Bichromophoric anthracene-porphyrin molecule namely 5-(4-nitrophenyl)-10,20-bisphenyl-15-(9-anthryl)porphyrin (AnNPP) was synthesized as a model to determine the short-range energy transfer between anthracene and porphyrin moieties. Switching of Förster to Dexter mechanism was studied in the free base (AnNPP) and protonated form of AnNPP (PAnNPP). The complete protonation of AnNPP was carried out by using hydrogen chloride (HCl) and was confirmed by UV–vis spectroscopy. The steady-state fluorescence spectroscopy shows that anthracene quantum yield is decreased in both AnNPP and PAnNPP. This was further proved by measuring fluorescence-lifetime using time correlated single photon counting (TCSPC) technique. A remarkable decrease in fluorescence-lifetime of anthracene is observed for both AnNPP and PAnNPP. Quantum chemical calculations were performed for AnNPP and PAnNPP to support the short distance energy transfer with switching of excited state energy transfer (EET) mechanism. The comparison of the EET rates reveals that the Förster mechanism is followed in AnNPP, whereas the Dexter mechanism is predominant in PAnNPP.
This paper describes the fabrication of thin films of porphyrin and metallophthalocyanine derivatives on different substrates for the optochemical detection of HCl gas and electrochemical determination of L-cysteine (CySH). Solid state gas sensor for HCl gas was fabricated by coating meso-substituted porphyrin derivatives on glass slide and examined optochemical sensing of HCl gas. The concentration of gaseous HCl was monitored from the changes in the absorbance of Soret band. Among the different porphyrin derivatives, meso-tetramesitylporphyrin (MTMP) coated film showed excellent sensitivity towards HCl and achieved a detection limit of 0.03 ppm HCl. Further, we have studied the self-assembly of 1,8,15,22tetraaminometallophthalocyanine (4α-MTAPc; M = Co and Ni) from DMF on GC electrode. The CVs for the self-assembled monolayers (SAMs) of 4α-Co II TAPc and 4α-Ni II TAPc show two pairs of well-defined redox couple corresponding to metal and ring. Using the 4α-Co II TAPc SAM modified electrode, sensitive and selective detection of L-cysteine was demonstrated. Further, the SAM modified electrode also successfully separates the oxidation potentials of AA and CySH with a peak separation of 320 mV.
Adsorption of a heteroaromatic dithiol, 2,5-dimercapto-1,3,4-thiadiazole (DMT), on Au surface from a completely deaerated aqueous solution leads to the formation of a multilayer assembly via hydrogen bonding with water molecules, whereas adsorption from acetonitrile, ethanol, dimethyl sulfoxide, and chloroform solutions leads to the formation of a monolayer. Cyclic voltammetry (CV), attenuated total reflectance (ATR) infrared spectroscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to characterize the monolayer and multilayer assemblies of DMT on Au surface. Since DMT contains two S−H groups, it chemisorbs on Au surface through one of its two S−H groups, while the other S−H group is pointing away from the surface. The presence of free S−H groups on the Au surface was confirmed by CV, ATR-FT-IR, and XPS. It is presumed that the free S−H groups of DMT on the Au surface form a hydrogen bond with the water molecules. Subsequently, DMT molecules in solution form a hydrogen bond with the water molecules attached with DMT on Au surface, and this type of hydrogen bonding network goes on increasing when the soaking time of the Au surface in an aqueous solution of DMT increases. The involvement of water molecules in the multilayer formation was confirmed from the appearance of a broad stretching band around 3300 cm−1 corresponding to the hydrogen bonded O−H in the ATR-FT-IR spectrum, in addition to a binding energy peak at 533.4 eV due to hydrogen-donating water molecules in the O 1s region of XPS. The absence of a stretching band characteristic for S−S at 537 cm−1 in the Raman spectrum confirmed that the multilayer assembly was not formed via S−S linkage. However, exposure of DMT multilayer to air shows a stretching band characteristic of S−S, indicating that aerial oxidation leads to the formation of S−S bond. SEM images show that DMT forms a leaflike structured multilayer assembly on Au surface.
Charge-transfer (CT) complexes formed between aromatic thiol donors (thiophenol (TP), benzene-1,4-dithiol (BDT), p-aminothiophenol (ATP), p-hydroxythiophenol (HTP), and p-toluenethiol (TTP)) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as an acceptor were studied spectrophotometrically in dichloromethane. Addition of aromatic thiols in dichloromethane to DDQ leads to the formation of colored solutions that exhibit a very broad absorption band in the range 440-800 nm and a band in the region 300-400 nm. On the basis of the energies of LUMO and HOMO from quantum mechanical calculations, the broad band observed in the visible region was assigned to the pi*(a2) <-- pi(b1) transition and a band observed between 300 and 400 nm was assigned to the pi*(a2) <-- pi(a2) transition. The solid CT complexes of aromatic thiols and DDQ were prepared and characterized by FT-IR spectroscopy. The stoichiometry of the CT complexes was determined by Job's continuous variation method. The association constant (KCT), molar extinction coefficient (epsilon), oscillator strength (f), and transition dipole moment (micro) values were calculated from the electronic spectra. The vertical ionization potentials (ID) of the donors were calculated from their corresponding lambdaCT. Quantum mechanical (QM) calculations were performed to determine the ionization potential and the energies of the highest occupied molecular orbital (HOMO) of donors and lowest unoccupied molecular orbital (LUMO) of an acceptor.
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