How to convert a string into a structured list

Block[{Times = Sequence},
 ToExpression@ StringRiffle@ Characters@ "{ab{c{de}}f}"
 ]

(*  {a, b, {c, {d, e}}, f}  *)

str = "{ab{c{de}}f}";
str2 = StringReplace[str, x : LetterCharacter ~~ y : LetterCharacter :> x ~~ "," ~~ y];

Block[{Times = Sequence}, ToExpression[str2]]

{a, b, {c, {d, e}}, f}

str = StringReplace["{ab{c{de}}f}",x : LetterCharacter :> "," <> x <> ","];
Quiet[ToExpression[str] /. Null :> Nothing]

(*{a, b, {c, {d, e}}, f})*