Sulfone-fluorescein (Scheme 1,a; X = SO2) is the sulfonebridged analogue of a long-established green-emitting fluorescent dye fluorescein (3',6'-dihydroxyfluoran). Introduction of a strongly electron-deficient bridging group has been previously demonstrated to shift the absorption and emission maxima, mainly by lowering the LUMO energy level of the fluorophore, into the red range (>600 nm) preferred for higher light penetration depth and lower phototoxicity. Unlike the spirolactone variants with modified xanthene core (thiofluorescein, 2 carbofluorescein 3 , Si-fluorescein and their halogenated versions 5 ), only the derivatives with 2-alkyl-or 2-alkoxyphenyl pendant ring have been reported for bora-fluorescein and phospha-fluorescein, 1 while sulfone-bridged fluorone analogues of fluorescein remain unknown outside of patent literature. A concise synthetic strategy to access sulfone-fluoresceins would involve nucleophilic addition of aryllithium or arylmagnesium reagents to the keto group of appropriately substituted 9H-thioxanthen-9-one 10,10-dioxides. These heterocycles have found use in organic electronics as acceptor units in building blocks for thermally activated delayed fluorescence emitters, in particular in challenging recent applications such as phosphorescent and circularly polarized OLEDs. Thioxanthone 10,10-dioxides have previously been prepared through nucleophilic 12 or electrophilic ring closure of diaryl sulfones obtained in a multistep sequence, or, by far most commonly, via oxidation of pre-assembled thioxanthones, thus limiting the scope of compatible functional groups. Instead, we envisaged an alternative synthetic approach involving a Pd-catalyzed sulfonylative homocoupling of benzophenones bearing the leaving groups in 2,2'-positions, readily accessible via established methods 15 (Figure ). For this, we were inspired by the recent reports of Wu, Wang and Jiang 17 relying on sodium dithionite as a masked "SO2 2-" synthone in their preparation of alkyl aryl sulfones, since the reported systems employing other SO2 surrogates 18 (K2S2O5, DABSO, or formamidinesulfinic acid) consistently failed when tested with our substrates. Gratifyingly, after a brief investigation of the reaction conditions (Table ), it was identified that the target thioxanthone 10,10-dioxides formed in good yields (>70% for many examples, see Scheme 2) with an air-stable and inexpensive Pd(II) catalyst Pd(dppf)Cl2 when DMSO was used as the reaction solvent with mild heating (80 °C). Distinct from the reported conditions, no addition of an external base or quaternary ammonium salts was needed. At least 1.5 equiv. of Na2S2O4 was required to achieve complete conversion of the starting material, but higher excess of dithionite was well tolerated and found necessary in certain cases. Lowering the Pd catalyst loading substantially decreased the reaction yields. The reaction was sensitive to the nature of solvent, and even though other dipolar aprotic solvents (in particular DMF) were suitable, the reproducibility suffered because of the significant induction time as determined by means of in situ IR spectroscopy (see Figure ). The substrates bearing strong electron-withdrawing groups yielded xanthones (e.g., 1m', 1o') as the major products, likely via competing hydrolysis of aryl triflates to phenols if the main reaction pathway was slowed down, and the presence of aryl halides (as in 1r) other than fluorine was not tolerated. Most peculiarly, and in stark contrast with previous reports, the transformation was only successful in intramolecular version; otherwise, alternative reactivity with formation of mixtures containing diaryl sulfides (2p) or disulfides (2q) was noticed, while other bridged (1t,u) or simple aryl triflates (1v) were unreactive. The required presence of a coordinating 2-carbonyl group suggested the initial formation of an oxidative addition complex (Scheme 3, i) stabilized via chelation, which should then undergo an insertion of SO2 2-(sulfoxylate dianion arising by disproportionation from dithionite dissociated into two sulfur dioxide radical anions ) generating the corresponding Pd(0)-chelated arylsulfinate 20 intermediate (ii). In the absence of an intramolecular electrophile (i.e. in the case of mono-triflate 1u), the intermediate complex dissociates to the free sulfinate product (iii). This product was trapped by alkylation with excess of CH3I, leading to the aryl methyl sulfone 2u in 58% yield. Otherwise, the second (intramolecular) oxidative addition of an aryl triflate (for 1a) or bromide (for 1a') leads to the intermediate (iv), which then undergoes reductive elimination of 2a regenerating the active catalyst. In a control experiment with 2-bromobenzophenone, it was confirmed that only aryl triflates underwent the initial oxidative addition under these reaction conditions, as no formation of 2u was detected upon addition of excess CH3I electrophile. However, an alternative Scheme 3. Proposed Reaction Mechanism. and more complex mechanism involving addition of sulfur dioxide radical anion to (i) with formation of Pd(I) species instead of (ii) cannot be ruled out. The robustness of the proposed method allowed us to prepare 3 (Scheme 4), the key building block for the synthesis of sulfonefluoresceins, on a multigram scale by demethylation of 2h in a three-step sequence starting from the commercial UV absorber benzophenone-6