How to delcare it

sqlcmd = new SqlCommand("backup database  " + dbname + " to disk='" + destdir + "\\" + DateTime.Now.ToString("ddMMyyyy_HHmmss") + ".bak'", con);

string filename = "D:\\Backup\\DateTime.Now.ToString("'ddMMyyyy_HHmmss'").Bak";

I have taken the database backup in specifi folder with above sqlcmd, but how to declare the path adress in string filename, the error is getting in above line code for DateTime.Now.ToString("'ddMMyyyy_HHmmss'").Bak";