Nodal-plane model of the excited-state intramolecular proton transfer of 2-(o-hydroxyaryl)benzazoles

doi:10.1016/S1010-6030(99)00025-8

Journal of Photochemistry and Photobiology A: Chemistry

Volume 122, Issue 3, 31 March 1999, Pages 151-159

https://doi.org/10.1016/S1010-6030(99)00025-8 Get rights and content

Abstract

The excited-state intramolecular proton transfer of 2-(o-hydroxyaryl)benzazoles has been studied by emission spectroscopy. Although both 2-(2-hydroxy-3-aryl)benzazole and 2-(1-hydroxy-2-aryl)benzazole consist of an aryl moiety, a benzazol-2-yl group and an OH group, their emission properties are very different for the reasons that can be explained in terms of our previously offered nodal-plane model.

Introduction

Much attention has been directed from both experimental and theoretical viewpoints to the excited state intramolecular proton transfer (ESIPT) of hydrogen-bonded molecules 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. ESIPT is a very simple chemical process readily accessible for both accurate measurement and quantitative theoretical analysis. The study of ESIPT is at the forefront of the revolution in the understanding of chemical reactions at the molecular level. It also plays an important role both in fundamental biochemistry and in practical application: the ESIPT of hypericin and related molecules is of current interest because of the anti-viral (especially anti-human-immunodeficiency virus (HIV)) and the anti-tumor activities of these molecules [11]. A molecule showing ESIPT will also be useful in future optical memories and switches 12, 13, 14. Furthermore, ESIPT is also of great interest in view of its relevance to fluorescence probes for biomolecules 15, 16, to UV absorbers for polymers [17]and cosmetics [18], to proton transfer laser-dye [19]and to radiation detection scintillators [20].

We have investigated the dynamic processes in the low-lying excited states of o-hydroxybenzaldehyde (OHBA) and its related molecules (OHBAs) by using spectroscopic and computational methods 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. We have constructed a simple theoretical model of the ESIPT of OHBAs based on the nodal pattern of the wave function 25, 26. Although this nodal-plane model is a qualitative one, it allows us to recognize the important features of ESIPT immediately and provides useful information about the reaction mechanisms. Our nodal-plane model is applicable to photochemical reactions in various excited states of various molecules 37, 38. Many chemists have cited it 5, 8, 10, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, and the usefulness of our explanation is recognized by many researchers 5, 8, 39, 40, 41, 44, 48, 49.

In this article, we discuss the application of our nodal-plane model to the ESIPT of the 2-(o-hydroxyaryl)benzazoles (2-(o-hydroxynaphtyl)benzazoles and 2-(o-hydroxyanthryl)benzothiazoles) whose structures are shown in Fig. 1. Although the ESIPT of 2-(o-hydroxynaphtyl)benzazoles was studied previously 50, 51, 52, 53, 54, 55, 56, the dependence of ESIPT on the structure of the isomers (Fig. 1) has not yet been investigated. Section 2briefly summarizes the understanding of ESIPT by using our nodal-plane model. Section 3describes experiments with 2-(o-hydroxyaryl)benzazoles and notes that the results of those experiments are consistent with our nodal-plane model. The last section summarizes our conclusions. The work reported in this paper was presented at the 17th IUPAC Symposium on Photochemistry [57].

Section snippets

Nodal-plane model of ESIPT

The understanding of the mechanism of ESIPT in a simple molecule can lead to valuable insights into the behavior of complex systems. OHBA is the simplest example of an aromatic molecule with an intramolecular hydrogen bond, and it can exist in two tautomeric forms, the keto form and the enol form (proton-transferred form). The relative stability of each form depends on the electronic state of the molecule (Fig. 2): in the ground state (S₀ state), the keto form is stable, in the first excited $^{1} ($

ESIPT of 2-(o-hydroxyaryl)benzazoles

The ESIPT of 2-(2-hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) has been studied extensively 30, 31, 69, 70, 71, 72, 73, 74. In the S₀ state, the enol form is stable, and in the S^(π)₁ state, ESIPT yields the keto form. As shown in Scheme. 1, the mechanism of ESIPT in the S^(π)₁ states of HBO and HBT can be explained like that of the ESIPT in the S^(π)₁ state of OHBA. In this section, we will show the absorption and fluorescence spectra of 2-(o

Conclusions

The ESIPT of 2-(o-hydroxyaryl)benzazoles has been studied by emission spectroscopy. Although both 2-(2-hydroxy-3-aryl)benzazole and 2-(1-hydroxy-2-aryl)benzazole consist of an aryl moiety, a benzazol-2-yl group and an OH group, their emission properties are very different for reasons that can be explained in terms of our nodal-plane model 25, 26. Our nodal-plane model provides a simple but rather powerful concept for the understanding of ESIPT. Using it, we can predict the stabilization due to

Acknowledgements

We are indebted to Dr. Keishi Ohara and Mr. Arinobu Nakamura of Ehime University for their experimental support. We also thank Professor Satoshi Okazaki of Kyoto University and Professors Takashi Manabe and Hidenori Hayashi of Ehime University for their cooperation in the measurement of nanosecond transient absorption spectra. Shin-ichi Nagaoka expresses his sincere thanks to Professor Noboru Hirota of Kyoto University for his continuous encouragement. This work was partly supported by

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Nodal-plane model of the excited-state intramolecular proton transfer of 2-(o-hydroxyaryl)benzazoles

Abstract

Introduction

Section snippets

Nodal-plane model of ESIPT

ESIPT of 2-(o-hydroxyaryl)benzazoles

Conclusions

Acknowledgements

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