Abstract
Perylenediimide (PDI) compounds with no substituents in their core are widely used as the active units of thin-film organic lasers. Recently, bay-substituted PDIs (b-PDIs) bearing two sterically hindering diphenylphenoxy groups at the 1,7-bay positions have received great attention because they show red-shifted emission with respect to bay-unsubstituted PDIs, while maintaining high photoluminescence (PL) quantum yields and low amplified spontaneous emission (ASE) thresholds even at high doping rates. However, their ASE photostability is relatively low compared to that of state-of-the-art PDIs. Thus, the design of b-PDIs with improved ASE photostability remains a challenge. Here, the synthesis of two b-PDIs with phenyl-type substituents at the imide positions is reported. Complete characterization of their optical properties, including absorption, PL, ASE, and transient spectroscopy, supported also by quantum chemical calculations, is performed with the dyes diluted in either a liquid solvent or a polystyrene film. Film experiments were accomplished at very low doping rates, to resemble the isolated molecule behavior, and also in a range of increasing doping rates, to investigate concentration quenching effects. The reported b-PDIs show improved ASE photostability (3-fold) with respect to b-PDIs with aliphatic-type substituents at the imide positions, whilst they show more propensity toward aggregation.
Highlights
Organic materials have been demonstrated to be suitable media for laser applications.[1,2] the large oscillator strength of electronic transitions associated with the excitation of π-electrons, the adoption of solution-based methods for lowcost integration with other technologies,[3] and the lower toxicity of organic materials compared to their inorganic counterparts[4] make them attractive for practical applications.[5,6] In addition, the bottom-up synthesis techniques used in organic chemistry allow for fine control of the optoelectronic properties of the organic materials through the design of the molecular structure.[7]
Prior to analyzing the experimental optical properties, the structural properties of the bay-substituted baysubstituted PDIs (b-PDIs) derivatives were studied from a computational perspective by means of density functional theory (DFT) and semiempirical (GFN2-xTB) calculations
The three b-PDI derivatives exhibit PDI cores slightly distorted from planarity due to the diphenylphenoxy groups attached to the 1,7-bay positions
Summary
Organic materials have been demonstrated to be suitable media for laser applications.[1,2] the large oscillator strength of electronic transitions associated with the excitation of π-electrons, the adoption of solution-based methods for lowcost integration with other technologies,[3] and the lower toxicity of organic materials compared to their inorganic counterparts[4] make them attractive for practical applications.[5,6] In addition, the bottom-up synthesis techniques used in organic chemistry allow for fine control of the optoelectronic properties of the organic materials through the design of the molecular structure.[7]. Different synthetic strategies have been developed to improve the properties of PDIs toward lowering the optical pump energy needed (i.e., the threshold) for amplified spontaneous emission (ASE) and laser operation, and to tune the emission wavelength of PDIs. In a first step, substitution at the imide positions, leaving the bay positions of the perylene core unsubstituted (i.e., bay-unsubstituted or u-PDIs), intended to improve PDI solubility and increase the amount of PDI that could be incorporated into the polymer matrix without significant photoluminescence (PL) quenching.[12−14] Following this strategy, the best results in terms of energy threshold were obtained for a 2,6-diisopropylphenyl-substituted PDI (PDI-O) dispersed in a polystyrene (PS) matrix at 1 wt % (with a 2−3 kW cm−2 threshold).[12] Even so, for concentrations typically above ∼1 wt %, molecular interactions/aggregates appear, leading to concentration-induced quenching and the associated reduction of the emission efficiency.[12,15−17] the absorption and emission spectra of these derivatives remain unchanged with respect to that of the unsubstituted PDI core because of the electronic nodal character of the imide nitrogens.[7] This constitutes a handicap since it limits the Received: January 29, 2021 Revised: April 29, 2021 Published: May 28, 2021
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