Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad

Geometry-Independent Ultrafast Energy Transfer in Bioinspired Arrays Containing Electronically Coupled BODIPY Dimers as Energy Donors

In photosynthetic light-harvesting complexes, strong interaction between chromophores enables efficient absorption of solar radiation and has been suggested to enable ultrafast energy funneling to the reaction center. To examine whether similar effects can be realized in synthetic systems, and to determine the mechanisms of energy transfer, we synthesized and characterized a series of bioinspired arrays containing strongly-coupled BODIPY dimers as energy donors and chlorin derivatives as energy acceptors. The BODIPY dimers feature broad absorption in the range of 500-600 nm, complementing the chlorin absorption to provide absorption across the entire visible spectrum. Ultrafast (~10 ps) energy transfer was observed from photoexcited BODIPY dyads to chlorin subunits. Surprisingly, the energy-transfer rate is nearly independent of the position where the BODIPY dimer is attached to the chlorin and of the type of connecting linker. In addition, the energy-transfer rate from BODIPY dimers to chlorin is slower than the corresponding rate in arrays containing BODIPY monomers. The lower rate, corresponding to less efficient through-bond transfer, is most likely due to weaker electronic coupling between the ground state of the chlorin acceptor and the delocalized electronic state of the BODIPY dimer, compared to the localized state of a BODIPY monomer.

Surface hopping modeling of charge and energy transfer in active environments

An active environment is any atomic or molecular system changing a chromophore's nonadiabatic dynamics compared to the isolated molecule. The action of the environment on the chromophore occurs by changing the potential energy landscape and triggering new energy and charge flows unavailable in the vacuum. Surface hopping is a mixed quantum-classical approach whose extreme flexibility has made it the primary platform for implementing novel methodologies to investigate the nonadiabatic dynamics of a chromophore in active environments. This Perspective paper surveys the latest developments in the field, focusing on charge and energy transfer processes.

Ultrafast Intermolecular Interaction Dynamics between NIR-Absorbing Unsymmetrical Squaraines and PCBM: Effects of Halogen Substitution

Among near-infrared (NIR) dyes, squaraine derivatives are applied as efficient sensitizers in optoelectronic and biomedical devices due to their simple synthesis, intense absorption, and emission and exceptional photochemical stability. The fundamental understanding of the structure-property relationships of sensitizers provides the insight to increase the efficiency of such devices. Here, unsymmetrical squaraine derivatives (ABSQs) with donor-acceptor-donor (D-A-D') architectures having N,N-dimethyl amino anthracene and benzothiazole (ABSQ-H) halogenated with fluoride (ABSQ-F), chloride (ABSQ-Cl), and bromide (ABSQ-Br) were synthesized to understand the effect of halogen on the photophysical properties and intermolecular interaction dynamics with phenyl-C₆₁-butyric acid methyl ester (PCBM), which is used widely as an electron acceptor in bulk heterojunction-based devices. Interestingly, ABSQ-H exhibited intense absorption (ε ∼ 6.72 × 10⁴ M^-1 cm^-1) spectra centered at ∼660 nm. Upon halogen substitution, a bathochromic shift in the absorption spectra with an increase of molar absorptivity was observed (ε ∼ 8.59 × 10⁴ M^-1 cm^-1), which is beneficial for NIR light harvesting. The femtosecond transient absorption spectra of ABSQs revealed that the polarity of the solvent controlled the excited-state relaxation dynamics. Upon addition of PCBM, the fluorescence intensity and dynamics of halogenated ABSQs were quenched, and the formation of a squaraine radical cation was observed, reflecting the occurrence of intermolecular charge-transfer dynamics between ABSQs and PCBM. Thus, the observation of a bathochromic shift with intense absorption and an efficient intermolecular interaction with PCBM upon halogenation of ABSQs provide a design strategy for the development of unsymmetrical squaraine derivatives for bulk heterojunction-based optoelectronic devices.

Multi-stimuli programmable FRET based RGB absorbing antennae towards ratiometric temperature, pH and multiple metal ion sensing

A red-green-blue (RGB) multichromophoric antenna 1 consisting of energy donors naphthalimides and perylenediimides and a central aza-BODIPY energy acceptor along with two subchromophoric red-blue (RB 6) and green-blue (GB 12) antennae was designed that showed efficient cascade Förster resonance energy transfer (FRET). RGB antenna 1 showed pronounced temperature-dependent emission behaviour where emission intensities in green and red channels could be tuned in opposite directions by temperature giving rise to unique ratiometric sensing with a temperature sensitivity of 0.4% °C. RGB antenna 1 showed reversible absorption modulation selectively in the blue region (RGB ↔ RG) upon acid/base addition giving rise to pH sensing behaviour. Furthermore, RGB antenna 1 was utilized to selectively sense metal ions such as Co2+ and Fe3+ through a FRET turn-off mechanism induced by a redox process at the aza-BODIPY site that resulted in the selective spectral modulation of the red band (i.e., RGB → GB). Model antenna RB 6 showed white light emission with chromaticity coordinates (0.32, 0.33) on acid addition. Antennae 1, 6 and 12 also exhibited solution state electrochromic switching characterized by distinct colour changes upon changing the potential. Finally, antennae 1, 6 and 12 served as reversible fluorescent inks in PMMA/antenna blends whereby the emission colours could be switched or tuned using different stimuli such as acid vapour, temperature and metal ions.