If it is on seperate tickets you can checkin for the LH flight 23h prior to departure (of the LH flight) and print your BP online.
This should save you some time, but depending on your luggage situation 1h40m might be a bit risky. If SriLankan will interline your bags you should be fine. If you have to collect them in FRA and re-check them at LH T1 then I would leave more time.
If you travel with carry-on only 1h40m should be just about ok if there are no delays.