Try this:
DateTime.ParseExact(str, "yyyyMMddHHmmss", CultureInfo.InvariantCulture);
If the string may not be in the correct format (and you wish to avoid an exception) you can use the DateTime.TryParseExact
method like this:
DateTime dateTime;
DateTime.TryParseExact(str, "yyyyMMddHHmmss",
CultureInfo.InvariantCulture, DateTimeStyles.None, out dateTime);