Compounds capable of forming an intramolecular exciplex on photoirradiation of the compound in water comprise two exciplex forming partners, one being a donor moiety and the other an acceptor moiety, each having at least one aromatic nucleus and being connected by a saturated aliphatic chain having the flexibility to allow said partners to come into exciplex forming relationship. The compounds may be used as labels for oligonucleotides. Certain of the compounds display pH sensitive emission.
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