The development of light-controlled molecular motion, mimicking natural systems like chlorophyll and retinal, has opened avenues for photofunctional systems and materials. Artificial photoactive molecules have been developed for applications including photoactuators, photoredox catalysis, photopharmacology, and photoluminescence (PL). However, combining multiple light-responsive functions in a single molecule, particularly orthogonally, remains challenging due to potential detrimental interactions between the functional parts. Light-driven rotary molecular motors (MMs), performing continuous unidirectional 360-degree rotation via photoisomerization and thermal helix inversion (THI), are advanced molecular machines with applications ranging from nanoscale mechanical manipulation to artificial muscles. Tracking the location and rotation of MMs in complex environments is crucial for realizing their full potential. This work aims to address the challenge of integrating a photoluminescent dye with a rotary molecular motor to create a multifunctional system.

The literature review section highlights existing research on light-driven rotary molecular motors and photoluminescent dyes. It emphasizes the challenges in combining these functionalities into a single molecule, citing examples of photoactuators, photoredox catalysis, and photopharmacology research, and discusses the unique properties of BODIPY dyes, such as high PLQY, narrow PL band, high extinction coefficient, and excellent photostability, making them suitable candidates for integration into molecular motors. The review also discusses past efforts in creating hybrid systems and highlights the lack of successful examples that achieve both efficient motor rotation and bright photoluminescence simultaneously. This lack of success is largely attributed to the potential for detrimental interactions between the two components.

The researchers designed and synthesized a hybrid molecule, BODIPY/Motor, by attaching a BODIPY dye to a second-generation overcrowded alkene molecular motor. The almost perpendicular connection between the BODIPY and the motor core minimizes direct π-system conjugation while allowing for "moderate" interactions. The synthesis is detailed in Supplementary Fig. 1 and characterized by NMR, HRMS, and single-crystal X-ray analysis (Fig. 2a). The photochemical and thermal isomerization processes were characterized by ¹H NMR, showing a photostationary state (PSS) achieved after 20 min of 395 nm irradiation. Steady-state UV/vis absorption spectroscopy and PL spectroscopy were used to investigate the optical properties of BODIPY/Motor, with comparisons made to bare BODIPY and the motor. Time-resolved PL spectroscopy and femtosecond transient absorption spectroscopy were employed to study excited-state dynamics. The photoisomerization quantum yields (QYs) were measured in acetone and toluene with 390 nm excitation. Computational methods, including SF-TDDFT calculations, were used to analyze the ground and excited states potential energy surfaces (PESs), conical intersections (CIs), and excited-state dynamics, to understand the photochemical reactions and the role of solvents. Circular dichroism (CD) spectroscopy examined the chiroptical properties. The time-resolved PL spectroscopy was performed using a Hamamatsu C5680 streak camera equipped with a Ti:sapphire laser, utilizing different excitation wavelengths (400 nm and 510 nm). The solvents used included acetone and toluene to study the effect of solvent polarity on the system. Computational details include ground-state geometry optimizations and vertical excitation energy calculations using various computational methods and software (Q-Chem 5.3).

The BODIPY/Motor hybrid successfully maintained both motor rotation and bright photoluminescence. The relative magnitudes of these functions were controlled by solvent polarity and excitation wavelength. In polar solvents like acetone, the photoisomerization QY increased (22%), while the PLQY decreased (2.6%). In apolar toluene, the photoisomerization QY was 12%, similar to the bare motor, and the PLQY was 54%. The absorption spectrum showed contributions from both BODIPY and the motor, with a slight bathochromic shift in BODIPY/Motor compared to BODIPY. Calculations revealed that the S₀→S₁ transition was localized on BODIPY, while S₀→S₂ was on the motor. The motor rotation was observed even under green light (505 nm) excitation, highlighting a synergistic effect where BODIPY absorption triggers motor rotation. CD spectroscopy showed chirality transfer from the chiral motor to the achiral BODIPY, demonstrating light-controlled helicity modulation. Time-resolved PL spectroscopy showed faster excited-state depopulation in polar solvents due to a lower energy barrier towards the CI, favoring photoisomerization over PL. Theoretical calculations revealed a competition between photoluminescence and photoisomerization depending on the solvent used, showing a correlation between the average PL lifetime and solvent polarity.

The results demonstrate a successful integration of two distinct light-dependent functionalities in a single molecule, overcoming the challenges associated with potential detrimental interactions. The ability to tune the relative contributions of motor rotation and PL by changing the solvent polarity and excitation wavelength provides unprecedented control over the system's behavior. The observed synergistic effects, such as green-light-driven rotation and induced chirality in the BODIPY, showcase the potential of this design for advanced applications. The theoretical analysis helps to explain the underlying photophysical mechanisms driving the interplay between photoisomerization and photoluminescence. These findings suggest new avenues for the design of multifunctional hybrid molecules with applications in various fields such as nanoscale devices, bioimaging, and light-driven materials.

This study successfully created a light-driven molecular motor integrated with a photoluminescent dye, demonstrating tunable motor rotation and bright photoluminescence. The synergistic interactions between the components enable lower-energy excitation and chirality transfer. This work provides insights into the photophysics of such systems and opens pathways for developing advanced light-driven multifunctional hybrid molecules with diverse applications.

The study primarily focuses on two specific solvents, acetone and toluene, and further investigation with a wider range of solvents might provide a more comprehensive understanding of solvent effects. While theoretical calculations provide valuable insights, they are based on approximations and may not fully capture the complexity of the system's dynamics. The investigation of the long-term stability and potential degradation of the hybrid molecule under continuous irradiation is also crucial for future applications.