Complementarity and internal consistency between magnetic and optical properties for the manganese(II) copper(II) heterodinuclear compound [Mn(Me6-[14]ane-N4)Cu(oxpn)](CF3SO3)2 (Me6-[14]ane-N4 = (.+-.)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclo