<jats:title>Abstract</jats:title><jats:p>Isoindigo, an electron‐withdrawing building block for polymeric field‐effect transistors, has long been considered to be non‐fluorescent. Moreover, using electron‐deficient heterocycle to replace the phenyl ring in the isoindigo core for better electron transport behaviour is synthetically challenging. Here we report the syntheses of a series of tetrazaisoindigos, including pyrazinoisoindigo (<jats:bold>PyrII</jats:bold>), pyrimidoisoindigo (<jats:bold>PymII</jats:bold>) and their hybrid (<jats:bold>PyrPymII</jats:bold>), and the investigation on their photophysical and electric properties. Proper flanking groups need to be chosen to stabilize these highly electron‐deficient bislactams. Both <jats:bold>PyrII</jats:bold> and <jats:bold>PymII</jats:bold> derivatives show lower LUMO energy levels than that of naphthalene bisimide (<jats:bold>NDI</jats:bold>). Interestingly, <jats:bold>PyrII</jats:bold> is instinctively unstable and can be easily reduced, while both <jats:bold>PymII</jats:bold> derivatives are stable. More surprisingly, <jats:bold>PymII</jats:bold> derivatives are highly fluorescent and their photoluminescence quantum yields are around 40 %, 133 times higher than that of reported isoindigo derivatives. <jats:bold>UV‐vis</jats:bold> spectroscopic results and theoretical calculations show that strong intramolecular hydrogen‐bond exists in <jats:bold>PymII</jats:bold>, which prohibits it from non‐radiative decay and accounts for its fluorescent behaviour. <jats:bold>PymII</jats:bold> derivatives are n‐type semiconductors, while <jats:bold>Ph‐PyrII</jats:bold> and the hybrid show balanced ambipolar charge transport behaviour, all among the best isoindigo derivatives. Our study not only discloses the structure‐property relationship of tetrazaisoindigos, but also provides novel electron‐deficient monomers for conjugated polymers.</jats:p>

<jats:p>We combine on-the-fly trajectory surface hopping simulations and the doorway–window representation of nonlinear optical response functions to create an efficient protocol for the evaluation of time- and frequency-resolved fluorescence (TFRF) spectra and anisotropies of the realistic polyatomic systems. This approach gives the effective description of the proper (e.g., experimental) pulse envelopes, laser field polarizations, and the proper orientational averaging of TFRF signals directly from the well-established on-the-fly nonadiabatic dynamic simulations without extra computational cost. To discuss the implementation details of the developed protocol, we chose cis-azobenzene as a prototype to simulate the time evolution of the TFRF spectra governed by its nonadiabatic dynamics. The results show that the TFRF is determined by the interplay of several key factors, i.e., decays of excited-state populations, evolution of the transition dipole moments along with the dynamic propagation, and scaling factor of the TFRF signals associated with the cube of emission frequency. This work not only provides an efficient and effective approach to simulate the TFRF and anisotropies of realistic polyatomic systems but also discusses the important relationship between the TFRF signals and the underlining nonadiabatic dynamics.</jats:p>

<jats:p>As a widely-used sunscreen compound, the caffeic acid (CA) shows the strong UV absorption, while the photoinduced reaction mechanisms behind its photoprotection ability are not fully understood. We try to investigate the photoinduced internal conversion dynamics of CA in order to explore the photoprotection mechanism. The most stable CA isomer is selected to examine its nonadiabatic dynamics using the on-the-fly surface hopping simulations at the semi-empirical level of electronic-structure theory. The dynamics starting from different electronic states are simulated to explore the dependence of the photoinduced reaction channels on the excitation wavelengths. Several S1/S0 conical intersections, driven by the H-atom detachments and the ring deformations, have been found to be responsible for the nonadiabatic decay of the CA. The simulation results show that the branching ratios towards these intersections are modified by the light with different excitation energies. This provides the valuable information for the understanding of the photoprotection mechanism of the CA compound.</jats:p>

<jats:p>The nonadiabatic dynamics of methyl nitrate (CH3ONO2) is studied with the on-the-fly trajectory surface hopping dynamics at the ADC(2) level. The results confirmed the existence of the ultrafast nonadiabatic decay to the electronic ground state. When the dynamics starts from S1 and S2, the photoproducts are CH3O+NO2, consistent with previous results obtained from the experimental studies and theoretical dynamics simulations at more accurate XMS-CASPT2 level. The photolysis products are CH3O+NO2 at the ADC(2) level when the dynamics starts from S3, while different photolysis products were obtained in previous experimental and theoretical works. These results demonstrate that the ADC(2) method may still be useful for treating the photolysis mechanism of CH3ONO2 at the long-wavelength UV excitation, while great caution should be paid due to its inaccurate performance in the description of the photolysis dynamics at the short-wavelength UV excitation. This gives valuable information to access the accuracy when other alkyl nitrates are treated at the ADC(2) level.</jats:p>

<jats:p>Photolysis reaction channels of CH<jats:sub>3</jats:sub>ONO<jats:sub>2</jats:sub> are obtained in the nonadiabatic dynamics simulations.</jats:p>