**1. Introduction**

The design of molecules featuring two or more chromophoric units is of great interest [1–4]. One can envisage new applications of such systems by taking advantage of the interactions that may develop between the chromophoric units, i.e., energy transfer [5,6], or electron transfer pathways [7], enabling the design of light harvesters with broadband absorption and high *pseudo*-Stokes shifts [8–11], or photosensitizers for photovoltaic devices [12,13] mimicking the natural photosynthesis [14,15]. It is crucial to the design of multichromophoric systems to have flexible functionalization methods at one's disposal that are able to tailor the targeted analogues. In this regard, both boron dipyrromethene dyes (BODIPYs) [16] and coumarins [17] are in themselves two of the more widely used fluorophores (Figure 1), and hence have optimal candidates to design multichromophore ongoing energy transfer processes. Indeed, the former luminophores stand out due to their stability, chemical versatility and tunable photophysical signatures [18,19], whereas the latter fluorophores can be also deeply functionalized and display spectral bands at higher energies than the BODIPY core. Thus, both chromophores are complementary from a spectral point of view, and hence are suitable building blocks to be combined in a single molecular structure towards the promotion of intramolecular energy transfer hops.

Despite the fact that coumarin-BODIPY hybrids are known, and some examples of these dyads can be found in the literature applied as chemosensors [20,21], energy transfer cassettes [22,23] or fluorescent probes for bioimaging [24,25], the photonic performance, and hence the practical applicability of these multifunctional molecular assemblies, can be still improved. Indeed, one of the main drawbacks of

these multichromophores is the low absorbance of the coumarin core [26], which falls in the ultraviolet (UV) region and it is usually masked (or at least overlapped) under the more energetic transitions of the BODIPY unit. This feature hampers the application of these dyads, for instance as broadband energy transfer cassettes since, while the energy transfer from the coumarin to the BODIPY is effective, the light harvesting is not greatly improved by the presence of coumarin.

**Figure 1.** Structure of boron dipyrromethene dyes (BODIPYs) and coumarins.

From the synthetic point of view, the utilization of salicylaldehyde as a versatile building block is well-documented [27]. Herein, we show that such an attractive starting material can be rendered fluorescent by attaching it to a BODIPY fragment, thereby opening up new possibilities for the synthesis of more complex products. In this first example, we used the BODIPY-containing salicylaldehyde to prepare BODIPY-coumarin hybrids. Thanks to this methodology, we have been able to decorate the chromene π-system of the coumarin with a battery of aromatic moieties (from functionalized aryl groups with electron donors, i.e., methoxy, or acceptors, i.e., cyano, groups, to pyridine, naphthalene, modified stilbenes, triphenylamine, or benzothiophene). The computationally-aided spectroscopic analysis of this set of dyads allows the selection of the best structural modification at the coumarin subunit to enhance the light harvesting ability along the UV-yellow spectral region towards applying these dyads as energy transfer cassettes. π

π Furthermore, in view of the conformational flexibility of these dyads around the linking bond between the 8-position of the BODIPY and the coumarin, we anticipated that they could behave as molecular rotors [28–30]. Thus, we have conducted additional measurements at different temperatures and controlled viscosities (increasing the amount of ethylene glycol in the medium) to test the viability of these hybrids as fluorescent sensors to monitor the viscosity of the surrounding environment.

### **2. Results and Discussion**
