half-life of caffeine molecules I am interested in the caffeine content in various coffees.

Will day-old coffee have much less caffeine?  I am refering to espresso drinks such as the Starbucks Americano or the drip coffees also available there.  Does refrigeration make much difference to the half-life?  What about re-heating?

How about instant?  I have seen charts comparing caffeine content in various fresh coffees but not instant.  What is the typical caffeine content of instant coffee?  What is the half-life of caffeine in instant coffee on the shelf as a room temperature solid?
According to Sigma-Aldrich, pure caffeine has a shelf-life of four years at room temperature, or many years at 2-8°C.

A caffeine solution can be stable for months even at moderately high temperatures.

So, essentially, your day old coffee has still all its caffeine in it, although probably it does not taste that well, but that's caffeine unrelated.

As a side note, after drinking coffee/tea/Coke/etc. it will take ~1-2 hours for blood caffeine levels to peak. Caffeine half-life in the body is ~3-6 hours.