conjugated polymers derived from carbazole and tetrazine. The ma terials analyzed were synthesized at the Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Paraná (UFPR). The polymer LaPPS80 is based on the carba zole–tetrazine combination (2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-9-(tridecan 7-yl)-9H-carbazole- alt-3,6-bis(4-octylthiophen-2-yl)-1,2,4,5-tetrazine), whereas LaPPS81 is derived from condensed carbazole–tetrazine (5,11-bis(2-ethylhexyl)dihydroindolo[3,2 b]-carbazole-alt- 3,6-bis(4-octylthiophen-2-yl)-1,2,4,5-tetrazine). It is worth noting that LaPPS81 can be obtained through different synthetic routes and is therefore denoted throughout this work as LaPPS81-C and LaPPS81-S. This study proposes a novel application of these materials as selective photodetectors in the ultraviolet–visible region. Spectroscopic characterization by optical absorption, photoluminescence (PL), and photoluminescence excitation (PLE) revealed that the transition from solution to the solid state (thin films) induces strong intermolecular interactions. These interactions are evidenced by Stokes shifts on the order of approximately 59 nm, 101 nm, and 139 nm, as well as by pronounced red-shifts in the emission spectra. These results indicate the formation of molecular aggregates and delocalized excited states, which play a key role in charge transport in these polymeric systems. Photoresistor devices with planar architecture and interdigitated fluorine-doped tin oxide (FTO) electrodes were fabricated and characterized. Morphological and electrical analyses showed that the oligomer LaPPS81-C, due to its low molar mass, does not form efficient percolation networks. In contrast, the polymers LaPPS80 and LaPPS81-S yielded func tional devices with distinct and complementary physical behaviors. LaPPS80 exhibited maximum sensitivity at 450 nm but showed the anomalous phenomenon of negative photoconductivity (NPC) under UV excitation (280–370 nm), attributed to the activation of deep recombination centers by high-energy photons. LaPPS81-S demonstrated positive photoconductivity, with high selectivity at 370 nm, and notable anisotropy in the response to light polarization, indicating a macroscopic molecular alignment that favors directional charge transport. Electrical stability tests (20 voltage sweep cycles) indicated the robustness of the devices, which maintained ohmic behavior without significant degradation or hysteresis. Analysis of the I–V curves suggested the predominance of direct tunneling injection mechanisms at low electric fields, with a transition to regimes consistent with field emission at higher voltages. Impedance spectroscopy corroborated the modulation of bulk resistance under illumination, indicating that LaPPS81 exhibits higher intrinsic conductivity compared to LaPPS80. Taken together, these results suggest that the LaPPS family holds potential for application in selective optical sensors, with responses that can be tuned through molecular engineering and morphological control.