How to delicately refuse a second date?

It's too late now, which is something your high IQ should have told you during the date itself. What you should have done is insisted during the date that you go dutch. That way if asked for a second date he wouldn't feel he was out much of anything if you kindly declined, saying you didn't think the chemistry was there. As it is, he has invested something in you and will feel used no matter what you say. So accept that and just be prepared next time you are in a similar situation on a first date.

There are ways around this, but believe it or not, I think most people appreciate honesty when it comes to this sort of thing, and you have a responsibility to not waste their time.

Recently, a work colleague asked me out, and I wasn't interested. I said that I respected him as a person, but I wasn't looking for more than friends at that point.

He said that he appreciated my honesty, and we've remained friends.

I know that is best cast scenario, but even if you don't respect him, or don't want to remain friends, saying something along those lines would be the most diplomatic way of going about it.

Just remember that it probably took a lot of courage for him to pursue you and that you should do your best to let him down as easily and cleanly as possible.

I would go with "You know, you seem like a really nice guy, but to be quite honest, I don't think we can click. But it was nice getting to know you". And say no to any follow ups for a second date. Honesty is always the best thing.

And perhaps you want to lay a little groundwork with the friend, just let him/her now that you don't plan on going out with the guy again and maybe that will head off him even asking.