Jisho

明く is present in the entry for あく when you click to it.

https://jisho.org/word/%E9%96%8B%E3%81%8F-1

開く is listed at the top of the entry because it is the most general way to write the word あく, but if you scroll to the bottom of the entry you'll see other forms, including 明く and 空く.

Additionally, the individual definitions tell you which way to write it is most common for that particular meaning. For example, definitions 5, 6, 7, and 8 all say that those meanings are especially associated with 明く.

It's very common for a single word to be written in a variety of ways due to how the Japanese language and kanji are related. The spoken Japanese language existed first, and it contained these various nuances within the same word. Then kanji were borrowed from China, and it became possible to write a single word with different kanji to capture those nuances.

If that doesn't explain what you wanted to know, just let me know.

Well i can get the nuance thing but relation is where im confused. Brightness relates to open only in 字音 or do they have another connection?

Kanji typically get used in verbs because the kanji had those meanings when used in Chinese originally. There should be some compounds from Chinese where 明 expresses a similar meaning to what is seen in the verb あく. 明 doesn't only mean "brightness" but it also means "dawn" and, by extension, the "opening" of the new day. My monolingual dictionary lists 7 definitions for 明, not just "brightness."

①あかるい。「明星」「清明」 ②あかり。あかりがつく。「明滅」「灯明」対滅 ③あきらか。あきらかにする。「明確」「証明」 ④さとい。かしこい。「明君」「賢明」 対①③④暗 ⑤あける。夜があける。また、つぎの。あす。「明晩」「未明」 ⑥神。また、神聖なもの。「神明」 ⑦みん。中国の王朝名。

So i can surmise that this etymological theory can be expressed and seen in all kanji. Thanks for your extensive reply.