Michal F. Rode

The luminescence properties of N ² ,3‐etheno‐2‐aminopurine (N ² ,3‐ε2APu) have been studied using steady‐state and time‐resolved spectroscopy. N ² ,3‐ε2APu was embedded in poly(vinyl alcohol) (PVA) polymer, which is known for effective immobilization of dissolved fluorophore molecules. Upon UV excitation, N ² ,3‐ε2APu shows a bright dark‐blue fluorescence with high fluorescence anisotropy, indicating efficient immobilization. Time‐resolved fluorescence measurements show a somewhat heterogeneous lifetime, requiring a lifetime distribution model to fit the data. The fit reveals a central lifetime of about 5 ns and a half‐width of the Lorentzian distribution of about 2 ns. We attribute this lifetime spread to the varying surroundings of fluorophore molecules in the polymer matrix. The N ² ,3‐ε2APu‐doped PVA films exhibit strong room‐temperature phosphorescence (RTP) centered at 500 nm, with a lifetime of approximately 0.15 s. The excitation at 330 nm results in negative phosphorescence anisotropy. Interestingly, N ² ,3‐ε2APu–doped PVA films can be directly excited to a triplet state at longer wavelengths (above 400 nm). This excitation results in a high positive phosphorescence anisotropy, reaching a value of 0.3 at about 450 nm. The ability to excite a molecule at longer wavelengths may find applications in the biomedical sciences, particularly in the study of photo‐chemically labile proteins and their complexes with emitting ligands.

Intrinsically fluorescent photoswitches (IFPSs) are crucial for super‐resolution microscopy techniques. This study explores the impact of substituents on fluorescence and photoswitching efficiency in hemiindigos (HIs) and hemithioindigos. A series of functionalized derivatives was synthesized by one‐step Knoevenagel condensations using a sustainable mechanochemical approach and characterized across various environments. Ortho‐OH‐substituted hemiindigo derivatives exhibited high fluorescence quantum yields (Φ_FL = 0.65) while retaining significant photoswitching capabilities (Φ_Z→E = 0.29 in THF) and fast recovery. These enhanced properties are attributed to the π‐electron‐donating characteristics of the OH substituents and the presence of intramolecular hydrogen bonds. The results of CC2 ab initio calculations revealed a detailed photoswitching mechanism: (1) initial population of the fluorescent ππ* singlet excited state minimum, (2) subsequent elongation of the C═O bond leading to depopulation of the ππ* state to the nπ* excited state minimum, (3) twisting of the double bond within the nπ* state, and (4) relaxation to the ground electronic state via a conical intersection. The analysis of the potential‐energy surfaces modulated by EDG substituents, both in the ππ* and nπ* excited states, provides insights into the structure‐property relationships of IFPSs, guiding future rational design.

Intrinsically fluorescent photoswitches (IFPSs) are crucial for super-resolution microscopy techniques. This study explores the impact of substituents on fluorescence and photoswitching efficiency in hemiindigos and hemithioindigos. A series of functionalized derivatives were synthesized by one-step Knoevenagel condensations using a sustainable mechanochemical approach and characterized across various environments. Ortho-OH-substituted hemiindigo derivatives exhibited high fluorescence quantum yields (ΦFL = 0.65) while retaining significant photoswitching capabilities (ΦZ→E = 0.29 in THF) and fast recovery. These enhanced properties are attributed to the -electron-donating characteristics of the OH substituents and the presence of intramolecular hydrogen bonds. The results of CC2 ab initio calculations revealed a detailed photoswitching mechanism: (1) initial population of the fluorescent ππ* singlet excited state, (2) subsequent elongation of the C=O bond leading to depopulation of the ππ* state to the nπ* excited state, (3) twisting of the double bond within the nπ* state, and (4) relaxation to the ground electronic state via a conical intersection. The analysis of potential-energy surfaces in the excited states provides insights into the structure-property relationships of IFPSs, guiding future rational design.

Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet ¹(π,π*) excited state deactivation pathway through nonadiabatic transition to the ¹(n,π*) excited- state, and subsequent deactivation to the ground electronic state (S₀), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited–state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T₁ state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C₍₂₎-O distance elongates by 0.16 Å in the ¹(n,π*) state, with respect to the ground state. Intermediate–state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques.

A novel, computationally designed, class of triangular-shape organic molecules with an inverted singlet–triplet (IST) energy gap is investigated with ab initio electronic structure methods. The considered molecular systems are cyclic oligomers and their common feature is electronic conjugation along the molecular rim. Vertical excitation energies from the electronic ground state to the lowest singlet and triplet excited states were computed, as well as vertical emission energies from these states to the ground state. The results underscore the significance of optimizing excited-state geometries to accurately describe the optoelectronic properties of IST molecules, in particular with respect to their application in OLEDs.

Indigo, a rich blue dye, is an incredibly photostable molecule that has survived in ancient art for centuries. It is also unique in that it can undergo both an excited-state hydrogen and proton transfer on the picosecond timescale followed by a ground-state back transfer. Previously, we performed gas phase excited-state lifetime studies on indigo to study these processes in a solvent-free environment, combined with excited-state calculations. We found two decay pathways, a fast sub-nanosecond decay and a slow decay on the order of 10 ns. Calculations of the excited-state potential energy surface found that both hydrogen and proton transfer are nearly isoenergetic separated by a 0.1 eV barrier. To further elucidate these dynamics, we now report a study with deuterated indigo, using resonance-enhanced multi-photon ionization and pump-probe spectroscopy with mass spectrometric isotopomer selection. From new calculations of the excited-state potential energy surface, we find sequential double-proton or hydrogen transfer, whereby the trajectory to the second transfer passes a second barrier and then encounters a conical intersection that leads back to the ground state. We find that deuteration only increases the excited-state lifetimes of the fast decay channel, suggesting tunneling through the first barrier, while the slower channel is not affected and may involve a different intermediate state. Graphical abstract

Macrocyclic tetra(naphthimidazole)resorcin[4]arenes exhibit large Stokes shifts upon embedding in PMMA matrix enabling their applications in luminescent solar concentrator technology.

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans–cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data.

Core‐fluorescent cavitands based on 2‐(2′‐resorcinol)benzimidazole fluorophores (RBIs) merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity, show excited state intramolecular proton transfer (ESIPT) and readily undergo deprotonation to form dianionic cavitands, capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts (due to emission from anionic double keto tautomers) and cation‐selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity. Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non‐macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non‐macrocyclic analogs are anionic double keto tautomers, which are the result of excited state intramolecular proton transfer (ESIPT) or excited state double proton transfer (ESDPT).