He was in Paris serving as minister to France.
Did Thomas Jefferson write the Constitution? No, he was actually a supporter of the smaller government structure originally proposed in the Articles of Confederation and did not want its revision to mean a stronger, more centralized union.
If Thomas Jefferson wrote the Constitution, he would have preferred leaving many of the enumerated powers in the hands of state governments and may have included checks and balances similar to those he envisioned when he drafted the Virginia Constitution.
However, we can say that he had INFLUENCE on those who help craft the contitution.
Without the influence of Thomas Jefferson, Bill of Rights mainstays such as the right to bear arms and religious freedom may not have held up so strongly.
Although someone like James Madison might be considered father of the Constitution, Thomas Jefferson would ascend to the presidency on principles that advocated for restraint of federal powers and a return to the the agrarian democracy he envisioned.