By itself, a full sun exposure should not have caused the plants any damage, though it can add to heat stress when plants are in containers. (Especially since the pot can get fairly hot, taxing the roots.) Could the containers have dried out too much at some point? Even one afternoon of being too dry could significantly scorch leaves. (If they were kept too wet, that can rot roots and similarly scorch foliage, but usually the foliage wilts and turns yellow-green before browning and falling off in that case.) Opportunistic infections like Botryosphaeria canker can also take advantage of stressed plants, though we can't tell if that was a factor in this case.

For the future, it's best to put new plants in the ground as soon as you're able; if the location for these dogwoods wasn't ready for planting yet, then keeping them in pots is more practical, but otherwise, being kept in containers instead of planted in the ground adds to the stress of plants, both in summer and winter. If the option is available, it's always best to put new plants in the ground as soon as possible so they can start establishing roots; in-ground soil won't change temperature as drastically or dry out as quickly as potting mix, and it also retains more nutrients.

We can't see enough detail in the photo to guess if that plant will recover, but if you have the option, plant all of them as soon as possible, and monitor them for watering needs for the rest of summer and autumn. (Winter, too, if it's mild enough and we don't get much rain or snow.) If you need to keep them in pots for awhile longer, putting them in part shade is a good idea, and just check on them regularly for watering needs; nothing else can be done to encourage regrowth, and if they are able, they'll produce new foliage at some point later this summer.

Miri