Laser Flash Photolysis Study of the Reaction Mechanism in the Photochromism of 1-(Acyloxy)-2-methoxyanthraquinones

Abstract

Nanosecond laser flash photolysis has been used to study the primary processes in the photochromic reaction of five O-acylic derivatives of 1-hydroxy-2-methoxyanthraquinone with methyl (I), tolyl (II), phenyl (III), ethoxyl (IV), and diethylamino (V) groups in the migrating acyl. The triplet−triplet absorption spectra of the reactive triplet states of quinones IV and V were detected, and the rate constants of the primary photochemical step were measured. The temperature dependence of the rate constants of acyl migration in the triplet states of quinones IV and V was studied, and the Arrhenius parameters were determined. The rate constants of thermal acyl migration and their Arrhenius parameters were measured for compounds I−V. It was found that migrant nature significantly influences the activation energy of both thermal and photochemical reactions of acyl migration. The activation energy of thermal migration increases from 37.9 ± 0.2 kJ/mol for compound I up to 66.5 ± 0.7 kJ/mol for compound V. The photochemical process is characterized by considerably lower values of the activation energy (15.9 ± 0.8 and 26.0 ± 1.7 kJ/mol for compounds IV and V, respectively). It was confirmed in the present work that photochemical migration of acyl groups is an adiabatic process occurring on the triplet potential energy surface.

Study of Acyl Group Migration by Femtosecond Transient Absorption Spectroscopy and Computational Chemistry

“…In contrast to the case of 3 , the same transient absorption spectrum assigned to the triplet state of 2 a was detected upon femto- and nanosecond excitation of para -quinone 1a , which excludes its formation by the reexcitation of 2a or a spiro biradical similar to 5 . In addition, the quantum yield of 2a,b formation is close to unity, as it should be in the case of a very fast adiabatic reaction on the triplet PES. On the contrary, the quantum yield of 4 is much less than unity (0.32) 19 due to the decay of the biradical 5 to both isomers 3 and 4 .…”

“…This is possible if the extinction coefficients of 1a and 2a are known, and if one assumes that the photoexcitation of 1a leads to 2a with a quantum yield close to unity . Using the extinction coefficient of 2a at 510 nm (1.1 × 10 4 M -1 cm -1 ), 4c, we consecutively estimated the extinction coefficients of all of the proposed intermediates (Table ). It is clear that the bleaching of 1a is significant at λ < 420 nm and negligible at longer wavelengths (Figure a, Table ).…”

“…In all quinones under study ( 1a − c ) the precursors of the ana -quinone products ( 2a − c ) were detected and assigned to the triplet excited states of 2a − c (Scheme ). In the case of electron-donating substituents in the migrating carbonyl ( 1b,c ) group, several short-lived intermediates were also detected and tentatively assigned to the triplet excited states of the para -quinones ( 3 1b,c *) . However, the time resolution of the system (∼3 ns) was not sufficient to detect 3 1a * nor to study the dynamics of the primary photophysical processes.…”

“…It was found that this process is thermally reversible and represents a new type of photochromic reaction of substituted anthraquinones . Unlike the photo-Fries rearrangement involving the homolytic cleavage of a carbon−heteroatom bond, this photochemical migration is a concerted adiabatic process which proceeds on the triplet potential energy surface. , An adiabatic mechanism of acyl migration was proposed based primarily on the analysis of energy correlation diagrams and the dependence of the reactivity on the chemical structure 4b. This mechanism was later supported 4c by the recognition of 9-acetoxy-2-methoxy-1,10-anthraquinone in its triplet excited state ( 3 2a ), as the precursor of the ground state 2a .…”

“…A series of O-acylic derivatives of 1-hydroxy-2-methoxyanthraquinone with methyl ( 1a ), ethoxyl ( 1b ), and diethylamino ( 1c ) groups in the migrating carbonyl moiety were studied using nanosecond laser flash photolysis . In all quinones under study ( 1a − c ) the precursors of the ana -quinone products ( 2a − c ) were detected and assigned to the triplet excited states of 2a − c (Scheme ).…”