Modulation of the ESIPT emission of benzothiazole type dye incorporated in silica-based hybrid materials

Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Substituent effect on ESIPT mechanisms and photophysical properties of HBT derivatives

Substituent effects on excited-state intramolecular proton transfer (ESIPT) and photophysical properties of 2-(2-Hydroxyphenyl) benzothiazole (HBT) derivatives have been theoretically unveiled via the density functional theory (DFT) and time-dependent DFT (TDDFT). The optimized geometrical configurations and normal mode analyses confirm that the proton transfer processes are more reactive in excited state. Through calculating the activation energies and rate constants of ESIPT processes, finding that the processes are increasingly inactive when substituent group changes from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂. In addition, the photophysical properties analyses indicate the vertical transition energies are in good agreement with those observed in experiment. Note that all the absorption and emission maxima of enol and keto forms have the significant red-shift. In order to clarify the substituent effect on ESIPT and photophysical properties, we draw the frontier molecular orbitals (FMOs) isosurfaces and calculate the distances of electrons and holes and atomic charges. It follows that the intramolecular charge transfer (ICT) degrees are increasingly enlarged as substituting from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂ groups, which not only causes the red-shift of absorption and emission of enol and keto forms, but also affects the charge distribution of proton donor and acceptor, inhibiting the occurrence of ESIPT processes.

Synthesis, Optical and Electrical Characterization of Amino-alcohol Based Sol-gel Hybrid Materials

This manuscript describes the synthesis and characterization of five new organic–inorganic hybrid (OIH) sol-gel materials that were obtained from a functionalized siloxane 3-glycidoxypropyltrimethoxysilane (GPTMS) by the reaction with the new Jeffamine^®, namely three different diamines, i.e., EDR-148, RFD-270, and THF-170, a secondary diamine, i.e., SD-2001, and a triamine, i.e., T-403. The OIH sol-gel materials were characterized by UV-visible absorption spectrophotometry, steady-state photoluminescence spectroscopy, and electrochemical impedance spectroscopy. The reported OIH sol-gel materials showed that, with the exception of the samples prepared with Jeffamine^® SD-2001, the transmittance values ranged between 61% and 79%. Regarding the capacitance data, the values reported changed between 0.008 and 0.013 nF cm⁻². Due to their optical and electrical properties these new OIH materials show promising properties for applications as support films in an optical sensor area such as fiber sensor devices. Studies to assess the chemical stability of the OIH materials in contact with cement pastes after 7, 14, and 28 days were also performed. The samples prepared with THF–170 and GPTMS, when compared to the samples prepared with RFD-270 and T-403, exhibited improved behavior in the cement paste (alkaline environment), showing promising properties for application as support film in optical fiber sensors in the civil engineering field.

Applications of Nanomaterials Based on Magnetite and Mesoporous Silica on the Selective Detection of Zinc Ion in Live Cell Imaging

Functionalized magnetite nanoparticles (FMNPs) and functionalized mesoporous silica nanoparticles (FMSNs) were synthesized by the conjugation of magnetite and mesoporous silica with the small and fluorogenic benzothiazole ligand, that is, 2(2-hydroxyphenyl)benzothiazole (hpbtz). The synthesized fluorescent nanoparticles were characterized by FTIR, XRD, XRF, ¹³C CP MAS NMR, BET, and TEM. The photophysical behavior of FMNPs and FMSNs in ethanol was studied using fluorescence spectroscopy. The modification of magnetite and silica scaffolds with the highly fluorescent benzothiazole ligand enabled the nanoparticles to be used as selective and sensitive optical probes for zinc ion detection. Moreover, the presence of hpbtz in FMNPs and FMSNs induced efficient cell viability and zinc ion uptake, with desirable signaling in the normal human kidney epithelial (Hek293) cell line. The significant viability of FMNPs and FMSNs (80% and 92%, respectively) indicates a potential applicability of these nanoparticles as in vitro imaging agents. The calculated limit of detections (LODs) were found to be 2.53 × 10⁻⁶ and 2.55 × 10⁻⁶ M for Fe₃O₄-H@hpbtz and MSN-Et₃N-IPTMS-hpbtz-f1, respectively. FMSNs showed more pronounced zinc signaling relative to FMNPs, as a result of the more efficient penetration into the cells.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Synthesis of 2-acyl-benzo[1,3-d]selenazoles via domino oxidative cyclization of methyl ketones with bis(2-aminophenyl) diselenide

The synthesis of unprecedented 2-acyl-benzo[1,3-d]selenazoles is presented using bis(2-aminophenyl) diselenide and aryl methyl ketones under conventional heating and microwave irradiation.

Excited-State Intramolecular Proton-Transfer Properties of Three Tris(N-Salicylideneaniline)-Based Chromophores with Extended Conjugation

The synthesis and photophysical properties of three tris(N-salicylideneaniline) (TSA) compounds containing 1,3,5-triarylbenzene, -tristyrylbenzene, and -tris(arylethynyl)benzene core units are reported. The TSA compounds underwent efficient excited-state intramolecular proton transfer (ESIPT) in solution and in solid state due to the preformed C=N⋅⋅⋅H-O hydrogen-bonded motifs of the structures. Steady-state fluorescence emission spectra of the TSA molecules revealed dual bands only in DMSO, and large Stokes shifts in other polar aprotic and protic solvents. Femtosecond transient absorption spectroscopic measurements in THF revealed lifetime values in the range of 14-16 ps for the excited-state keto-tautomer. The TSA compounds are also responsive to metal ions (Cu²⁺ and Zn²⁺ ) in DMSO, exhibit enhanced aggregate-induced emission (AIE) properties in DMSO/water mixtures, and are highly luminescent in the solid state.

How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

Synthesis and fluorescence properties of benzoxazole-1,4-dihydropyridine dyads achieved by a multicomponent reaction

Photoactive ESIPT dyads with a high Stokes' shift were obtained by a multicomponent one-pot Hantzsch synthesis.

Low-temperature spectra of the analogues of 10-hydroxybenzo[h]quinoline as an indication of barrierless ESIPT

The absorption and fluorescence spectra of two analogues of 10-hydroxybenzo[h]quinoline (10-HBQ), namely, 1-hydroxy-7-methylbenzo[c]acridine (HMBA) and 4-hydroxybenzo[c]phenanthridine (HBPA), were studied in n-alkane matrices at 5 K. Considerable energy separation between the onsets of the spectra and broadening of the bands was an indication that intramolecular proton transfer (ESIPT) takes place at such a low temperature. DFT and ab initio methods were used to calculate the electronic transition energies and oscillator strengths and the vibronic structure of the electronic spectra. Shortcomings in our knowledge of the shape of the potential energy surface for ESIPT systems are highlighted in the context of the discussion of the shape of the electronic spectra. The π-expansion of the 10-HBQ chromophore achieved by adding a benzene moiety at various positions adjacent to the pyridine ring led to compounds possessing diverse photophysical properties, ranging from the non-ESIPT strongly fluorescent molecule of 10-hydroxy-1-azaperylene to weakly emitting (or nonemitting) molecules, where ESIPT occurs very efficiently.

Excited state intramolecular proton transfer in electron-rich and electron-poor derivatives of 10-hydroxybenzo[h]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo[h]quinoline were prepared via a straightforward strategy comprising formation of the benzo[h]quinoline skeleton followed by C-H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo[c]acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08-0.45 ps).