1,6-Diazapyrene: A Novel, Well-Defined, Small-Size Prototype System for Nitrogen-Containing PAHs

The quest for nitrogen-doped (N-doped) polycyclic aromatic hydrocarbons (PAHs) requires well-defined prototype systems to understand the relationship between the structure and the resulting photophysical and photochemical properties. To this end, a novel, simple, and small compound, 1,6-diazapyrene, was synthesized. In-depth analysis, employing optical spectroscopy and (time-dependent) density functional theory, (TD-)DFT, elucidates the optical excitations on the basis of MO symmetry, energy, and topology considerations; the study further unveils the photophysical and photochemical deactivation kinetics after photoexcitation, revealing extreme changes against pyrene as well as against the well-known 2,7-diazapyrene isomer. The high sensitivity of the aza-substitution position to generate such changes is considered as highly relevant for the targeted design of N-doped PAHs in general.

Theoretical Study on the Internal Conversion Decay Pathways of Bithiophene-Fused Isoquinolines

In this study, the radiative and nonradiative decay pathways from the first singlet excited states (denoted as S1) of three bithiophene-fused isoquinolines were investigated by using the mixed-reference spin-flip time-dependent density functional theory approach. These isoquinolines, which are prepared via [2 + 2 + 2] cycloaddition reactions between three types of bithiophene-linked diynes and nitriles, exhibit different fluorescence quantum yields in response to the positions of their sulfur atoms. The decay processes, including the fluorescence emission and internal conversion, were considered. In the internal conversion pathway, the minimum energy conical intersection structures between the ground and first singlet excited states (denoted as S0/S1 MECI) of the ring strain for the isoquinoline skeleton and the ring opening of the thiophene skeleton were systematically explored. Dewar-type ring strain resulted in the smallest energy barrier from the equilibrium geometries of the ground state (denoted as S0) to the MECI structures between the S0 and S1 states. The energy difference between the three types of bithiophene-fused isoquinolines at the transition state geometries of the S1 state varies owing to the steric effects between the methyl groups and the hydrogen atom of the thiophene ring, and the excitation energy increases owing to a decrease in aromaticity. In addition, the oscillator strengths of the S0 and S1 states were evaluated at the equilibrium geometries of the S1 state to determine the contribution of the fluorescence process. The obtained theoretical results are consistent with the experimental results.

Excited state dynamics of azanaphthalenes reveal opportunities for the rational design of photoactive molecules

Various photoactive molecules contain motifs built on aza-aromatic heterocycles, although a detailed understanding of the excited state photophysics and photochemistry in such systems is not fully developed. To help address this issue, the non-adiabatic dynamics operating in azanaphthalenes under hexane solvation was studied following 267 nm excitation using ultrafast transient absorption spectroscopy. Specifically, the species quinoline, isoquinoline, quinazoline, quinoxaline, 1,6-naphthyridine, and 1,8-naphthyridine were investigated, providing a systematic variation in the relative positioning of nitrogen heteroatom centres within a bicyclic aromatic structure. Our results indicate considerable differences in excited state lifetimes, and in the propensity for intersystem crossing vs internal conversion across the molecular series. The overall pattern of behaviour can be explained in terms of potential energy barriers and spin-orbit coupling effects, as demonstrated by extensive quantum chemistry calculations undertaken at the SCS-ADC(2) level of theory. The fact that quantum chemistry calculations can achieve such detailed and nuanced agreement with experimental data across a full set of six molecules exhibiting subtle variations in their composition provides an excellent example of the current state-of-the-art and is indicative of future opportunities for rational design of photoactive molecules.

Transient spectroscopic insights into nitroindole's T1 state: Elucidating its intermediates and unique photochemical properties

Indoles are notable for their distinct photophysical and photochemical properties, making them useful indicators in biological systems and promising candidates for a variety of pharmaceutical applications. While some indoles exhibit room temperature phosphorescence, such a phenomenon has not been observed in nitroindoles. Typically, adding of a nitro group into aromatic compounds promotes ultrafast intersystem crossing and increases the formation quantum yield of the lowest excited triplet (T1). Therefore, understanding the reactivity of nitroindoles' T1 states is imperative. This study investigated the physical properties and chemical reactivities of the T1 state of 6-nitroindole (3HN-6NO2) in both polar aprotic and protic solvents, using transient absorption spectroscopy. Our results demonstrate the basicity and acidity of 3HN-6NO2, emphasizing its potential for protonation and dissociation in mildly acidic and basic conditions, respectively. Furthermore, 3HN-6NO2 has a high oxidizing capacity, participating in electron transfer reactions and proton-coupled electron transfer to produce radicals. Interestingly, in protic solvents like alcohols, 3HN-6NO2 dissociates at the -NH group and forms N-H…O hydrogen-bonded complexes with the nitro group. By identifying transient absorption spectra of intermediates and quantifying kinetic reaction rate constants, we illuminate the unique properties of the T1 state nitroindoles, enriching our understanding of their photophysical and photochemical behaviors. The results of this study have significant implications for their potential application in both biological systems and materials science.

Non-Radiative Deactivation in Isolated Quinoline

The photophysics of the S2 1(ππ*) state of the polycyclic aromatic nitrogen-containing hydrocarbon (PANH) quinoline is investigated in a free jet using a picosecond laser system. A [1 + 1] multiphoton ionization spectrum yields the S2 origin at around 32 200 cm-1 and reveals several vibronic bands. In time-resolved experiments, quinoline is then excited between 312.2 and 279.7 nm. Probe wavelengths of 351 and 263.5 nm are employed. The dynamics is monitored by time-resolved photoelectron imaging. The images reveal a short-lived band at high electron kinetic energies with a ps lifetime and a band at lower electron kinetic energies that shows an offset at long delay times. In comparison with previous work, the offset is assigned to ionization from the T1 state. Lifetimes decrease from 45 ps at the S2 origin to 11 ps at +3550 cm-1. Most likely, the S2 1(ππ*) state deactivates by internal conversion to the S1 1(nπ*) state, followed by intersystem crossing to the triplet manifold.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Excited state hydrogen or proton transfer pathways in microsolvated n-cyanoindole fluorescent probes

The sensitivity of the fluorescence properties of n-cyanoindole (n-CNI) fluorescent probes to the microenvironment makes them potential reporters of protein conformation and hydration. The fluorescence intensity of 5-CNI, 6-CNI, and 7-CNI is quenched when exposed to water solvent whereas substitution on position 4 of indoles dramatically increases it. A potential mechanism for this sensitivity to water may be similar to that found in indole. The fluorescence of indole is found to be quenched when interacting with water and ammonia solvent molecules via radiationless decay through an S1 (πσ*)/S0 conical intersection caused by excited state proton or hydrogen transfer to the solvent molecules. In this study we examine this fluorescence quenching mechanism along the N-H bond stretch of n-CNI probes using water cluster models and quantum mechanical calculations of the excited states. We find that n-CNI-(H2O)1-2 clusters form cyclic or non-cyclic structures via hydrogen bonds which lead to different photochemical reaction paths that can potentially quench the fluorescence by undergoing internal conversion from S1 to S0. However, the existence of a high energy barrier along the potential energy surface of the S1 state in most cases prevents this from occurring. We show that substitution on position 4 leads to the highest energy barrier that prevents the fluorophore from accessing these non-radiative channels, in agreement with its high intensity. We also find that the energy barrier in the S1 state of non-cyclic 5-CNI-(H2O)1-2 excited complexes decreases as the number of water molecules increases, which suggests great sensitivity of the fluorescence quenching on the aqueous environment.

Singlet O2 from Ultraviolet Excitation of the Quinoline-O2 Complex

We report results from experiments with the quinoline-O₂ complex, which was photodissociated using light near 312 nm. Photodissociation resulted in formation of the lowest excited state of oxygen, O₂ a ¹Δ_g, which we detected using resonance enhanced multiphoton ionization and velocity map ion imaging. The O₂⁺ ion image allowed for a determination of the center-of-mass translational energy distribution, P(E_T), following complex dissociation. We also report results of electronic structure calculations for the quinoline singlet ground state and lowest energy triplet state. From the CCSD/aug-cc-pVDZ//(U)MP2/cc-pVDZ calculations, we determined the lowest energy triplet state to have ππ* electronic character and to be 2.69 eV above the ground state. We also used electronic structure calculations to determine the geometry and binding energy for several quinoline-O₂ complexes. The calculations indicated that the most strongly bound complex has a well depth of about 0.11 eV and places the O₂ moiety above and approximately parallel to the quinoline ring system. By comparing the experimental P(E_T) with the energy for the singlet ground state and the lowest energy triplet state, we concluded that the quinoline product was formed in the lowest energy triplet state. Finally, we found the experimental P(E_T) to be in agreement with a Prior translational energy distribution, which suggests a statistical dissociation for the complex.

Substitution enables significant new decay channels for a non-canonical amino acid

UV-Vis absorption spectra and emission peaks of indole and 7-fluoroindole are measured and it is observed that 7-fluoroindole quenches the fluorescence signals significantly compared to indole. This observation is elucidated via reconstruction of the potential energy surfaces, determination of the conical intersections, and dynamical studies. It is observed that a single fluorine substitution on indole leads to the appearance of several accessible low-energy conical intersections that cause fast nonradiative decay. In this paper, we have investigated the nonradiative processes of Ind and 7F-Ind theoretically using high-level methods, such as EOM-EE-CCSD, SA-CASSCF, MS-CASPT2/6-311++G(d,p) levels of theory, to study these pathways and their feasibility.

The quantum chemical solvation of indole: accounting for strong solute-solvent interactions using implicit/explicit models

In this paper, we investigate the efficacy of different quantum chemical solvent modelling methods of indole in both water and methylcyclohexane solutions. The goal is to show that one can yield good photophysical properties in strongly coupled solute-solvent systems using standard DFT methods. We use standard and linearly-corrected Polarisable Continuum Models (PCM), as well as explicit solvation models, and compare the different model parameters, including the choice of density functional, basis set, and number of explicit solvent molecules. We demonstrate that implicit models overestimate energies and oscillator strengths. In particular, for indole-water, no level inversion is observed, suggesting a dielectric medium on its own is insufficient. In contrast, energies are seen to converge fairly rapidly with respect to cluster size towards experimentally measured properties in the explicit models. We find that the use of B3LYP with a diffuse basis set can adequately represent the photophysics of the system with a cluster size of between 9-12 explicit water molecules. Sampling of configurations from a molecular dynamics simulation suggests that the single point results are suitably representative of the solvated ensemble. For indole-water, we show that solvent reorganisation plays a significant role in stabilisation of the excited state energies. It is hoped that the findings and observations of this study will aid in the choice of solvation model parameters in future studies.

Novel bipyrazolo[1,5-a]pyridine luminogens with aggregation-induced emission enhancement properties

A novel 3,3'-bipyrazolo[1,5-a]pyridine molecular scaffold was obtained as a product of serendipity. Both photophysical characterisations and HOMO-LUMO theoretical calculations indicate its potential as a promising fluorophore with notable intramolecular charge transfer. Nonetheless, the emission properties of this compound suffer from the typical aggregation-caused quenching effect. To overcome this situation, we introduced additional diaryl groups onto the skeleton and synthesised a series of 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines via palladium-catalysed intermolecular C-H/C-H bond cross-coupling reaction in 35-62% yields. This series of tailor-made luminogens with twisted π-structures display aggregation-induced emission enhancement properties.

Origin of the Aggregation-Induced Phosphorescence of Platinum(II) Complexes: The Role of Metal-Metal Interactions on Emission Decay in the Crystalline State

Discerning the origins of the phosphorescent aggregation-induced emission (AIE) from Pt(II) complexes is crucial for developing the broader range of photo-functional materials. Over the past few decades, several mechanisms of phosphorescent AIE have been proposed, however, not have been directly elucidated. Herein, we describe phosphorescence and deactivation processes of four class of AIE active Pt(II) complexes in the crystalline state based on experimental and theoretical investigation. These complexes show metal-to-ligand and/or metal-metal-to-ligand charge transfer emission in crystalline state with different heat resistance against thermal emission quenching. The calculated energy profiles including the minimum energy crossing point between S₀ and T₁ states were consistent with the heat resistant properties, which provided the mechanism for AIE expression. Furthermore, we have clarified the role of metal-metal interaction in AIE by comparing two computational models.

Efficient enumeration of bosonic configurations with applications to the calculation of non-radiative rates

This work presents algorithms for the efficient enumeration of configuration spaces following Boltzmann-like statistics, with example applications to the calculation of non-radiative rates, and an open-source implementation. Configuration spaces are found in several areas of physics, particularly wherever there are energy levels that possess variable occupations. In bosonic systems, where there are no upper limits on the occupation of each level, enumeration of all possible configurations is an exceptionally hard problem. We look at the case where the levels need to be filled to satisfy an energy criterion, for example, a target excitation energy, which is a type of knapsack problem as found in combinatorics. We present analyses of the density of configuration spaces in arbitrary dimensions and how particular forms of kernel can be used to envelope the important regions. In this way, we arrive at three new algorithms for enumeration of such spaces that are several orders of magnitude more efficient than the naive brute force approach. Finally, we show how these can be applied to the particular case of internal conversion rates in a selection of molecules and discuss how a stochastic approach can, in principle, reduce the computational complexity to polynomial time.

Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.

Combined Spectroscopic and TD-DFT Analysis to Elucidate Substituent and Acidochromic Effects in Organic Dyes: A Case Study on Amino- versus Nitro-Substituted 2,4-Diphenylquinolines

A combined spectroscopic and TD-DFT case study was performed, to identify a robust method to calculate the complex near UV/Vis absorption spectra of various amino- vs. nitro-substituted 2,4-diphenylquinolines, which vary strongly under neutral and successively acidic conditions. For this, different DFT functionals were tested for geometry optimization and the TD part to calculate the neutral and different protonated species in a fast screening approach, i. e. using single point calculations in an implicit solvent. Offset-corrected M06HF, hitherto only applied to polymers, was identified as a suitable method to reproduce the absorption spectra in a reasonable fashion for all different substitution pattern and all different protonated species at different pH values; moreover, the method properly predicts the energetic ordering of low-lying n-π* and ππ* transitions, which is decisive for the non-/emissive nature of the different compounds. In all, this might provide a valuable tool for computer-aided design of related classes of compounds.