Why do small structural changes lead to big different on fluorescence yields?
Boron-dipyrromethene (BODIPY) dyes have excellent structural and spectroscopic properties, such as intense and narrow emission bands, which are tunable across a broad spectral range. To this end, BODIPY dyes have been extensively used in various applications, including organic photovoltaics, optical sensors and fluorescence labeling. Over the last two decades, research and application of BODIPY dyes have grown significantly compared to other conventional dyes like cyanines and rhodamines. Despite the progress on understanding their structural modification, the relationship between their luminescence properties and molecular structures remains largely underexplored. This is specifically important for rational design for special and specific applications.
Generally, BODIPY dyes undergo non-radiative decays caused by free intermolecular motions such as distortion, vibrations and rotations. Non-radiative decay is assumed to dominate the energy relaxation state of the co-twisted structure. Building on this assumption, recent pioneering studies on the underlying mechanisms behind the non-radiative decay of BODIPY have majorly focused on the intersystem crossing (ISC) mechanism caused by heavy atom effects. Previous findings revealed that benzo[a]-fused BODIPY exhibits stronger fluorescence while benzo[b]-fused analogue 3 displays virtual non-fluorescence characteristics. Nevertheless, the effects of benzo-fused positions on fluorescence yield still remain unclear. Moreover, there is limited information about why benzo[a] and benzo[b] BODIPY derivatives display significant behavioral differences.
It is speculated that non-fluorescence of chromophores such as analog 3-4 is due to free phenyl group rotation at the meso-position. Interestingly, the virtual non-fluorescence characteristics of BODIPY 5, despite its rigid structure, suggest that molecular motion is not the primary factor affecting fluorescence yields. This has raised questions on the possible existence of other factors affecting the energy decay of BODIPY. A typical non-radiative decay route arguably comprises ISC, enabling singlet and triplet wave functions to mix with the help of spin-orbit coupling (SOC). However, the exact origin of the SOC observed in the [b]-position also remains an illusion that requires thorough studies to elucidate.
Inspired by the previous results, Dr. Zhigang Ni, and Professor Hua Lu from Hangzhou Normal University investigated the origin of the large differences in fluorescence yields between benzo[a] and benzo[b] BODIPY derivatives. The aim was to provide a better understanding of these issues based on ab initio calculations. Their work is currently published in the journal, Physics Chemistry Chemical Physics.
Results revealed that benzo[a]-BODIPY chromophores exhibited relatively higher fluorescence yields. In contrast, the non-fluorescence observed in benzo[b]-fused BODIPY 3-5 was mainly due to larger SOC and smaller singlet-triplet energy gaps. Additionally, benzo[b]-fusion partially contributed to HOMO but fully contributed to LUMO, thus exacerbating the differences in the impact of SOC on S1 S0 and T2 S0 transitions. On the other hand, benzo[a]-fusion equally contributed to both HOMO and LUMO to minimize the effects of SOC between S1 and T2, leading to higher fluorescence yields.
In summary, the research team studied how fluorescence yields of BODIPYs are affected by benzo-fusion at [a] and [b] positions and provided more insights on the various issues raised in the past studies. Based on the quantum calculations, the differences in fluorescence yields were attributed to the differences in the nature of SOC between S1 and T1 as well as the differences in the contribution made by benzo[a] and benzo[b] positions to HOMO. Thus, the results provided a new approach for regulating excited state properties. In a statement to Advances in Engineering, Professor Hua Lu the lead and corresponding author explained their findings provided a fundamental understanding of the ISC mechanism that could benefit the rational design of heavy atom-free triplet organic chromophores for specific applications.
Xu, N., Xiao, Y., Ni, Z., Gai, L., Zhou, Z., & Lu, H. (2021). Rationalizing the effect of benzo-fusion at [a] and [b] positions of BODIPY on fluorescence yields. Physical Chemistry Chemical Physics, 23(32), 17402-17407.