Well, from what I've read, plants store certian chemicals (salts, sugars) in the roots at night, and when photosynisith begins in the morning, these chemicals move up into the tissue of the plant. When night comes on again, these chemicals are sent back to the roots to be stored.
.. that's not how it works. During photosynthesis, the light reactions convert light energy into high energy carriers, including ATP and NADPH. These go on to power the Calvin cycle, which generates glucose. Energy therefore is stored as ATP/NADPH during the light reactions, and later transferred to generate glucose during the dark/Calvin cycle.This is stored in the cells.. not the roots.
So when you chop down before first light, these chemicals won't be in your product.
Generally people who grow weed will flush their plants before they harvest, this will get rid of any "chemicals" still in your plant.. if you mean chlorophyll, that will break down when you cure it.
(As to giving extra dark time, I've also read and tried something called 'shock curing' where you place the the plant in darkness for 3-5 days, and give it cold, ice water. Seemed to really make the plant extra sticky.)