Chaiwongwattana S, Škalamera Đ, Došlić N, Bohne C, Basarić N
Institute of Science, Suranaree University of Technology, 30000 Nakhon Ratchasima, Thailand.
Phys Chem Chem Phys. 2017 Oct 25;19(41):28439-28449. doi: 10.1039/c7cp05472f.
Photophysical properties and excited state intramolecular proton transfer (ESIPT) reactivity for anthrol carbaldehydes 1-5 have been investigated computationally and experimentally by steady-state and time-resolved fluorescence and laser flash photolysis (LFP). 1,2-Disubstituted anthrol carbaldehydes 1 and 2 are not ESIPT reactive, contrary to naphthol analogues. The main deactivation channels from S for 1 and 2 are fluorescence (Φ = 0.1-0.2) and intersystem crossing (ISC) to almost isoenergetic T states. The triplet states from 1 and 2 were detected by LFP (in N-purged CHCN, τ = 15 ± 2 μs for 1, and τ = 5.5 ± 0.1 μs for 2). In contrast, 2,3-disubstituted anthrols 3-5 undergo efficient barrierless ultrafast ESIPT. However, the typical dual emission from locally excited states and ESIPT tautomers were not observed since ESIPT proceeds via a conical intersection with S delivering the keto-tautomer in the hot ground state. Therefore, anthrols 3-5 are about ten times less fluorescent compared to 1 and 2, and the emission for 3-5 originates from less-populated conformers that cannot undergo ESIPT. Keto-tautomers for 3-5 were detected in CHCN by LFP (λ = 370 nm, τ = 30-40 ns). The difference in ESIPT reactivity for 1-3 was fully disclosed by calculations at ADC(2)/aug-cc-pVDZ level of theory, and particularly, by calculation of charge redistribution upon excitation to S. Only 2,3-disubstituted anthrols exhibit polarization in S that increases the electron density on the carbonyl and decreases this density on the phenolic OH, setting the stage for ultrafast ESIPT.
通过稳态和时间分辨荧光以及激光闪光光解（LFP），对蒽醛1 - 5的光物理性质和激发态分子内质子转移（ESIPT）反应活性进行了计算和实验研究。与萘酚类似物相反，1,2 - 二取代的蒽醛1和2不具有ESIPT反应活性。从S态到1和2的主要失活通道是荧光（Φ = 0.1 - 0.2）和系间窜越（ISC）到几乎等能量的T态。通过LFP检测到了1和2的三重态（在氮气吹扫的乙腈中，1的τ = 15 ± 2 μs，2的τ = 5.5 ± 0.1 μs）。相比之下，2,3 - 二取代的蒽3 - 5经历了高效的无障碍超快ESIPT。然而，由于ESIPT是通过与S态的锥形交叉进行的，在热基态下产生酮式互变异构体，因此未观察到来自局部激发态和ESIPT互变异构体的典型双重发射。因此，与1和2相比，蒽3 - 5的荧光约低十倍，并且3 - 5的发射源于较少数量的不能进行ESIPT的构象异构体。通过LFP在乙腈中检测到了3 - 5的酮式互变异构体（λ = 370 nm，τ = 30 - 40 ns）。通过在ADC(2)/aug - cc - pVDZ理论水平上的计算，特别是通过计算激发到S态时的电荷重新分布，充分揭示了1 - 3在ESIPT反应活性上的差异。只有2,3 - 二取代的蒽在S态表现出极化，增加了羰基上的电子密度并降低了酚羟基上的电子密度，为超快ESIPT奠定了基础。
