Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. , We show here that thanks to of fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Intramolecular excimers of open forms of 2H-benzopyran, 2H- and 3H-naphthopyrans in solution: TD-DFT/DFT analysis

We hypothesized that in the open forms of diphenyl-substituted photochromic compounds, immediately after the photoinduced cleavage of the C-O bond, one of the phenyl rings forms a stack with an aromatic system at the other end of the alkyl linker. The formation of these intramolecular stacking excimers is made possible by a known increase in the rotational mobility of the linker double bonds in the excited state. The flexibilities of linkers are analyzed in terms of changes in their dihedral angles. After internal conversion, stacking is partially broken but it still prevents the recovery of the C-O bond. This explains the significantly greater stability of the open forms of diphenyl-substituted benzopyran and naphthopyran compared to dimethyl-substituted ones. Open structures with residual stacking are intermediate and by thermal motion, they transform into stable trans-trans and cis-trans forms. However, in our opinion, they significantly affect the equilibrium of closed and open isomers in solution. In particular, the calculated stacking energy of 2H-naphthopyran turned out to be higher than that of 3H-naphthopyran, which partly explains the significantly greater stability of the open form of the former. For the DFT/TD-DFT calculations performed in this work, the following functionals were selected to provide reliable stacking for the open forms of all three studied compounds: APFD, M052X, M06HF, as well as B3LYP with D3(BJ) Grimme dispersion correction. The 6-311++G(d,p) basis set and cyclohexane solvent modeled using IEFPCM were also used.

Eclipsed and Twisted Excimers of Pyrene and 2-Azapyrene: How Nitrogen Substitution Impacts Excimer Emission

Due to their unique photophysical and electronic properties, pyrene and its analogues have been the subject of extensive research in recent decades. The propensity of pyrene and its derivatives to form excimers has found wide application in various fields. Nitrogen-substituted pyrene derivatives display similar photophysical properties, but for them, excimer emission has not been reported to date. Here, we use time-dependent density functional theory (TD-DFT) calculations to investigate the low-lying exciton states of dimers of pyrene and 2-azapyrene. The excimer equilibrium structures are determined and the contribution of charge transfer (CT) excitations and intermolecular interactions to the exciton states is disclosed using a diabatization procedure. The study reveals that the dimers formed by the two molecules have quite similar exciton-state patterns, in which the relevant CT contributions govern the formation of excimer states, along with the La/Lb state inversion. In contrast with pyrene, the dipole-dipole interactions in 2-azapyrene stabilize the dark eclipsed excimer structure and increase the barrier for conversion into a bright twisted excimer. It is suggested that these differences in the nitrogen-substituted derivative might influence the excimer emission properties.

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for time-dependent Density Functional Theory (TD-DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well-developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrids-particularly with large fractions of Fock exchange-are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between each assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

Monomer and Excimer Emission in Electrogenerated Chemiluminescence of Pyrene and 2,7-Di-tert-butylpyrene Associated with Electron Transfer Distance

Electrogenerated chemiluminescence (ECL) is a light emission phenomenon caused by electrochemically generated radical anions (R•-) and cations (R•+), in which the ion annihilation results in the formation of a pair of excited (R*) and ground state (R) of a luminescent molecule. Here, the ECL properties of pyrene (Py) and 2,7-di-tert-butylpyrene (di-t-BuPy) are reported. It was found that at a commonly employed concentration (1 mM), the ECL spectra were time-dependent because of increasing the oligomer emission and increasing the concentration of R near R*, leading to an enhancement of the excimer emission. At a low concentration range (20-30 μM), the shape of the ECL spectra containing the monomer and excimer emission was determined by isolated pairs of R* and R, which were generated through ion annihilation of R•- and R•+. It was found that in the ECL of Py and di-t-BuPy originated from the isolated pairs of R•- and R•+, 58 and 48% of the excited states were the excimer states, respectively. Diffusion equation analysis indicates that the lower excimer formation in the case of di-t-BuPy is because of a farther initial separation distance between R* and R, i.e., a longer electron transfer distance between the radical ions. The Marcus model for the electron transfer kinetics suggests that the farther electron transfer distance is mainly caused by the larger molecular size, which resulted in a smaller reorganization energy of the solvent acetonitrile molecule. Taking advantage of the photophysical and electrochemical properties of Py and di-t-Bu Py, the monomer and excimer emission in ECL is discussed.

Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design

The complex photokinetics of donor-acceptor-donor triads with varying flexible spacer lengths (n = 4-10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady-state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in a liquid solution, dynamic charge-transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli-responsive high-contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in-depth analysis of complex photophysics can be put to practical use in materials science.

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for Time-Dependent Density Functional Theory (TD-DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well-developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrids-particularly with large fractions of Fock exchange-are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between each assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

Isolated 2-hydroxypyrene and its dimer: a frequency- and time-resolved spectroscopic study

We investigated isolated 2-hydroxypyrene and its dimer in the gas phase by time- and frequency-resolved photoionisation with picosecond time-resolution.

Computer vision vs. spectrofluorometer-assisted detection of common nitro-explosive components with bola-type PAH-based chemosensors

Computer vision (CV) algorithms are widely utilized in imaging processing for medical and personal electronics applications. In sensorics CV can provide a great potential to quantitate chemosensors' signals. Here we wish to describe a method for the CV-assisted spectrofluorometer-free detection of common nitro-explosive components, e.g. 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT), by using polyaromatic hydrocarbon (PAH, PAH = 1-pyrenyl or 9-anthracenyl) – based bola-type chemosensors. The PAH components of these chemical bolas are able to form stable, bright emissive in a visual wavelength region excimers, which allows their use as extended matrices of the RGB colors after imaging and digital processing. In non-polar solvents, the excimers have poor chemosensing properties, while in aqueous solutions, due to the possible micellar formation, these excimers provide “turn-off” fluorescence detection of DNT and TNT in the sub-nanomolar concentrations. A combination of these PAH-based fluorescent chemosensors with the proposed CV-assisted algorithm offers a fast and convenient approach for on-site, real-time, multi-thread analyte detection without the use of fluorometers. Although we focus on the analysis of nitro-explosives, the presented method is a conceptual work describing a general use of CV for quantitative fluorescence detection of various analytes as a simpler alternative to spectrofluorometer-assisted methods.

Simplified computer vision-assisted algorithm for the excimer fluorescence "turn-off" detection of nitro-analytes in aqueous media is described.