Formally say "thank you for taking the time and effort in doing something"

Question

I hope the question is not too silly, but I think it may help others too.

I want to thank the professors that are writing reference letters to me. I thought I'd put this at the end of my email requesting the aforementioned letters:

1) Thank you for taking the time to write this letter.
2) Thank you for taking the time and effort in writing this letter.
3) Thank you for dedicating your time and energy into this.

Are these options acceptable? What would be a more natural way of saying this stressing not only the time they took to write it, but also the pains they took, having to lose time writing this?

I think this question has the benefit of being useful in other cases. I just realized I don't really know how to thank people for something like this formally in English.

Related questions:
How to say thank you for the ....?
'Thank you for taking your time writing' or 'Thank you for taking your time to write' ?
Should I say "Thank you for your kindly attention" or "Thank you for you kind attention"

EDIT: I should have emphasized this better, but I felt it was very important to thank the referees for the effort (not only the time spent) put into writing the letter. So I wanted that the answer to this question could highlight this desire. And thank you for your answers! All of them are excellent, it's a shame I can pick only one as the correct answer.

Answer 1

Instead of ending the letter with these sentences, a correspondence reply letter usually start with these lines.

"Thank you for taking the time to write this letter..."

This is perfectly natural and a very commonly used sentence, used to start off a reply letter.

"Thank you for taking the time and effort in writing this letter."

This would be more suitable if the initial letter was very long, and may have given you the impression that the writer took a great deal of effort to write it.

"Thank you for dedicating your time and energy into this."

Although not wrong, this sounds a little too dramatic. Thanking him for spending 'energy' on writing letters (although he may have), sounds awkward and very dramatic.

Answer 2

• I would like to take this opportunity to thank you for writing this reference on my behalf.
• Thank you so very much for writing this reference on my behalf.
• Thank you for taking the time to write this reference on my behalf.

I would avoid time and effort, etc. It's patronizing in my opinion in this context.

Answer 3

ORIGINAL: Thank you for taking the time and effort in writing this letter.

I don't like this one. We don't "take the effort in" writing a letter, we "put effort into" writing a letter. I suggest removing the word taking:

REVISED: Thank you for your time and effort in writing this letter.

---

ORIGINAL: Thank you for dedicating your time and energy into this.

I agree completely with the earlier answer – I think that sounds too flowery.

---

As for getting your point across simply, I'd suggest adding a second sentence to your first:

Thank you for taking the time to write this letter. I know you are busy, so I really appreciate your time and effort.

I thinks that flows more natural than trying to express both gratitude and "pains" in the same opening sentence.