How to redirect language prefix to language domain (TLD)
I had the same issue and solved it by putting a 301 redirect in the htaccess file:

# 301 rewrite for all prefixed language nodes to their own domain
RewriteRule ^en/(.*)$ http://example.com/$1 [L,R=301]
RewriteRule ^de/(.*)$ http://example.de/$1 [L,R=301]


Then I generated a new sitemap and inserted it in the google webmaster tools. I'll monitor the performance impact of this solution. Update is coming in two weeks!