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[Display omitted] • The excited state behavior of a group of newly developed fluoropurine analogues of aza-heterocycles, triazole pyrimidinyl fluorophores, has been comprehensively studied. • The reaction energy profiles indicate that ESIPT is difficult to happen. The large Stokes shift of the fluorophores, therefore, is reconsidered. • Based on the FMOs results, the large Stokes shift is attributed to an ICT process occurring in the excited state, the rationality of which is further confirmed by the fluorescence spectra results. Fluoropurine analogues are a kind of unnatural bases, which are widely used in chemistry, biological science, pharmacy and other fields. At the same time, fluoropurine analogues of aza-heterocycles play an important role in medicinals research and development. In this work, the excited state behavior of a group of newly developed fluoropurine analogues of aza-heterocycles, triazole pyrimidinyl fluorophores, was comprehensively studied. The reaction energy profiles indicate that excited state intramolecular proton transfer (ESIPT) is difficult to happen, which is further proved by fluorescent spectra results. This work proposed a new and reasonable fluorescence mechanism based on the original experiment, and found that the large Stokes shift of the triazole pyrimidine fluorophore is due to the intramolecular charge transfer (ICT) process of the excited state. Our new discovery is of great significance for the application of this group of fluorescent compounds in other fields and the regulation of fluorescence properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ESIPT of monomer PTI induces keto → enol transfer and the red-shift of fluorescent spectra. • The strengthening of intermolecular hydrogen bond hinders the occurrence of ESIPT of PTI. • The large Stokes shift of PTI is induced by hydrogen bond strengthening while not ESIPT. The fluorescent detection mechanism of 2-(4-nitro-1,3-dioxoisoindolin-2-yl) acetic acid (CORM3-green) on CO-Releasing Molecule-3 (CORM-3) is theoretically studied. Upon reaction with CORM-3, the non-fluorescent CORM3-green is transferred to the keto form of 2-(4-amino-1,3-dioxoisoindolin-2-yl)acetic acid (PTI) to produce strong fluorescence peak located at 423 nm. This peak red-shifts to 489 nm, which is induced by the strengthening of intermolecular hydrogen bond (HB) between PTI and water molecules and attributed to the experimentally observed fluorescence emission at 503 nm. This result is dramatically different from previous reports that the experimental fluorescence corresponds to the proton transferred enol form of PTI. To illustrate this confusion, the calculated fluorescence peak of PTI-Enol is located at 689 nm, which is much larger than that of experimental result. This result excludes the occurrence of excited state intramolecular proton transfer (ESIPT). It is concluded that intermolecular HBs hinders the formation of intramolecular HB and the ESIPT of the keto form of PTI. This conclusion confirms that experimental Stokes shift of 113 nm is mainly caused by the intermolecular hydrogen bonding rather than by ESIPT process. This work proposes a reasonable explanation for the detection mechanism of CORM3-green and experimental fluorescence phenomenon. [ABSTRACT FROM AUTHOR]

[Display omitted] • In high resolution spectra there are two separate peaks of fluorescence for EGF. • Those two peaks are observed simultaneously for other proteins and pure Trp. • The ratio between two peaks depends on the polarity of microenvironment. • The third peak appears in rather hydrophobic microenvironment only. • The energy of excited state of Trp stabilization is the same in different environments. Fluorescence spectra of proteins and peptides are traditionally used to get an information on self-association of proteins and peptides, on their tertiary and quaternary structure. In this study it was shown that there are just three peaks of tryptophan fluorescence (at ∼308, at ∼330, and at ∼360 nm) in rough unsmoothed spectra of fluorescence of pure tryptophan in different solvents that change their heights depending on the polarity of a solvent. Two separate peaks at ∼330 nm and ∼360 nm are especially prominent in the spectrum of human epidermal growth factor. In contrast, in smoothed (either mathematically, or physically) spectra of Trp-containing proteins a single maximum of fluorescence varies between 330 and 360 nm. The theory of tryptophan fluorescence is discussed in light of three discrete peaks existence. A stabilizing hydrogen bond with aromatic system of benzene ring in the excited state is proposed as the cause of emission at ∼360 nm bringing Trp to the destabilized ground state. Emission from the destabilized excited state has a maximum at ∼330 nm if the ground state is destabilized, as well as if both states are stabilized. If the excited state is destabilized, while the ground state is stabilized by purely hydrophobic interactions, emitted light should have a maximum at ∼308 nm. The degree of hydrophilicity of tryptophan microenvironment is proposed to be measured as the ratio between the peak at 360 nm and the peak at 330 nm if the observed shifts are not "horizontal", but "vertical". The process of dissociation of hemagglutinin trimers from pandemic Influenza A(H1N1) virus is described as an example of the advantages of the proposed method. [ABSTRACT FROM AUTHOR]

• Sensing mechanism of a fluorescent probe for thiols was theoretically confirmed. • Both S 0 and S 1 geometries are analyzed for the probe and its thiol complexes. • Calculated spectra are in good agreements with the reported reference. • Photoisomerization process could explain the weak fluorescence of the probe. The sensing mechanism of a reported fluorescence probe for cysteine, homocysteine and glutathione (Yin et al., 2018) has been investigated by time-dependent density functional theory. Experimental absorption and emission spectra of the probe before and after thiol addition were reproduced well by theoretical calculations, which validated the rationality of the method. Optimized geometries showed that the probe molecule had distinctly different geometries in its ground and excited states. It corresponded to the photoisomerization process and explained the weak fluorescence of the probe molecule. Moreover, by the potential energy curve scan, photoisomerization was further confirmed to be a spontaneous process with a barrier that barely existed. Frontier orbital analysis indicated that this photoinduced isomerization of the probe molecule derived from the antibonding character for lowest unoccupied molecular orbital at its C C double bond. In contrast, probe-thiol complexes exhibited similar geometries in their ground and excited states, which was responsible for the strong fluorescence of the probe with thiols. Due to distinct excited-processes, the probe can be used to sense thiols by monitoring the fluorescent change. [ABSTRACT FROM AUTHOR]

The 8-oxo-deoxyguanosine is the most abundant specie of the DNA oxidative damage. Despite the deleterious effects such as gene mutation it may cause, the 8-oxodG was also reported to have beneficial effect such as repairing the nearby cyclobutane pyrimidine dimer (CPD) after photoexcitation. Due to its strong biological relevance, the photoinduced excited state dynamics behavior of 8-oxo-deoxyguanosine is of particular interest. In this work, a theoretical investigation by combination of complete active space self-consistent field (CASSCF) ab initio calculations and on-the-fly nonadiabatic dynamics simulations are implemented to provide intrinsic deactivation mechanism of its free base 8-oxoguanine after being excited to the S 1 and S 2 states. Two minimum energy conical intersections (MECIs) characterized by the C3-puckered motion with attractive chiral character are located, which contribute appreciably to the S 1 state deactivation process. When the system is being excited to the S 2 state directly, a S 2 → S 1 → S 0 two-step decay pattern is proposed. A nearly planar S 2 /S 1 intersection plays a significant role in the S 2 → S 1 decay process. The subsequent S 1 state relaxation process is also dominated by the C3-puckered deformation motion. One decay time is estimated to be 704 fs, which compares well with the experimental observation of 0.9 ± 0.1 ps in solvents. Particular illustration is the fact that the MECIs configurations we located bear an exceptional resemblance with previous reported thymine, cytosine and guanine, suggesting that the current work could lend support for better understanding of the non-natural nucleobases and derivatives. Unlabelled Image • Provide a comprehensive picture of the deactivation mechanism of the 8-oxo-G system on the S 1 and S 2 states. • Provide support for better understanding of the photochemical and photophysical properties of non-natural nucleobases. • Provide reasonable theoretical explanation to the experimental observations of the 8-oxo-G system. [ABSTRACT FROM AUTHOR]

The ferulic acid (FA) is a kind of phenolic acid widely exists in nature plants. Apart from its medicinal values, the FA is also widely applied in cosmetic industry. Recently, it was found to have potential applications in commercial sunscreens for its strong photostability and photoprotection property from harmful UV rays. Such excellent property lies in the ultrafast decay process of the FA system when exposure to the UV light, but the underlying detailed relaxation pathway is still less clear-cut. In the current work, high-level ab initio electronic structure calculations and on-the-fly surface hopping dynamics simulations were employed to explore the photoinduced decay mechanism of the FA system both on the S 1 and S 3 states in the gas phase. The results provide a reasonable explanation for the wavelength dependent decay patterns of FA system. The S 1 state decay pathway is driven by a re-emission process to dissipate excess energy. While for the S 3 state deactivation process, the pathway is dominated by a non-adiabatic process driven by the internal conversion process through the conical intersection regions. A S 3 -S 1 -S 0 two step decay pattern is proposed, and the pathways are mainly driven by a puckering distortion motion of the aromatic ring and a twisting motion around the bridging double bond. The calculation results contribute to a better understanding of detailed dynamics behavior of the FA deactivation process, and provide theoretical guidance for further design of efficient and environmentally friendly sunscreens. Unlabelled Image • Provide fundamental and intrinsic photochemical properties of FA in gas phase • Provide the photoprotection mechanism of FA after exposure to the UV light • Explain the wavelength dependent deactivation mechanism of FA [ABSTRACT FROM AUTHOR]

In spite of the pivotal role of excited state electronic structures as regulation of photoinduced electron transfer (PET) process, the effect of excited state conformation on PET remains elusive. Here we exploit distinguishable emission characters of trans and cis singlet excited states of donor-acceptor-donor ensemble MTPAAZO to reveal that its PET efficiency and rate are closely depended on its singlet excited state conformation. The PET process occurs solely in cis conformation of MTPAAZO singlet excited states. Novel molecule (MTPA) 2 Ab as-designed with similar structure of MTPAAZO cis singlet excited states shows high PET efficacy and rate, leading to long-lived CS states. Our findings enable the rational design of the novel molecules with highly efficient PET process suitable for charge separation applications. Unlabelled Image • Different fluorescence emission features from the trans and cis singlet excited states were identified. • Cis → trans conversion in ground states and trans → cis isomerization in singlet excited states were discovered. • The PET process was observed only in cis conformation of MTPAAZO singlet excited states. [ABSTRACT FROM AUTHOR]

The excited state dipole moment of 2-[(4-methoxyphenyl)ethynyl]-3-(1-methyl-1 H -indol-3-yl)-quinoxaline has been determined in ethyl acetate solution using the method of thermochromic shifts. Three different models have been tested and are compared to each other and to the results of ab initio calculations at the coupled cluster CC2/cc-pVTZ and SCS-CC2/cc-pVTZ level of theory. Good agreement is obtained for solvent polarity functions as defined by Bilot and Kawski (L. Bilot and A. Kawski, Z. Naturforsch. , 1962, 17A , 621 − 627), while the original Lippert-Mataga type polarity functions fail to describe the excited state dipole moment correctly. We report an excited state dipole moment of the title compound of 8.5 D and a change upon excitation of 5.5 D. • The S1 dipole moment of the target molecule has been determined in ethylacetate solution from thermochromic shifts. • The title compound has a large dipole moment change upon excitation (>5 Debye). • Improvement of methods in solution by exchanging solvatochromic shifts with thermochromic shifts in a single solvent. • Exchange of the ill defined Onsager radius in the theory by measured cavity volume. • Comparison of experimental results with ab initio calculations at the coupled cluster level of theory with cc-pVTZ basis set. [ABSTRACT FROM AUTHOR]

Molecular motion plays an important role in the reverse intersystem crossing of thermally activated delayed fluorescence (TADF) materials, since the conformation varies as the molecule vibrates, leading to potential changes in the energies of excited states. Although many theoretical simulations have researched the relationship between the excited states and the molecular conformations, there are still few experimental results showing the energy level difference between different long-lived excited states. Herein, a novel method for measuring spectrally resolved luminescence lifetimes is proposed to detect the energy splitting of the long-lived excited states of a classical TADF molecule, BTZ-DMAC. A set of the time-gated luminescence spectra with different delay times were captured by a spectrograph equipped on an auto-phase-locked system, and then used for lifetime analysis at each wavelength. Unlike traditional measurement techniques, the proposed novel method does not require ultrafast laser, high-speed detector and any phase matching circuitry, thus significantly reducing the cost. This method revealed a definite energy gap between the two excited states of BTZ-DMAC with different lifetimes, indicating different conformations caused by molecular vibration. This low-cost method could be also used to detect many other luminescence materials for investigating the detail mechanisms of multiple excited states. Unlabelled Image • Spectrally resolved luminescence lifetime detection was achieved on an auto-phase-locked system. • No ultrafast laser, high-speed detector and phase matching circuitry were needed. • This method revealed an energy splitting of the long-lived excited states of a TADF molecule, BTZ-DMAC. • The method could be used to study other luminescence mechanisms with multiple excited states. [ABSTRACT FROM AUTHOR]