Saving a large integer in Ocaml

I am trying to convert a large base-10 number to base-16 and then to hex. The error I am getting is Ocaml is saying the integer is too big.

utop # let large_int = 11515195063862318899931685488813747395775516287289682636499965282714637259206269 ;;Error: Integer literal exceeds the range of representable integers of type int

I tried using Int64 as well and a similar issue.

utop # let large_int = Int64.of_string "11515195063862318899931685488813747395775516287289682636499965282714637259206269";;    Exception: Failure "Int64.of_string".

This is the larger code I am trying to make work. It works as expected with the example integer below.

(* convert from base10_to_base16 *)let rec split_digits n =  if n = 0 then [] else    let (d, rest) = n mod 256, n / 256 in    d :: split_digits restlet decode_base10 message =  let bytes_arr = List.rev (split_digits message) in  let decoded_message = String.concat "" (List.map (fun x -> String.make 1 (Char.chr x)) bytes_arr) in  decoded_messagelet base10_message = 310400273487 (* base-10 number *)let decoded_message = decode_base10 base10_message