7-Hydroxyquinoline-8-carbaldehydes. 1. Ground- and Excited-State Long-Range Prototropic Tautomerization

Abstract

Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H and (13)C NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular "frame" (7-hydroquinolines, 7-HQs) and a proton "crane" (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, OH (7-quinolinol)) and NH (7(1H)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, HMMQC, shows only a trace of the NH form in highly polar and/or protic media). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV-vis spectroscopic properties of 7-HQCs with those of 7-HQs, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted 7-HQCs.

Excited‐State Intramolecular Proton Transfer: Photoswitching in Salicylidene Methylamine Derivatives

“…This property is termed molecular photochromism. [11][12][13][14][15][16][17] In recent decades, systems that exhibit the phenomenon of excited-state intramolecular proton transfer (ESIPT) have been of considerable applied interest as photostabilizers and sunscreens for the protection of organic polymers and biological tissues against damage that can be caused by the UV component of sunlight. [5] Over the years, most research effort in this field has been focused on two classes of intramolecular reactions related to photochromism.…”

“…More recently, a new class of photochromic systems subjected to a light-induced intramolecular proton-transfer (PT) process has been invoked. [11][12][13][14][15][16][17] In recent decades, systems that exhibit the phenomenon of excited-state intramolecular proton transfer (ESIPT) have been of considerable applied interest as photostabilizers and sunscreens for the protection of organic polymers and biological tissues against damage that can be caused by the UV component of sunlight. [18][19][20][21] Of these systems, photochromic Schiff bases [22] represent a special group of ESIPT systems, for which depopulation routes of the excited molecule are particularly complex.…”

8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design

“…The theoretical design was later expanded by using a variety of tautomeric (7-hydorxyquinolines [ 37 , 38 , 39 , 40 , 41 , 42 ], 3-OH and 3-NH 2 pyridines [ 43 ]) and crane (carbaldehydes [ 37 , 39 ], carboxamides [ 40 , 42 ], pyridines [ 38 , 43 ] and pyrimidines [ 41 ]) sub-units. The experimentally studied systems are limited to a few examples of 7-hydroxyquinoline ( 4 ) [ 39 ], 3-hydroxypyridine [ 44 , 45 , 46 , 47 ] and amide [ 48 ] tautomeric sub-units. The common feature of these systems is the excited state proton transfer either from E1K* or from K2E* ( Scheme 3 ) to K1K* or K2K* respectively, which results to a conical intersection in the transition state between K1K* and K2K* (tautomeric and crane sub-units being perpendicular).…”

On the nonadditivity of the substituent effect in homo-disubstituted pyridines

“…[2][3][4] The sEDA and pEDA descriptors were next modified to evaluate heteroatom or heteroatomic group incorporation effect into a cyclic systems (sEDA(II) and pEDA (II)), [5] to estimate substituent effect caused by a doublebonded substituent (sEDA(=) and pEDA(=)), [6] and to determine the heteroatom incorporation effect into a ring junction position (sEDA(III) and pEDA(III)). [7] The sEDA and pEDA descriptors of the classical substituent effect, now often referred by us as sEDA(I) and pEDA(I), were used to study a variety of problems: aromaticity of different systems, [7][8][9][10][11][12][13][14][15][16][17][18][19] stability and tautomerism, [20][21][22] vibrational and nuclear magnetic resonance spectra, [23][24][25] optical molecular switches, [26][27][28][29] halogen bond problems, [30] reaction course analysis, [31] and general issues connected to the substituent effect. [32][33][34] One of the general problems of the substituent effect is its nonadditivity.…”