Mechanism of gadolinium doping induced room-temperature phosphorescence from porphyrin

The spin-orbit coupling mechanism was generally used to explain heavy atom effect induced room-temperature phosphorescence (RTP). Here, we demonstrate that the mechanism of RTP induced by Gd³⁺ from hematoporphyrin monomethyl ether (HMME) is due to the mixing of singlet (S) and triplet (T) states. The spin-forbidden transition between S and T states was partly allowed due to the states mixing, as indicated by the direct absorption corresponding to transition from S₀ to T₁, which was observed for the first time from RTP. The quantum yield of T₁ was determined to be 0.80, and the percent of each energy transfer process was determined. Meanwhile there is no nonradiative relaxation from HMME to Gd³⁺ because of the large energy gap between the excited and ground states of Gd³⁺. The special energy level of Gd³⁺ as well as the states mixing between S and T states produced the strong phosphorescence emission. The population and deactivation of triplet states in heavy atom induced RTP can be used to analyze the mechanism of phosphorescent emission.