The reason for the phosphorescence lifetime difference is often considered to be related to the El-Sayed rule . During the research for this article, we asked social networks what would be the cause of this phenomenon. The El-Sayed rule was the most common answer, indicating this explanation's popularity among the chemistry community. Nevertheless, despite exhaustive research, we could not locate any reference that convincingly showed how the El-Sayed rule would apply in this case. The closest we found was Olmsted and El-Sayed's analysis of BA phosphorescence .