Effects of micelles on excited state intramolecular proton transfer activities of 2-hydroxy 1-naphthaldehyde

Excited state behavior of pyrrole 2-carboxyldehyde: theoretical and experimental study

Photophysical and photochemical dynamics of excited state proton transfer reaction have been reported for Pyrrole 2-carboxyldehyde (PCL). Experimental and theoretical observations yield all possible signatures of intramolecular and intermolecular proton transfer in an excited state. Dual emission (~325 nm, ~375 nm) on photo excitation indicates the existence of more than one species in an excited state. Computed reaction pathway and two-dimensional potential energy profile in the ground state reveals a single minimum corresponding to normal form (E). Dual minima in excited state energy profile shows the existence of two species, one normal and other zwitterionic (Z*) species. A large Stokes shifted emission at ~375 nm in hydrocarbon medium reveals the existence of zwitterionic species due to Excited state intramolecular proton transfer (ESI(ra)PT). Excited State Intermolecular proton transfer (ESI(er)PT) is observed in a hydroxylic environment around 430-490 nm. pH variation in hydroxylic medium suggests the formation of anion (A((-))) from Z*.

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: part 2. 2-Hydroxy-1-naphthaldehyde

The S(1)← S(0) fluorescence excitation spectrum of jet-cooled 2-hydroxy-1-naphthaldehyde (2H1N) with origin at 26,668 cm(-1) has been measured. Nine totally symmetric modes and three non-totally symmetric modes have been assigned in the excitation spectrum. Ab initio calculations indicate that 2H1N undergoes a planarity change upon excitation, which may account for the unusual intensity of non totally symmetric vibrational modes in the excitation spectrum. A number of low intensity features were observed on the low energy side of the origin which have been assigned to the 2H1N dimer rather than different ground state confomers of 2H1N. The origin of the S(1)← S(0) electronic transition of the dimer lies at ~26,401 cm(-1); combinations of two low frequency intermolecular modes of the dimer (59 cm(-1) and 17 cm(-1)) were also observed. The occurrence of excited state intramolecular proton transfer (ESIPT) in 2H1N cannot be proven on the basis of this work. A comparison of the (photo)physical properties of 2H1N with 1-hydroxy-2-naphthaldehyde (1H2N) [A. McCarthy and A.A. Ruth, PCCP, 2011, 13, 7485-7499 (Part 1)], however, indicate the plausibility of an ESIPT process in 2H1N. The strength of the intramolecular hydrogen bond (IMHB) in 2H1N was computed as ~10.6 kcal/mol, a value comparable to the IMHB strength of 1H2N. The establishment of a lower limit on the state lifetimes of 2H1N, of ~1.8 ps, indicates that any proposed ESIPT reaction in 2H1N may not proceed barrierlessly. Above an excess energy of ~1000 cm(-1), the intensity of the fluorescence excitation spectrum reduces significantly, indicating the onset of a non-radiative decay mechanism.

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength, aromaticity, and excited-state intramolecular proton transfer reaction

The inequivalence of substitution pair positions of naphthalene ring has been investigated by a theoretical measurement of hydrogen bond strength, aromaticity, and excited state intramolecular proton transfer (ESIPT) reaction as the tools in three substituted naphthalene compounds viz 1-hydroxy-2-naphthaldehyde (HN12), 2-hydroxy-1-naphthaldehyde (HN21), and 2-hydroxy-3-naphthaldehyde (HN23). The difference in intramolecular hydrogen bond (IMHB) strength clearly reflects the inequivalence of substitution pairs where the calculated IMHB strength is found to be greater for HN12 and HN21 than HN23. The H-bonding interactions have been explored by calculation of electron density ρ(r) and Laplacian ∇(2) ρ(r) at the bond critical point using atoms in molecule method and by calculation of interaction between σ* of OH with lone pair of carbonyl oxygen atom using NBO analysis. The ground and excited state potential energy surfaces (PESs) for the proton transfer reaction at HF (6-31G**) and DFT (B3LYP/6-31G**) levels are similar for HN12, HN21 and different for HN23. The computed aromaticity of the two rings of naphthalene moiety at B3LYP/6-31G** method also predicts similarity between HN12 and HN21, but different for HN23.

Photo-induced relaxation and proton transfer in some hydroxy naphthoic acids in polymers

Photophysical and photochemical properties of 3-hydroxy-2-naphthoic acid [3,2-HNA] and 1-hydroxy-2naphthoic acid [1,2-HNA] in different aprotic, protic, and ion exchange (Nafion) polymers have been described in this article. In both molecules, intramolecular hydrogen bond (IMHB) exists between OH and COOH functional groups. Both 3,2-HNA and 1,2-HNA form different emitting species in different polymeric media. Fluorescence characteristic of 3,2-HNA is found to depend on its concentration, nature of the microenvironment, and wavelength of excitation, while 1,2-HNA is less susceptible to these changes. 3,2-HNA exhibits dual fluorescence band (normal and large Stokes shifted) in aprotic and only a single large Stokes shifted fluorescence in protic polymers, while 1,2-HNA shows a single fluorescence band along with weak phosphorescence emission in these polymers. Both excited-state inter and intramolecular proton transfer (ESPT) take place in 3,2-HNA in aprotic and protic polymers, resulting in large Stokes shifted emission band. A competition between ESIPT and excimer formation is observed by the appearance of rise time on increasing the concentration of 3,2-HNA in protic polymer. In Nafion film, 3,2-HNA is present as a cationic as well as neutral species. The presence of extra protons in Nafion film facilitates excited-state intramolecular proton transfer (ESIPT) in the neutral species of 3,2-HNA and gives large Stokes shifted emission (10 500 cm(-1)). No such effect is observed in 1,2-HNA doped in Nafion film. It is observed that, depending on the position of the IMHB ring, the electronic spectra get modified and the strength of IMHB is affected by the micro-environment of the polymer which alters the photophysics of these molecules.