# Induction of J Aggregate-like Optical Transitions in Dihydroxyquinone by Coordination with Al(III)

## TL;DR

This study shows that forming metal complexes with Al(III) induces J-aggregate-like optical effects in dihydroxyquinone compounds, enhancing their fluorescence.

## Contribution

The paper demonstrates J-aggregate-like behavior in Al(III) complexes of dihydroxyquinones through experimental and computational methods.

## Key findings

- Al(III) complexation causes red-shifts in UV–vis and fluorescence spectra of QNZ and DHN.
- Fluorescence quantum yield and lifetime increase significantly for DHN upon Al(III) complexation.
- Computational analysis supports a head-to-tail chromophore arrangement with stronger J-type coupling in Al(III)2DHN2.

## Abstract

The metal-complex
formation of 1,4-dihydroxyanthraquinone (quinizarin,
QNZ) and 6,11-dihydroxy-5,12-naphthacenedione (DHN) with Al­(III) ions
is investigated by stationary and time-resolved emission spectroscopy
combined with quantum chemical calculations of optical properties.
UV–vis and fluorescence spectra revealed small red-shifts of
200 and 60 meV for the QNZ and DHN metal complexes, respectively.
The fluorescence quantum yield increases from 0.08 to 0.23 for QNZ,
while for DHN it changes from 0.24 to 0.79 upon complexation, suggesting
the presence of J-aggregate-like exciton coupling within the coordination
structure. The average fluorescence lifetime of QNZ varies from 0.65
ns of the free ligand to 2.77 ns, and in the case of DHN it goes from
1.57 to 2.61 ns after Al­(III) complexation. These results are consistent
with formation of a more rigid molecular structure which effectively
decreases the nonradiative rate constant. Confocal fluorescence microscopy
images of Al­(III) complexes adsorbed into the μmZeolite L structure
gave similar red-shifted J type emission. Density functional theory,
at the B3LYP/def2-TZVP level of theory, and the analysis of the electronic
transition dipole moment, calculated with TDDFT at the CAM-B3LYP/def2-TZVP
level, supports a near head-to-tail chromophore arrangement containing
two metal centers coordinated with two chromophores. The Al­(III)2DHN2 complex exhibits the stronger transition dipole
coupling and a more pronounced J-type character when compared with
Al­(III)2QNZ2 complex. The radiative rate constant
of Al­(III)2DHN2 is twice that of the single
DHN chromophore.