Sensitization of Nd³⁺ Luminescence by Simultaneous Two-Photon Excitation through a Coordinating Polymethinic Antenna

Abstract

We have designed and synthesized two new cyaninic Nd3+ complexes where the lanthanide emission can be induced from simultaneous two-photon absorption followed by energy migration. These complexes correspond to a molecular design that uses an antenna ligand formed by the functionalization of a heptamethine dye with 5-ol-phenanthroline or 4-phenyl-terpyridine derivatives. These complexes employ the important nonlinear optical properties of symmetric polymethines to sensitize the lanthanide ion. We verified that simultaneous biphotonic excitation indirectly induces the 4F3/2 → 4I11/2 Nd3+ emission using femtosecond laser pulses tuned below the first electronic transition of the antenna. The simultaneous two-photon excitation events initially form the nonlinear-active second excited singlet of the polymethine antenna, which rapidly evolves into its first excited singlet. This state in turn induces the formation of the emissive Nd3+ states through energy transfer. The role of the first excited singlet of the antenna as the donor state in this process was verified through time resolution of the antenna’s fluorescence. These measurements also provided the rates for antenna-lanthanide energy transfer, which indicate that the phenanthroline-type ligand is approximately five times more efficient for energy transfer than the phenyl-terpyridine derivative due to their relative donor–acceptor distances. The simultaneous two-photon excitation of this polymethine antenna allows for high spatial localization of the Nd3+excitation events.

Seeking Brightness in Molecular Erbium-Based Light Upconversion

“…This might suggest the operation of a L2 (dye) → Er energy transfer in the complex with a global rate constant = 7.7(2) × 10 8 s –1 and an efficiency of = 49(1)%. The latter interpretation corresponds to that previously proposed for rationalizing the titration of [(IR780-Phen)] + with Nd­(DBM) 3 in acetone, where = 2.5 × 10 9 s –1 and = 75% were reported for [(IR780-Phen)­Nd­(DBM) 3 ] + (Figure c) …”

“…This might suggest the operation of a L2(dye) → Er energy transfer in the complex with a global rate constant 2c). 85 However, as a control experiment, we additionally recorded the dye-centered π(S 1 ) emission lifetime in [L2Y(hfac) 3 ] + , and we found that it indeed closely mirrors that found for [L2Er(hfac) 3 ] + with τ π(S1) = 670(13) ps (Figure S27). One concludes that the shortening of the dye-centered emission lifetime in going from [L2] + to [L2Er(hfac) 3 ] + does not reflect the operation of the sensitizer to activator energy transfer as proposed in ref 85, but it is mainly due to the heavy atom effect and spin−orbit coupling (Z = 68 for erbium), which allow n o n -n e g l i g i b l e i n t e r s y s t e m c r o s s i n g w i t h…”

“…However, as a control experiment, we additionally recorded the dye-centered π­(S 1 ) emission lifetime in [ L2 Y­(hfac) 3 ] + , and we found that it indeed closely mirrors that found for [ L2 Er­(hfac) 3 ] + with τ π(S1) = 670(13) ps (Figure S27). One concludes that the shortening of the dye-centered emission lifetime in going from [ L2 ] + to [ L2 Er­(hfac) 3 ] + does not reflect the operation of the sensitizer to activator energy transfer as proposed in ref , but it is mainly due to the heavy atom effect and spin–orbit coupling ( Z = 68 for erbium), which allow non-negligible intersystem crossing with = 7.7(2) × 10 8 s –1 (efficiency = 49(1)%). Because of the smaller spin–orbit constant of yttrium ( Z = 39), the latter rate constant is slightly reduced and reaches = 7.0(2) × 10 8 s –1 (efficiency = 47(1)%) in [ L2 Y­(hfac) 3 ] + .…”

“… It is worth stressing here that the huge absorption of the dye at 801 nm is difficult to make compatible with the use of the comparative method applied here with front-face geometry for estimating the upconverting quantum yields (see Appendix 3). Since the same method was used for [ L1 Er­(hfac) 3 ] + and [ L2 Er­(hfac) 3 ] + , comparison remains pertinent, and Table collects experimental upconversion quantum yields normalized for P = 25 W·cm –2 (assuming the expected linear dependence of Φ up with P in the absence of the saturation phenomenon). , …”

“…With this in mind, cyanine-based sensitizers relevant to the family of closed-chain cationic streptopolymethines, , which display among the largest available NIR absorption coefficients ( > 10 5 M –1 ·cm –1 ), ,, have been regularly grafted onto the surface of lanthanide-doped nanoparticles for rising ETU processes. ,, Inspired by these successes, miniaturization to reach the molecular size was first attempted by Hyppännen and co-workers, who measured solution-based NIR-to-visible upconversion assigned to an ETU mechanism operating in prospective [IR-806]­[Er­(tta) 4 ] ion pairs formed in chloroform (Figure a) . Beyond their connections to terpyridine- or phenanthroline-binding units for exploring the non-linear two-photon sensitization of [Nd­(dibenzoylmethane) 3 ] in ethanol, followed by light-downshifting (Figure c), a cationic cyanine has been connected only once, to the best of our knowledge, to a terdentate unit for inducing molecular upconversion in the mononuclear [ L1 Er­(hfac) 3 ] + complex according to the ETU mechanism (Figure b) . Further improvements of the upconversion quantum yields and brightness in dye-sensitized molecular complexes require (i) an increasing number of cyanine sensitizers per activator and/or (ii) a closer approach between the cyanine sensitizer and the lanthanide emitter.…”