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