Incorporation of electron withdrawing {TAIL ~ groups} on the acyl substituent at the 10 position of the leuco dye such that it is electron withdrawing overall results in a dramatic improvement in the thermal and light stability of the leuco form, and, generally, the stronger {HEAD ~ the electron} withdrawing character of the acyl substituent, the more stable is the leuco form 
Incorporation of electron withdrawing { TAIL ~ groups } on the acyl substituent at the 10 position of the leuco dye such that it is electron withdrawing overall results in a dramatic improvement in the thermal and light stability of the leuco form , and , generally , the stronger { HEAD ~ the electron } withdrawing character of the acyl substituent , the more stable is the leuco form
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH