*3.4. Spectroscopic Measurements*

The photophysical properties were registered using quartz cuvettes with optical pathways of 1 cm in diluted solutions (around 2 × 10−<sup>6</sup> M), prepared by adding the corresponding solvent to the residue from the adequate amount of a concentrated stock solution in acetone, after vacuum evaporation of this solvent. Ultraviolet-visible (UV-vis) absorption and fluorescence spectra were recorded on a Varian model CARY 4E spectrophotometer (Agilent Technologies, Santa Clara, CA, US), and an Edinburgh Instruments (Livingston, England) spectrofluorimeter (model FLSP920), respectively. Fluorescence quantum yields (φ) were obtained using PM546 as a reference (Exciton, φ<sup>r</sup> = 0.85 in ethanol). Radiative decay curves were registered with the time correlated single-photon counting technique, as implemented in the aforementioned spectrofluorimeter. Fluorescence emission was monitored at the maximum emission wavelength by using a microchannel plate detector (Hamamatsu C4878) of picosecond time-resolution (20 ps), after excitation with a Fianium pulsed laser (time resolution of around 150 picoseconds). The fluorescence lifetime (τ) was obtained after the deconvolution of the instrumental response signal from the recorded decay curves by means of an iterative method. The goodness of the exponential fit was controlled by statistical parameters (chi-square) and the analysis of the residuals. Radiative (kfl) and non-radiative (knr) rate constants were calculated as follows: kfl = φ/τ; knr = (1 − φ)/τ.

## *3.5. Computational Simulations*

Ground state energy minimizations were performed using a functional range-separated hybrid wb97xd within the Density Functional Theory (DFT), using the triple valence basis set with a polarization and a diffuse function (6-311+g\*). The optimized geometries were taken as a true energy minimum using frequency calculations (no negative frequencies). The conformational search around the linkage bond between the coumarin fragment and the BODIPY at 8-positions suggests that the aforementioned geometry corresponds to the most stable conformer. The absorption profile was simulated with the Time Dependent (TD-DFT) method using the same calculation level and basis set. The Polarizable Continuum Model (PCM) was considered to have a solvent effect (cyclohexane) in all the calculations. All the calculations were performed in Gaussian 16, using the "arina" computational resources provided by the UPV-EHU.

#### **4. Conclusions**

Salicylaldehyde was efficiently functionalized with a BODIPY unit via a Liebeskind-Srogl cross-coupling reaction. In a first example of the application of **3** as a building block, a *meta*-bromophenyl BODIPY-coumarin was prepared from which 10 novel analogues were attained using the Suzuki reaction. The addition of coumarins, functionalized with aromatic moieties, to the *meso* position of BODIPYs is a suitable synthetically accessible strategy to ameliorate the light harvesting ability of BODIPY-coumarin hybrids. The proper selection of the functional aromatic groups to extend the π-system of the chromene core, enables the enhancement of the absorption probability at the UV-blue region, providing a more efficient and broader light collection for the subsequent excitation energy transfer to the BODIPY. The low fluorescence response of the herein reported dyads is attributed to the conformational freedom of the coumarin around the key *meso* position and the activation of electron transfer processes when electron-rich groups are tethered at the coumarin subunit.

However, the detrimental impact of the conformational flexibility on the fluorescence response of the BODIPY-coumarin hybrids paves the way to apply them as molecular rotors for the monitorization of the environmental microviscosity. In fact, in those dyads not undergoing PET, the fluorescence efficiency and lifetime progressively increases and lengthens, respectively, with the viscosity of the media. Therefore, these BODIPY-coumarin dyads behave as versatile molecular rotors to quantify the viscosity following the changes of the fluorescence signatures upon excitation almost along the whole UV-yellow spectral region, owing to their broadband light harvesting and ensuing energy transfer.

**Supplementary Materials:** The following are available online; synthetic details and characterization data (IR, NMR, HRMS) of each compound, <sup>1</sup>H and <sup>13</sup>C-NMR spectra, photophysical data (Table S1), absorption spectra (Figure S1) and computed absorption spectra (Figure S2).

**Author Contributions:** E.E.-P., C.I.B.-H. and J.G.B.-G. synthesized the compounds and carried out their characterization (IR, NMR, HRMS). T.A. conducted the photophysical measurements. I.L.-A. supervised the spectroscopy study. J.B. carried out the theoretical simulation and wrote the original draft. E.P.-C. designed the compounds, supervised the organic synthesis and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Spanish Ministerio de Economia y Competitividad (project MAT2017-83856-C3-3-P), Gobierno Vasco (project IT912-16), CONACyT (grants 253623, 123732) and Dirección de Apoyo a la Investigación (DAIP-UG CIIC318/2019).

**Acknowledgments:** The authors thank SGIker of UPV/EHU for technical support with the computational calculations, which were carried out in the "arina" informatic cluster. E.E.-P. and J.G.B.-G. thank CONACyT (Mexico) for graduate fellowship.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
