Dear sir

if you ask the question related to plant tissue culture  kindly refer this article

Evaluation of different carbon sources for high frequency callus culture

with reduced phenolic secretion in cotton (Gossypium hirsutum L.) cv.

SVPR-2

You can use liquid Nitrogen or Phosphate buffer for the same.

As my experience bring your sample in LN2 and keep it in -80oC up to your work.

In cases where a leaf sample is to be collected from another region, storing in -80 deg. C is not an option nor using liquid Nitrogen....In such case, is there any chemical method to prevent the release of phenol content in that leaf ?

Yes! in that case the phosphate buffer of pH 8 will be a good option.