There is no wildcard character in custom date and time format that it can parse every possible character in your string.
You can add your format the first character of your string like;
string s = "1140421164500";
Console.WriteLine(DateTime.ParseExact(s, s[0] + "yyMMddHHmmss",
CultureInfo.InvariantCulture));
Output will be;
4/21/2014 4:45:00 PM
Here a demonstration
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