Dynamic quenching of porphyrin triplet states by two-photon absorbing dyes: Towards two-photon-enhanced oxygen nanosensors

Abstract

Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [1]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern-Volmer quenching constants were correlated with the corresponding ET driving forces (ΔG ET ), calculated using the Rehm-Weller equation. FRET-pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.

Generation of Phosphorescent Triplet States via Photoinduced Electron Transfer: Energy and Electron Transfer Dynamics in Pt Porphyrin–Rhodamine B Dyads

“…Based on our previous spectroscopic and electrochemical investigations of Pt porphyrins, 18 we conjectured that a suitable dyad could be constructed from Rhodamine B (RhB + ) and a PtP (Pt(II) tetraarylporphyrin) with appropriately tuned oxidation potential. From the synthetic point of view, tetraarylporphyrins with various substitution patterns can be conveniently generated using well-established synthetic protocols.…”

“…56 Peripheral substituents in the meso -aryl rings practically do not affect spectroscopic features of PtP’s, while significantly altering their electrochemical potentials. 18 This feature gives the desired flexibility of tuning the level of the RP state without affecting the rest of the chromophoric system.…”

“…On the other hand, the ability to generate an emissive triplet state via ET, as opposed to having to deal with ET to prevent unwanted phosphorescence quenching – a problem we encountered in construction of two-photon oxygen probes, 13 – would greatly extend the choice of antenna chromophores suitable for construction of these useful sensors. 18 …”

Two-Photon Absorbing Nanocrystal Sensors for Ratiometric Detection of Oxygen

“…27-32 We now report the design of an oxygen sensor incorporating a semiconductor nanocrystal (NC) that acts as both a scaffold, intensity standard and two-photon sensitizer for the Os(II)PP complexes (via FRET). Whereas polypyridyl complexes have been used pervasively as one-photon sensors, they have not been prominently exploited within a two-photon sensing construct owing to their relatively low two-photon absorption cross-sections.…”

Dendritic Encapsulation of Redox‐Active Units

“…[11][12][13][14][15] Encapsulation may also be a key feature in light harvesting and energy funnels [16][17][18][19][20][21][22][23][24][25][26][27] and nonlinear optical effects. [28][29][30][31] The reactivity of catalytic sites can also be tuned by dendritic encapsulation.…”