The switchable phosphorescence and delayed fluorescence of a new rhodamine-like dye through allenylidene formation in a cyclometallated platinum(ii) system.

Shunan Zhao,Véronique Guerchais,Julien Boixel,Yifan Zhu,Keith Man-Chung Wong,Ling Li

doi:10.1039/d1sc02787e

Abstract

A new rhodamine-like alkyne-substituted ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. The chemo-induced “ON–OFF” switching capabilities on the spirolactone ring of the Rhodyne ligand with an end-capping platinum(ii) metal centre can modulate the interesting acetylide–allenylidene resonance. The long-lived 3IL excited state of Rhodyne in its ON state as an optically active opened form was revealed via steady-state and time-resolved spectroscopy studies. Exceptional near-infrared (NIR) phosphorescence and delayed fluorescence based on a rhodamine-like structure were observed at room temperature for the first time. The position of the alkyne communication bridge attached to the platinum(ii) unit was found to vary the lead(ii)-ion binding mode and also the possible resonance structure for metal-mediated allenylidene formation. The formation of a proposed allenylidene resonance structure was suggested to rationalize these phenomena.

Highlights

Transition metal complexes have attracted much interest due to a variety of potential applications,[1,2,3,4,5,6,7] associated with the capability of promoting an emissive triplet excited state at room temperature.[8,9] the controllable population of the triplet excited state by an external stimulus has become an intense research topic of interest, dedicated to the examples of pH- and photo-activatable photodynamic therapy (PDT) reagents,[10,11,12] molecular logic gates[13] and data storage,[14] or fundamental photochemistry.[15,16] The incorporation of switchable organic units into optical-active transition metal complexes is one common strategy to achieve metal-based photo-functional materials with “ON” and “OFF” states in a controllable manner.[17,18,19,20,21,22,23,24,25,26,27,28] The design of multi-chromophoric systems capable of triplet sensitization and emission withRhodamine derivatives are found in equilibrium between two isomers with very different spectroscopic properties,[29] i.e. colourless and non- uorescent ring-closed and highly coloured and uorescent ring-opened structures about the spirocarbon
Rhodamine was discovered over a century ago and widely investigated,[33] most research about rhodamine has focused on its photophysics and photochemistry from the uorescence of singlet excited states, studies about rhodamine triplet excited states are relatively limited.[34,35]
Our group has recently developed a versatile strategy to generate a rhodamine triplet state as organelletargeting photosensitizers for efficient photodynamic therapy (PDT) through the ligation of rhodamine tethered chelate into transition metal systems.[43,44]. Such a long-lived triplet excited state of rhodamine is commonly found in a dark state at room temperature which can only be observed by transient absorption spectroscopy

Summary

Introduction

Transition metal complexes have attracted much interest due to a variety of potential applications,[1,2,3,4,5,6,7] associated with the capability of promoting an emissive triplet excited state at room temperature.[8,9] the controllable population of the triplet excited state by an external stimulus has become an intense research topic of interest, dedicated to the examples of pH- and photo-activatable photodynamic therapy (PDT) reagents,[10,11,12] molecular logic gates[13] and data storage,[14] or fundamental photochemistry.[15,16] The incorporation of switchable organic units into optical-active transition metal complexes is one common strategy to achieve metal-based photo-functional materials with “ON” and “OFF” states in a controllable manner.[17,18,19,20,21,22,23,24,25,26,27,28] The design of multi-chromophoric systems capable of triplet sensitization and emission withRhodamine derivatives are found in equilibrium between two isomers with very different spectroscopic properties,[29] i.e. colourless and non- uorescent ring-closed and highly coloured and uorescent ring-opened structures about the spirocarbon. The ring-opened form of 2 obtained by addition of acid into the solution resulted in a colour change to magenta, which is attributed to the appearance of new structured absorption bands at 532 nm and 573 nm and an increase in absorbance at 432 nm (Fig. 2b).

Results

Conclusion