A prototype for a new class of fluorescent molecular rotors (FMRs), namely 4-(diphenylamino) phthalonitrile (DPAP), was synthesized and its sensitivity towards solvent polarity and viscosity probed using photophysical and computational methods. DPAP is characterized by a pronounced fluorosolvatochromism in polarity-dependent absorption, emission and fluorescent lifetime experiments.At the same time, a strong viscosity response is observed, especially in polar and protic solvents.Quantum mechanical calculations assisted in interpreting the unusual solvent sensitivity of DPAP in terms of its high flexibility, giving rise to solvent-independent, barrier-free rotations. As a matter of fact, the modus operandi in DPAP contrasts that of traditional FMRs involving twisted intramolecular charge transfer (TICT) states. The influence of this unusual flexible character on excitation and emission energies was studied using computational methods upon considering twisting of the molecule in solvents of different polarity. Furthermore, a detailed characterization of the excited state profile was attained using time resolved spectroscopy techniques. In particular, a contrasting deactivation pattern of the intramolecular charge transfer (ICT) state was observed in low and high polar media. Moreover, in low and medium polar solvents strong emission is accompanied by triplet excited state formation, while in high polar and protic solvents the ICT state is highly stabilized and decays primarily non-radiatively.Notably, a viscosity increase in the latter solvents hampers rotations leading to a strong emission enhancement. This latter behavior, coupled to a remarkable solvatochromic character, makes DPAP a promising probe for biological and environmental sensing and imaging applications.
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, CHCl 3 , and acetonitrile. Analogously, DPAP/PC films exposed to polar and highly polymer-interacting solvents, that is, toluene, THF, and CHCl 3 , 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.
Since 2004 the field of graphene research has attracted increasing interest worldwide. Especially the integration of graphene into microelectronic devices has the potential for numerous applications. Therefore, we summarize the current knowledge on this aspect. Surveys show that considerable progress was made in the field of graphene synthesis. However, the central issue consists of the availability of techniques suitable for production for the deposition of graphene on dielectric substrates. Besides, the encapsulation of graphene for further processing while maintaining its properties poses a challenge. Regarding the graphene/metal contact intensive research was done and recently substantial advancements were made towards contact resistances applicable for electronic devices. Generally speaking the crucial issues for graphene integration are identified today and the corresponding research tasks can be clearly defined.
Among the plethora of recently proposed molecular sensors, those belonging to the class of fluorescent molecular rotors (FMRs) have attracted much attention owing to their peculiar photophysical properties that enable an unprecedented sensitivity towards environmental microviscosity. The usual FMR synthetic design prescribes chromophores characterized by an intramolecular rotation between two well-defined excited states, a locally excited state and a twisted internal charge transfer (TICT) state, where the sensing capabilities arise from a dual competition of the corresponding radiative/non-radiative decay processes. However, we have recently demonstrated a different modus operandi of a new subclass of solvatochromic FMRs, which exploit a solvent-independent, barrier-free intramolecular rotation of the excited dye. The rotational dynamics is modulated by local viscosity and, in turn, manifested through a variable spectral signal. In order to translate the same rotational mechanism in a versatile sensor of polarity and viscosity, we designed and thoroughly characterized a novel FMR, namely 4-(triphenylamino)-phthalonitrile (TPAP). Remarkably, in addition to a high sensitivity versus solvent polarity and viscosity, TPAP is also able to form stable fluorescent nanoparticles characterized by aggregation-induced emission, via a simple sonochemical treatment. Such peculiar features are tested in different applications aiming at illustrating its capability to report on solvatochromic and vapochromic effects, as well as to provide detailed intracellular information through bioimaging studies.
bcFluorescent organic nanoparticles (FONs) based on aggregation-induced emission (AIE) are receiving increasing attention owing to their simple preparation, enhanced optical properties, and a wide range of applications. Therefore, finding simple methods to tune the structural and emissive properties of FONs is highly desirable. In this context, we discuss the preparation of highly emissive, amorphous AIE spherical nanoparticles based on a structurally-simple molecular rotor and their sonochemical transformation into rhomboidal nanocrystals. Interestingly, the ultrasound-induced modification of the morphology is accompanied by a remarkable enhancement in the stability and emission of the resulting nanocrystals. Detailed characterization of both spherical and rhomboidal nanoparticles was carried out by means of several microscopic, crystallographic, and spectroscopic techniques as well as quantum mechanical calculations. In a nutshell, this work provides a unique example of the ultrasound-induced switching of morphology, stability, and emission in FONs.In recent years, the interest in fluorescent organic nanoparticles (FONs) 1 has been triggered by important advances in their preparation, characterization, and applications in the area of photonic materials, 2 optoelectronics, 3 (bio)chemical sensing 1a,4 and in vitro/ in vivo bio-imaging, 1a,5 just to mention a few. Among the plethora of molecular building blocks used for the preparation of FONs, those showing ''aggregation-induced emission'' (AIE) are particularly interesting and promising. 6 In contrast to planar organic molecules, which present ''aggregation-caused quenching'', AIE systems are flexible and non-planar. They are usually poorly emissive in solution, but strongly fluorescent upon aggregation due to restricted intramolecular rotation. 7 One of the simplest and most successful strategies for the preparation of AIE nanoparticles encompasses on the reprecipitation method. 8 Implicit is a solvent-exchange process, which mostly produces amorphous nanoparticles. Notably, their size and morphology is tunable by varying experimental conditions including temperature, concentration, aging time, nature and ratio of solvents, etc. However, crystalline FONs are often preferred, owing to their higher rigidity and stability especially in view of biological and optoelectronic applications. In this work, we focused on the aggregation of an easy-to-prepare molecular rotor, namely 4-(diphenylamino)-phthalonitrile (DPAP), leading to fluorescent, amorphous spherical nanoparticles and, subsequently, even stronger fluorescent rhomboidal nanocrystals. More precisely, a reprecipitation method was employed for the synthesis of AIE DPAP nanoparticles. The latter were subsequently transformed into flat rhomboidal nanocrystals of a high aspect ratio via short ultrasonication. Remarkably, such an unprecedented, ultrasound-induced modification of the nanoparticle morphology increased the stability -from a few hours to months -and the emission -3-fold -of the resulting nanocrys...
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