If your game allows characters to be created with traits, Pathfinder includes two social traits that do this, and there's a few other similar effects.
Student of Philosophy, social trait from PPC: Quests and Campaigns
You can use your Intelligence modifier in place of your Charisma modifier on Diplomacy checks to persuade others and on Bluff checks to convince others that a lie is true. (This trait does not affect Diplomacy checks to gather information or Bluff checks to feint in combat).
This doesn't apply to absolutely all uses of Diplomacy, but it does extend the benefit to some uses of Bluff as well.
Clever Wordplay, social trait from PPC: Pathfinder Society Primer
Choose one Charisma-based skill. You attempt checks with that skill using your Intelligence modifier instead of your Charisma modifier.
This allows you to substitute Intelligence for Charisma entirely on any one Charisma-based skill, such as Diplomacy.
The Empiricist, investigator archetype from PRG: Advanced Class Guide
The Empiricist's Ceaseless Observation class feature allows that:
At 2nd level, an empiricist uses his Intelligence modifier instead of the skill’s typical ability for all Disable Device, Perception, Sense Motive, and Use Magic Device checks. He can also use his Intelligence modifier instead of Charisma on any Diplomacy checks made to gather information.
Which broadly allows you to substitute Intelligence on several skills, and specifically on Diplomacy checks to gather information but not other Diplomacy checks.
Ironclad Logic, feat from PRG: Ultimate Intrigue
This doesn't actually let you substitute Intelligence for Charisma generally, but it does represent leveraging your high Intelligence (Int 19 required) for a bonus to Diplomacy checks:
You gain a +4 bonus on Diplomacy checks whenever you add your Charisma modifier on those checks.
If you have at least 10 ranks in Diplomacy and an Intelligence score of 27 or more, this bonus increases to +8.
Inside Man, slayer talent from Advanced Options: Slayer Talents & Lethalities (third-party, Rogue Genius Games)
Besides a bunch of free class skills, the Slayer gets to substitute Intelligence on Diplomacy checks:
Additionally the slayer uses his Intelligence modifier, rather than Charisma modifier, with Diplomacy checks.