Indolonaphthopyrans

An indolenaphthopyran compound represented by Formula (I),

• wherein,
• R1 and R2 each independently have a steric A value,
• wherein at least one of R1 or R2 has a steric A value of at least 0.6, and at least one of R1 and R2 is, in each case independently, alkyl, alkoxy, or perfluoroalkyl,
• R3 and R4 each independently have a Hammett 6p value, wherein at least one of R3 and R4 is, in each case independently, hydrogen, alkyl, acyclic alkyl amine, alkoxy, fluoro, or a nitrogen-containing heterocycle selected from morpholino, piperidino, and pyrrolidino,
• wherein the indolenaphthopyran compound has a calculated electronic steric factor of at least −3.3, wherein the electronic steric factor (ESF) is calculated in accordance with the following equation,
• ESF=[(sum of Hammett σp values for R3 and R4))×10]+(sum of steric A values for R1 and R2)
• m is 0 to 4, n is 0 to 4;
• and
• R5 independently for each m and R6 independently for each n are:
• alkyl;
• a perfluoroalkyl group;
• or
• alkoxy.