Jefferson was ambassador to France at the time.
He was trying to keep his head during the French Revolution.
Meanwhile, I believe it was James Madison that penned much of the Constitution, albeit not in a vacuum.
The Declaration of Independence was penned by Thomas Jefferson, but he had a Committee of Five that edited it for him. He actually had a first draft which included an anti-slavery theme.
The Constitution was put together by the committee present at Independence Hall. Each man had a significant part to play in it. George Washington was the one who wrote to many of the men continually, years before it took place, revving up interest in establishing a constitution. When the men finally got together they used their collective knowledge to draft the new Constitution. Some members had a heavier hand, but it was a collective effort.