Keep them watered well...clematis like to have their roots stay moist..especially water well with the temps being so hot right now..I think they are just in shock and should perk back up..stick your finger in the soil each morning and if it seems to be drying out water long and deep...Jeanne

I agree with Jeanne's comment, clematis are specialist plants, they need good drainage, plenty of free draining water and should be planted with a couple of buds below the soil level. They also need to be well mulched so as to ensure roots are kept cool. They're fussy at planting time but hardy once established. The symptoms you describe are those of 'clematis wilt' but as you've just planted yours that is unlikely. Give them a good water and if they don't respond in a day or so consider 'wilt' and cut back to a bud or two just above the soil - seems that's the only way to save them, and if it is 'wilt' and they survive they should then be immune to future attack. Good luck.

If they eventually turn black then it has wilted ...otherwise it's just shock and good advice on planting deeper and using mulch..keeping the mulch away from the vine for future reference when planting clematis...Jeanne

I am a total beginner , but I can tell you I encountered the same situation. My clems planted about 6 weeks ago were droopy to the point of falling off the trelis, and this every day starting from midday on, in spite of being watered daily and well mulched. They are both Rouge Cardinal, lovely and seemingly floriferous clems.
It must be transplant shock, as they did keep on growing and each continuously gives me new blooms. Lately ( about a week ago, after much patience with them) they kind of stopped the drooping, they still do it on an occasional really hot day though. None of them wilted, apart from a couple of leaves here and there that turned black/brown and I removed. Jeanne said they would grow out of it, and they are apparently just doing it. It takes time, so don't abandon them.

I so appreciate the encouraging posts!!