Photophysics of 1,8-naphthalimide/Ln(iii) dyads (Ln = Eu, Gd): naphthalimide → Eu(iii) energy-transfer from both singlet and triplet states

Abstract

Transient absorption and time resolved luminescence spectroscopy were used to study photophysical processes in the macrocycle-appended 1,8-naphthalimide compound H₃L, and its Eu(III) and Gd(III) complexes Eu·L and Gd·L, in particular the naphthalimide–Eu(III) energy-transfer process. In all cases aggregation of the naphthalimide chromophores results in a low-energy emission feature in the 470–500 nm region in addition to the naphthalimide fluorescence; this lower-energy emission has a lifetime longer by an order of magnitude than the monomer naphthalimide fluorescence. Transient absorption spectroscopy was used to measure the decay of the naphthalimide triplet excited state, which occurs in the range 30–50 μs. In Eu·L, partial energy-transfer from the naphthalimide chromophore results in sensitized Eu(III)-based emission in addition to the naphthalimide-based fluorescence features. Time-resolved measurements on the sensitized Eu(III)-based emission reveal both fast (∼10⁹ s⁻¹) and slow (∼10⁴ s⁻¹) energy-transfer processes from the naphthalimide energy-donor, which we ascribe to energy-transfer occurring from the singlet and triplet excited state of naphthalimide respectively. This is an unusual case of observation of sensitization of Eu(III)-based emission from the singlet state of an aromatic chromophore.