Role of Hydrogen Bonding in the Photophysical Properties of Isomeric Tetrapyridylporphyrins in Aprotic Solvent

Simultaneous exfoliation and functionalization of MoS2 with tetrapyridyl porphyrin

Molybdenum disulfide (MoS2) attracts the attention of the scientific community due to its thickness dependent properties. To fully exploit these features, it is necessary to produce the material in mono or few-layers on a large scale. Several methodologies have been developed for this purpose, the most promising one being liquid phase exfoliation (LPE). LPE allows obtaining good quality exfoliated MoS2 in a simple and scalable manner. Herein we report the simultaneous exfoliation and functionalization of MoS2 in chloroform using a specific porphyrin, namely tetrapyridyl porphyrin. We have corroborated that the exfoliation of MoS2 in the volatile solvent increases in the presence of the porphyrin due to the different interactions between them, obtaining dispersions with good concentrations. Additionally, the optical properties of the porphyrin are modified by these interactions. The characterization carried out by several techniques supports the hypothesis that the interactions occur through the pyridyl rings of the porphyrin and the molybdenum atoms of the material.

Ultrafast Time-Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy

A comprehensive study of ultrafast molecular relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solvent mixture. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate constants associated with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is observed at low-frequency mode, ca. 26 cm(-1) region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful analysis of time profiles of up-converted signals at different emission wavelengths helps to reveal detail molecular dynamics. The observed lifetimes are as indicated: A very fast decay component with 80 ± 20 fs observed at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time constants are due to intramolecular vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, respectively. The weighted average of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An additional ∼20 ± 2 ps rise component is observed in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model analysis of TA data exclusively generate the true spectra of each excited species/state with their respective lifetimes along with microscopic rate constants associated with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are observed which are associated with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, respectively, when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), respectively, as expected. The TA results parallel the fluorescence up-conversion studies, and both the results not only compliment each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx, and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.

Diffusive and non-diffusive photo-induced proton coupled electron transfer from hydrogen bonded phenols to meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin

Enhanced reductive fluorescence quenching of meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin (H2F20TPP) by two different phenols, 4-methoxy phenol (4-MeOPhOH) and 2,6-dimethoxy phenol (2,6-DiMeOPhOH) in the presence of various pyridine bases in dichloromethane solution is studied using steady state and time resolved fluorescence spectroscopic methods by employing time correlated single photon counting (TCSPC) and fluorescence up-conversion techniques. An enhanced quenching behaviour of H2F20TPP is observed when phenols are hydrogen bonded to various pyridine bases. Quenching observed in the steady state and time resolved studies in the nanosecond time domain follows second order kinetics and generates quenching rate constants and hydrogen bond equilibrium constants, the latter of which agree quite closely with those obtained from independent spectroscopic measurements. A significant kinetic deuterium isotope effect is observed, indicating the importance of proton movement in the quenching processes. This quenching effect is attributed to be due to a tri-molecular transition state involving H2F20TPP and a hydrogen bonded phenol complex, in which electron transfer from phenol to excited H2F20TPP is concerted with proton movement from the phenol to hydrogen bonded base. Observed quenching behaviours are rationalized by invoking diffusion controlled proton coupled electron transfer. Fluorescence up-conversion studies in the 100 ps time domain confirm ultrafast PCET for 4-MeOPhOH and base pairs which fall in a non-diffusive regime.

A high-performance and simple method for rapid and simultaneous determination of dihydroxybenzene isomers

A novel idea on the basis of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (THPP) functionalized carbon nanotubes (CNTs) film was developed to simulate the chromatographic separation, which was able to rapidly and simultaneously detect dihydroxybenzene isomers based on their different oxidation potentials and corresponding currents. The mechanism involved in the recognition and isolation of the dihydroxybenzene isomers was explored and confirmed by UV spectra and density functional theory. It was proven that the current method for simultaneously detecting and isolating dihydroxybenzene isomers was ascribed to the priority of porphyrin film to induce oxidation of the three kinds of isomers, and the priority depends to a large extent on the different tendencies of dihydroxybenzene to interact with the peripheral hydroxyl group and with the extended π-conjugated ring of porphyrin. Most important of all, porphyrin functionalized CNTs film allows for the formation of well-defined interface and provides an advantageous and high-performance platform for the simultaneous determination and isolation of dihydroxybenzene isomers.