**Ready Access to Molecular Rotors Based on Boron Dipyrromethene Dyes-Coumarin Dyads Featuring Broadband Absorption**

**Ernesto Enríquez-Palacios <sup>1</sup> , Teresa Arbeloa <sup>2</sup> , Jorge Bañuelos 2,\* , Claudia I. Bautista-Hernández <sup>1</sup> , José G. Becerra-González <sup>1</sup> , Iñigo López-Arbeloa <sup>2</sup> and Eduardo Peña-Cabrera 1,\***


Academic Editor: Goze Christine

Received: 15 January 2020; Accepted: 8 February 2020; Published: 12 February 2020

**Abstract:** Herein we report on a straightforward access method for boron dipyrromethene dyes (BODIPYs)-coumarin hybrids linked through their respective 8- and 6- positions, with wide functionalization of the coumarin fragment, using salicylaldehyde as a versatile building block. The computationally-assisted photophysical study unveils broadband absorption upon proper functionalization of the coumarin, as well as the key role of the conformational freedom of the coumarin appended at the *meso* position of the BODIPY. Such free motion almost suppresses the fluorescence signal, but enables us to apply these dyads as molecular rotors to monitor the surrounding microviscosity.

**Keywords:** molecular rotors; BODIPY; viscosity sensors; dye chemistry; energy-electron transfer
