According to this news release,

Otto Frank wrote the letters in 1941 in a despairing effort to get his family out of Nazi-occupied Amsterdam, before finally hiding the family, including Anne, in secret rooms in an Amsterdam office building for two years until they were betrayed, Time magazine said Thursday.

The sheaf of Otto Frank's letters, about 80 documents in total, show him seeking escape routes to Spain, exit visas from Paris and help to get to the United States or Cuba, all in vain.

The New York Institute will release the letters on February 14th after two years of difficulty with "concerns over copyright and other legal issues."