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Abstract A proper understanding of excited state properties of indole derivatives can lead to rational design of efficient fluorescent probes. The optically active and excited states of a series of substituted indoles, where a substituent was placed on position four, were calculated using equation of motion coupled cluster and time dependent density functional theory. The results indicate that most substituted indoles have a brighter second excited state corresponding to experimental absorption maxima, but a few with electron withdrawing substituents absorb more on the first excited state. Absorption on the first excited state may increase their fluorescence quantum yield, making them better probes. Electronic structure methods were found to predict the energies of the systems with electron withdrawing substituents more accurately than those with electron donating substituents. The excited states of both states correlated well with electrophilicity, similar to the experimental trends for the absorption maxima. Overall, these computational studies indicate that theory can be used to predict excited state properties of substituted indoles, when the substituent is an electron withdrawing group. 

The excited-state dynamics of o-nitrophenol have been explored using trajectory surface hopping nonadiabatic dynamics combined with floating occupation molecular orbital complete active space configuration interaction. We focus on the effect of excitation energy on the subsequent dynamics. The absorption spectrum of o-nitrophenol has two peaks, centered at 3.9 eV (∼320 nm) and 5.1 eV (∼240 nm), and we performed dynamics starting from each of these peaks. The results show that even though the relaxation time constants are similar for the two excitation windows, the underlying dynamics are different. When exciting to the low energy peak, the dynamics are dominated by intramolecular proton transfer followed by internal conversion to the ground state, while exciting to the high-energy peak leads to fast internal conversion to the first excited state and slower decay to the ground state. In this case, intramolecular proton transfer does not occur as frequently, and many trajectories decay to the ground state through conical intersections without proton transfer. By calculating spin–orbit coupling values along the trajectories, we also show that intersystem crossing is possible. Based on the Landau–Zener probability formula, we estimate that there is about a 30%–40% probability that intersystem crossing will occur within 1 ps. 

Time-resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255 nm (4.9 eV) and probed with vacuum ultraviolet (VUV) pulses at 153 nm (8.1 eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump–probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation through a process of internal conversion and intersystem crossing. The combination of photoelectron and photoion spectroscopy, together with computational results, provides strong evidence of intersystem crossing that is difficult to establish with only a single technique.