Energy transfer in coumarin-sensitised lanthanide luminescence: investigation of the nature of the sensitiser and its distance to the lanthanide ion

Abstract

A series of lanthanide complexes [Ln(dpxCy)3](3-) have been synthesised. The ligands are composed of a coordinating dipicolinic acid backbone decorated with a polyoxyethylene arm fitted with a coumarin moiety at its extremity. The nature of the coumarin as well as the length of the linker have been varied. Upon excitation at 320 nm, the coumarin exclusively acts as an antenna while the dipicolinic acid core is not excited. Upon excitation below 300 nm, both parts are excited. With europium as a metal centre, the relaxation of the europium ion (intrinsic quantum yield Φ(Eu)(Eu) and radiative lifetime τr) is constant for all the studied ligands. Therefore, the observed differences in overall quantum yield (Φ(Eu)(L)) in such systems come exclusively from the variation of the terminal coumarin. The overall quantum yields of the studied complexes are low (Φ(Eu)(L) < 2% in aqueous solution). In order to rationalise the mechanism of the energy transfer and to improve the sensitisation efficiency (ηsens), the distance between the coumarin sensitiser and the lanthanide centre was explored in solution and compared to the solid state. In the solid state, a dramatic effect was confirmed, with an improvement of 80% in the quantum yield Φ(Eu)(L) for short linkers ((-CH2CH2O-)n with n = 1 compared to n = 3). By monitoring the lifetime decay of the excited state of the lanthanide cation with nanosecond vs. microsecond time-resolved spectroscopy at low temperature, the sensitisation of the lanthanide ions by coumarin derivatives was demonstrated to mainly occur through the singlet excited state of the coumarin and not via the usual triplet pathway. No evidence of a different behaviour at room temperature was found by transient triplet-triplet absorption spectroscopy.

Multiplex Detection of Enzymatic Activity with Responsive Lanthanide‐Based Luminescent Probes

“…These preliminary data do not enable the assignment of either a triplet [16] or a singlet-mediated [17] Lnsensitization pathway. Such low-energy excitation is desirable for biological applications.…”

“…The low-temperature lifetime of the triplet state shortened in EuCou (299 ms) and TbCou (616 ms). These preliminary data do not enable the assignment of either a triplet [16] or a singlet-mediated [17] Lnsensitization pathway. The TbCou emission was slightly sensitive to oxygen, probably because of energy back transfer from the Tb excited state to the coumarin triplet.…”

Sensitised LnIII Emission and Excited‐State Dynamics of Cofacial ‘Pacman’ Porphyrin Terpyridine Complexes

“…Clearly, these values are much shorter than for the corresponding Lu III complex. This may be due to presence of an S 1 ‐mediated energy‐transfer pathway, which has recently been highlighted as a potentially important pathway for sensitised emission from Yb III and Nd III cations . Alternatively, the decreased S 1 excited‐state lifetime may be due to the increased efficiency of intersystem crossing (ISC) in the presence of the paramagnetic metal centre, which has also been shown to facilitate mixing of the nominal S 1 and T 1 excited states, which relaxes the spin‐selection rule and, hence, enhances the rate of ISC …”

“…This may be due to presence of an S 1 -mediated energy-transfer pathway, which has recently been highlighted as ap otentially important pathway for sensitised emissionfrom Yb III and Nd III cations. [44][45][46] Alternatively,t he decreased S 1 excited-state lifetime may be due to the increased efficiency of intersystemc rossing (ISC) in the presence of the paramagnetic metal centre, which has also been shown to facilitate mixing of the nominal S 1 and T 1 excited states, which relaxes the spin-selection rule and, hence,e nhances the rate of ISC. [47,48] To test both thesep ossibilities, we have undertaken additional fs-TAm easurements by using a[ Zn(PXT)Ln(NO 3 ) 3 ]c omplex with Ln = Gd.…”

Contribution of Energy Transfer from the Singlet State to the Sensitization of Eu3+ and Tb3+ Luminescence by Sulfonylamidophosphates

“…[6][7][8] Among thousands of publications on lanthanide complexes,o nly af ew have included evidenceo fadominant singlet (S 1 )p athway. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] Dominant singlet energy transfer has seldom been observed, in the sensitization of lanthanide ions, because intersystem crossing (S 1 !T 1 )i su sually very fast due to not only the rather small S 1 -T 1 energy difference (ca. 5000 cm À1 ), but also to the external heavy-atom effect induced by the lanthanide ion.…”