Pin Oaks prefer an acidic soil and chlorosis is not uncommon in a more basic soil profile. There are homeowner products available, usually in liquid form, once properly diluted, applied as a soil drench in the drip line.
A certified arborist in your area providing that service could treat your trees. Commercially this is done via injection into the soil in the drip line. Trunk injection is another option, but for the amount of iron needed the soil injection may be preferred or recommended. Best absorption is in the spring of the year as growth resumes. Iron is critical for the chlorophyll molecule which gives the leaves the dark green color we expect. this is a treatment that may need to be done annually or every other year, depending of severity.