[pgk]

By correlation of all above data, the most possible is that the background fluorescence is due to a structural conjugation (tautomerism) of the probe and not to an autoxidation reaction.  To detail, the phosphine group electron pair conjugates with the one carbonyl of the imide group and forms a phosphonium cation/enol anion system, through perylene double bonds. The said conjugation is stabilized by imide hydrogen bonding with DMSO, ethanol, water or any other hydrogen accepting solvent but destroyed by the acidification (protonation) of the so formed enol anion. Please, note that the said conjugation is also destroyed by the imide proton, in absence of imide hydrogen bonding. Besides, the said conjugation is slow and reaches a ratio of conjugated /non-conjugated probe that is equal to 0.03/0.83 within a week, in DMSO.
If the above hypothesis is right then, it must be an optimal pH that inhibits the probe conjugation fluorescence without seriously affecting the probe peroxidation. Besides, the right selection of the optimal pH avoids the use of reductive preservatives that might affect peroxidation. However, it is you who decides (and hardly works).

[STM]

@pgk

Thank you for your insight once more. Based on your points, I agree that the background fluorescence was not from an irreversible reaction like auto-oxidation of the probe. This is particularly so since the fluorescence of the oxidized product I synthesized was unaffected by the acidic buffer. Considering that dissolving the probe (the same probe solution which earlier had a background fluorescence in water) in an acidic medium completely shut out the background fluorescence, It therefore implies that the background fluorescence emanated from a temporary and reversible phenomenon of structural conjugation among the molecules of the probe.

Please can you refer me to materials where for reference purpose I can read about structural conjugation of phosphorus compounds with carbonyl groups to form phosphonium cations . Or any other material that can help me on this concept of conjugation of Phosphorus compounds to other function groups. Any material along this line will be highly helpful.

Thank you

[pgk]

You can do it by yourself. The structural conjugation of phosphorus compounds without and with carbonyl groups through double bonds, somehow goes like that:
Φ2P¨-CH=O  →  Φ2P+=CH-O-
Φ2P¨-CH=CH-CH=O  →  Φ2P+=CH-CH=CH-CH=CH-O-
Φ2P¨-CH=CH-CH=CH-CH=O  →  Φ2P+=CH-CH=CH-CH=CH-O-
And so on..
Try to apply the above in 3-diphenylphosphine succinimide, 3-diphenylphosphine glutarimide, an aromatic substituted diphenylphosphine phthalimide and an aromatic substituted diphenylphosphine perylenebisimide. It's not so difficult as you think. After the first or second assay, it willeasily go (chemical structures of the base molecule can be found in the web).
H-Bond stabilization:
Φ2P+=CH-CH=CH-CH=CH-O- -------- HOH
Φ2P+=CH-CH=CH-CH=CH-O- -------- HOR
Acidic destabilization:
Φ2P+=CH-CH=CH-CH=CH-O-   +    H+     →   Φ2P+=CH-CH=CH-CH=CH-OH  →
→  Φ2P¨-CH=CH-CH=CH-CH=O  + H+
DMSO stabilization:
Φ2P+=CH-CH=CH-CH=CH-O-   +    Me2S=O    →    Φ2P+=CH-CH=CH-CH=CH-O-S(Me2)-O-
Acidic destabilization:
Φ2P+=CH-CH=CH-CH=CH-O-S(Me2)-O- + H+   → Φ2P+=CH-CH=CH-CH=CH-O-S(Me2)-OH  →
→  Φ2P¨-CH=CH-CH=CH-CH=O  +  Me2S=O   +  H+
Take the look to the UV spectra of benzene, tiphenylphosphine, triphenylphosphine oxide and perylenebisimide from the web, in order to have an idea about the auxochromy of these functional groups, as well as the acidic medium interference.
Auxochromy of the conjugated form can be calculated by the Woodward–Fieser rules. Attention: Homoannular double bonds are transformed to heteroannular, during conjugation. Aromatic compounds follow a liitle different rules but the auxochromic values are about the same.
https://en.wikipedia.org/wiki/Woodward%27s_rules