Why might the ratio between two Q-band peaks in ZnTPP decrease with increasing temperature?

In ZnTPP (Zinc tetraphenylporphyrin), there are two Q-band emission peaks. When I run steady state emission measurements at different temperatures, the ratio of their respective peak intensities decreases with increasing temperature. This seems to me like an unusual behaviour compared to many other porphyrins, where the ratio remains constant.


Why is this?


The molecule has been around for a while so I would expect to find the answer in the literature, I am currently searching it and might post the answer if I find it before someone else answers.


