Photochemical Conversion of Phenanthro[9,10‑<i>d</i>]imidazoles into π‑Expanded Heterocycles

We discovered that phenanthro­[9,10-<i>d</i>]­imidazoles bearing a 2-halogenoaryl substituent at position 2 undergo swift photochemically driven direct arylation, leading to barely known phenanthro­[9′,10′:4,5]­imidazo­[1,2-<i>f</i>]­phenanthridines. The reaction is high-yielding, and it does not require any sensitizer or base. The discovered process is tolerant of a variety of substituents present both at positions 1 and 2; i.e., strongly electron-donating and electron-withdrawing substituents are tolerated as well as various heterocyclic units. Steric hindrance does not affect this process. The evidence gathered here indicates that S_RN1 mechanism is operating in this case with the formation of radical anion as a critical step, followed by heterolytic cleavage of a carbon–halogen bond. Also TfO groups were shown to undergo cyclization, which allows the use of salicylaldehydes in the construction of heterocyclic systems. Efficiency of this photochemically driven direct arylation has been demonstrated by the synthesis of two systems possessing 13 and 17 conjugated rings, respectively. Phenanthro­[9′,10′:4,5]­imidazo­[1,2-<i>f</i>]­phenanthridines are blue-emitters, and they exhibit strong fluorescence in solution and in the solid state in direct contrast to their precursors

Dynamics of Intramolecular Excited State Proton Transfer in Emission Tunable, Highly Luminescent Imidazole Derivatives

The enol–keto excited state dynamics of a series of emission tunable imidazole derivatives undergoing excited state intramolecular proton transfer (ESIPT) were determined by means of steady state and time-resolved spectroscopic techniques in different solvents at room temperature and at 77 K. Examination of the corresponding non-ESIPT compounds, with the proton transfer function deliberately blocked, was carried out for comparison. At room temperature, the ESIPT process in the examined samples, determined by picosecond streak camera experiments, had lifetimes ranging from less than 10 ps to ca. 100 ps, and the resulting keto forms deactivated with lifetimes less than 100 ps up to a few nanoseconds. Delayed luminescence detection at 77 K in solid glasses allowed the identification of the phosphorescence of the enolic form and, in a few cases, P-type delayed fluorescence was also seen. The phosphorescence lifetimes were in the range of seconds at 77 K. The enolic triplet excited state absorption at RT, determined by nanosecond laser flash-photolysis, displayed a maximum around 460–500 nm and lifetimes on the order of tens of microseconds. In a few cases, a broad band with a maximum around 420 nm was detected and tentatively ascribed to the triplet excited state of the keto form. Reaction rates with oxygen on the order of (2–4) × 10⁹ M^–1 s^–1 were measured

Gating That Suppresses Charge Recombination–The Role of Mono‑<i>N</i>‑Arylated Diketopyrrolopyrrole

Suppressing the charge recombination (CR) that follows an efficient charge separation (CS) is of key importance for energy, electronics, and photonics applications. We focus on the role of dynamic gating for impeding CR in a molecular rotor, comprising an electron donor and acceptor directly linked via a single bond. The media viscosity has an unusual dual effect on the dynamics of CS and CR in this dyad. For solvents with intermediate viscosity, CR is 1.5–3 times slower than CS. Lowering the viscosity below ∼0.6 mPa s or increasing it above ∼10 mPa s makes CR 10–30 times slower than CS. Ring rotation around the donor–acceptor bond can account only for the trends observed for nonviscous solvents. Media viscosity, however, affects not only torsional but also vibrational modes. Suppressing predominantly slow vibrational modes by viscous solvents can impact the rates of CS and CR to a different extent. That is, an increase in the viscosity can plausibly suppress modes that are involved in the transition from the charge-transfer (CT) to the ground state, i.e., CR, but at the same time are not important for the transition from the locally excited to the CT state, i.e., CS. These results provide a unique example of synergy between torsional and vibronic modes and their drastic effects on charge-transfer dynamics, thus setting paradigms for controlling CS and CR