Photoswitching of Emission in Naphthopyran Substituted Transition Metal Complexes

This project explores the synthesis of novel photochromic 3H-naphtho[2,1-b]pyrans, which have been decorated with pyridine units that serve as ligands in luminescent rhenium(I) complexes. It is envisioned that cycling the naphthopyrans would not only generate the coloured photomerocyanines but would also modulate the luminescence of the derived rhenium(I) complexes. Diverse synthetic strategies were devised to access the rhenium(I) complexes depending on the position and orientation of the pyridyl substituent on the naphthopyran ring. For the majority of the naphthopyrans which bear the pyridyl unit on the naphthalene moiety of the naphthopyran, Suzuki cross-coupling of either a bromo-, iodo- or trifloxy- substituted 3H-naphtho[2,1-b]pyran proved to be effective for introduction of the pyridyl unit. For the preparation of the 6-(pyridyl) substituted 3H-naphtho[2,1-b]pyran isomer, construction of a 1-bromo-4-(pyridyl)-2-naphthol was required, which was subjected Heck cross-coupling protocol to create the pyran ring. The 3-(4-pyridyl)-3H-naphtho[2,1-b]pyran was also obtained by a Heck cross-coupling strategy commencing from 1-phenyl-1-(4-pyridyl)prop-2-en-1-ol.

All of the new pyridyl substituted 3H-naphtho[2,1-b]pyrans exhibit good photochromic properties with the reversible generation of mostly red hues; the persistence of the photomerocyanines was dependent upon the location and orientation of the pyridyl unit. Spectroscopic measurements have shown that the novel rhenium(I) complexes exhibited both T- and P-type photochromism with an overall increase in colourability and variable half-times depending on both the point of attachment and orientation of the pyridyl unit on the naphthopyran core and also the nature of the bidentate ligands. Furthermore, the novel rhenium(I) complexes exhibited visible light induced photochromism due to a triplet photosensitization mechanism. Additionally, reversible quenching of the 3MLCT excited state emission occurred as a result of the photochromic reaction, which depended on the relative energies of the 3MLCT and 3IL excited states and on the extent of the photochromic reaction.

During the Suzuki cross-coupling of the pyridine unit to the naphthopyran nucleus, a ring contraction of the pyran ‘motif’ was observed. This reaction has been developed into an efficient base-mediated protocol to a diverse series of novel naphthofurans, naphthodifurans and benzo-fused indoles in generally good yields.