Fluorescent Polystyrene Films for the Detection of Volatile Organic Compounds Using the Twisted Intramolecular Charge Transfer Mechanism

Thin films of styrene copolymers containing fluorescent molecular rotors were demonstrated to be strongly sensitive to volatile organic compounds (VOCs). Styrene copolymers of 2-[4-vinyl(1,1'-biphenyl)-4'-yl]-cyanovinyljulolidine (JCBF) were prepared with different P(STY-co-JCBF)(m) compositions (m% = 0.10-1.00) and molecular weights of about 12,000 g/mol. Methanol solutions of JCBF were not emissive due to the formation of the typical twisted intramolecular charge transfer (TICT) state at low viscosity regime, which formation was effectively hampered by adding progressive amounts of glycerol. The sensing performances of the spin-coated copolymer films (thickness of about 4 µm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and favourable interaction with the polymer matrix such as THF, CHCl₃ and CH₂Cl₂. The vapochromic response was also reversible and reproducible after successive exposure cycles, whereas the fluorescence variation scaled linearly with VOC concentration, thus suggesting future applications as VOC optical sensors

Vapochromic features of new luminogens based on julolidine-containing styrene copolymers

We report on vapochromic films suitable for detecting volatile organic compounds (VOCs), based on new polystyrene copolymers containing julolidine fluorescent molecular rotors (JCAEM). Poly(styrene-co-hydroxyethylmethacrylate) copolymers functionalized with cyanovinyl-julolidine moieties of different compositions were prepared, (P(STY-co-JCAEM)(m) with m = 0.06-0.61). The sensing performance of the spin-coated copolymer films demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and a favourable interaction with the polymer matrix, such as Et2O and CH2Cl2. It is worth mentioning that the fluorescence decrease rate was 7 times faster than that of previously investigated julolidine-based fluorescent molecular rotors dispersed in PS films. This phenomenon was attributed to the better control of the JCAEM moiety distribution in the polymer matrix conferred by the covalent approach, combined with a minimal film thickness of 4 microns. These factors, in concert, strongly accelerate the deactivation pathways of the JCAEM units in the presence of VOCs which interact well. Overall, the present results support the use of julolidine-enriched styrene copolymers as effective chromogenic materials suitable for the fast detection of VOCs

Imidazole-Fused Enediynes by Selective C5–C4 Alkynylations of 4,5-Dibromoimidazoles

An efficient synthesis of symmetrical 1,2-disubstituted 4,5-dialkynylimidazoles by Sonogashira alkynylation of the corresponding 4,5-dibromo derivatives was developed. Moreover, through a careful tuning of the palladium ligand, unsymmetrical 1,2-disusbtituted 4,5-dialkynylimidazoles were also prepared through a regioselective C5 alkynylation of 4,5-dibromoimidazoles, followed by a second alkynylation involving the 4-bromo derivatives so obtained. This interesting class of imidazole-fused enediynes is also able to give thermal Bergman cycloaromatization (BC), as proved by DSC experiments

“N-alkyl diketopyrrolopyrrole-based fluorophores for luminescent solar concentrators: effect of the alkyl chain on dye efficiency”

We report on the preparation of luminescent solar concentrators (LSCs) made of poly(methyl methacrylate) (PMMA) thin films doped with six new diketopyrrolopyrrole (DPP) fluorophores obtained in good yields by using simple N-alkylation and direct C-H arylation synthetic strategies. Spectroscopic investigations in solution and in PMMA thin films combined with photocurrent measurements revealed that the branched alkyl chains were efficient in preventing DPP segregation from the PMMA matrix thanks to their higher steric hindrance. The aromatic substituent was found to expand DPP conjugation but favoured DPP adverse aggregation, thus affecting fluorescence emission and photocurrents of PMMA films. The worthwhile combination of the appropriate alkyl chain and aromatic moieties assured to achieve optical efficiency of 6.8% that was comparable to that of PMMA LSCs of the state-of-the art (7.2%)

Tuning of dye optical properties by environmental effects: a QM/MM and experimental study

The present work is aimed to a deeper investigation of two recently synthesized heteroaromatic fluorophores by means of a computational multilayer approach, integrating quantum mechanics (QM) and molecular mechanics (MM). In particular, dispersion of the title dyes in a polymer matrix is studied in connection with potential applications as photoactive species in luminescent solar concentrators (LSCs). Molecular dynamics simulations, based on accurate QM-derived force fields, reveal increased stiffness of these organic dyes when going from CHCl3 solution to polymer matrix. QM/MM computations of UV spectra for snapshots extracted from MD simulations show that this different flexibility permits to explain the different spectral shapes obtained experimentally for the two different environments. Moreover, the general spectroscopic trends are well reproduced by static computations employing a polarizable continuum description of environmental effects

Reversible vapochromic response of polymer films doped with a highly emissive molecular rotor

We report on a new vapochromic system suitable for sensing volatile organic compounds (VOCs) based on polymer films doped with 4-(diphenylamino)phthalonitrile (DPAP), a fluorescent molecular rotor sensitive to both solvent polarity and viscosity. Poly(methyl methacrylate) (PMMA) and polycarbonate (PC) films containing small amounts of DPAP (#0.1 wt%) were prepared and exposed to saturated atmospheres of different VOCs. DPAP/PMMA films show a good and reversible vapochromism when exposed to VOCs with high polarity index and favourable interaction with polymer matrices such as THF, CHCl3, and acetonitrile. Analogously, DPAP/PC films exposed to polar and highly polymer-interacting solvents, that is, toluene, THF, and CHCl3, show a gradual decrease and red-shift of the emission. In contrast to DPAP/ PMMA films, an unexpected increase and further red-shift of fluorescence are observed at longer exposure times as a consequence of an irreversible, solvent-induced crystallization process of PC. The vapochromism of DPAP-doped films is rationalized on the basis of alterations of the rotor intramolecular motion and polarity effects stemming from the environment, which, in concert, influence the deactivation pathways of the DPAP intramolecular charge transfer state. Overall, the present results support the use of DPAP-enriched plastic films as a new chromogenic material suitable for the detection of VOCs

Toward the design of alkynylimidazole fluorophores: computational and experimental characterization of spectroscopic features in solution and in poly(methyl methacrylate)

The possibilities offered by organic fluorophores in the preparation of advanced plastic materials have been increased by designing novel alkynylimidazole dyes, featuring different push and pull groups. This new family of fluorescent dyes was synthesized by means of a one-pot sequential bromination–alkynylation of the heteroaromatic core, and their optical properties were investigated in tetrahydrofuran and in poly(methyl methacrylate). An efficient in silico pre-screening scheme was devised as consisting of a step-by-step procedure employing computational methodologies by simulation of electronic spectra within simple vertical energy and more sophisticated vibronic approaches. Such an approach was also extended to efficiently simulate one-photon absorption and emission spectra of the dyes in the polymer environment for their potential application in luminescent solar concentrators. Besides the specific applications of this novel material, the integration of computational and experimental techniques reported here provides an efficient protocol that can be applied to make a selection among similar dye candidates, which constitute the essential responsive part of those fluorescent plastic materials

Preparazione e caratterizzazione di film a risposta ottica mediante dispersione di fluorofori organici in matrici polimeriche termoplastiche

Il presente lavoro di tesi consiste nella valutazione della risposta ottica nei confronti di stimoli esterni (temperatura, stress meccanici, agenti chimici o radiazione luminosa) di alcune miscele polimeriche costituite da matrici polimeriche commerciali, modificate con l’aggiunta di coloranti di natura organica. A seguito di sollecitazioni di varia natura, questi materiali compositi producono una variazione delle loro proprietà ottiche quali assorbimento, emissione e indice di rifrazione. I coloranti usati sono di natura aromatica ed eterociclica Le matrici polimeriche adoperate sono principalmente polimeri termoplastici di natura poliolefinica. Una prima fase del lavoro di tesi consiste nello studio delle proprietà ottiche dei coloranti in soluzione e nella determinazione delle loro variazioni a seguito di sollecitazioni esterne di varia natura. La seconda fase consiste nella preparazione di film polimerici mediante dispersione dei coloranti nella matrice polimerica in soluzione seguita da processi di “film casting” e, in alcuni casi, pressofusione Infine, attraverso misure spettroscopiche, sarà valutata la risposta ottica di tali sistemi a seguito di sollecitazioni meccaniche, chimiche e luminose

Fluorescent vapochromism in synthetic polymers

The detection of volatile organic compounds (VOCs) by means polymeric materials through fluorescence variations has attracted considerable interest over the past 10 years. The detection of VOCs is of great concern because of their undesirable impact on both the environment and human health. Moreover, the sensing of vapours of organic compounds such as explosives is one pressing issue in anti-terrorism and homeland security. In this mini-review, the current state of knowledge concerning the preparation of fluorescent polymers with vapochromic features is summarized. Two types of procedures are illustrated and thoroughly discussed along with their respective structure-property relationships: the first resides on the covalent insertion of chromophoric units into the macromolecule backbone or side chains; the second involves the physical dispersion of the fluorophore in a preformed polymer matrix. Herein we review the simplicity of the preparative routes available, and their influence over the properties of the resulting fluorophore-polymer systems, by focusing on the most illustrative examples described in the literature. Finally, conclusive remarks and future perspectives are presented and discussed

Hue-based quantification of mechanochromism towards a cost-effective detection of mechanical strain in polymer systems

HSV is a digital colour space easily accessible by the transformation of RGB images. In this work, the hue parameter H was used to assess mechanically induced colour changes of the aggregation-sensitive fluorescent dye 4,4′-bis-(2-benzoxazolyl)stilbene (BBS), thus implementing a cheap and reliable method for the detection of mechanical deformation in a polymer matrix