Triplet-triplet Extinction Coefficient and Triplet Yield. The extinction coefficient for 6MP's triplet-triplet absorption band was calculated using the singlet depletion method. This method, detailed in reference, is convenient for thiobases and has been previously used for 4thiothymidine, 4-thiouracil, 42 and 6-thioguanosine. Briefly, the excited triplet state concentration of 6MP, [ 3 M * ], can be obtained for any given time delay using equation (4):