Internal heavy atom effects in phenothiazinium dyes: enhancement of intersystem crossing via vibronic spin-orbit coupling

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods. The ISC rate constants are considerably increased when going from O towards Se while the fluorescence rate constants remain unchanged. For the three dyes, all accessible ISC channels are driven by vibronic spin-orbit coupling (SOC) between pi pi* states. The interplay between the ground and low-lying excited states has been investigated in order to determine the dominant relaxation pathways. In oxonine the relaxation to the ground state after photoexcitation in water proceeds essentially via fluorescence from the S-1(pi(H)pi(L)*) bright state (k(F) = 2.10 x 10(8) s(-1)), in agreement with the high experimental fluorescence quantum yield. In aqueous solution of thionine, the ISC rate constant (k(ISC) similar to 1 x 10(9) s(-1)) is one order of magnitude higher than fluorescence (k(F) = 1.66 x 10(8) s(-1)) which is consistent with its high triplet quantum yield observed in water (phi(T) = 0.53). Due to a stronger vibronic SOC in selenine, the ISC rate is very high (k(ISC) similar to 10(10) s(-1)) and much faster than fluorescence (k(F) = 1.59 x 10(8) s(-1)). This suggests selenine-based dyes as very efficient triplet photosensitizers.