The PrÄká¹›taprakÄÅ›a has the rule (8.25)
á¹£á¹hÄ-dhyÄ-gÄnÄṃ á¹hÄ·a-jhÄ·a-gÄ·Äḥ
(षà¥à¤ ा-धà¥à¤¯à¤¾-गानां ठाअ-à¤à¤¾à¤…-गाआः)
which explains Sanskrit dhyÄ changing to jhÄ·a. Hence
dhyÄyanti -> jhÄanti
dhyÄna -> jhÄṇa
The AbhidhÄnarÄjendraḥ has a rather long entry under jhÄṇa. It also derives the word from dhyÄna and cites the rule sÄdhyasadhyahyÄṃ jhaḥ instead, which I believe is from Hemacandra's grammar.
Hope this helps, NityanandaÂ