Azzam Charaf-Eddin

Choosing a Functional for Computing Absorption and Fluorescence Band Shapes with TD-DFT

The band shapes corresponding to both the absorption and emission spectra of a set of 20 representative conjugated molecules, including recently synthesized structures, have been simulated with a Time-Dependent Density Functional Theory model including diffuse atomic orbitals and accounting for bulk solvent effects. Six hybrid functionals, including two range-separated hybrids (B3LYP, PBE0, M06, M06-2X, CAM-B3LYP, and LC-PBE) have been assessed in light of the experimental band shapes obtained for these conjugated compounds. Basis set and integration grid effects have also been evaluated. It turned out that all tested functionals but LC-PBE reproduce the main experimental features for both absorption and fluorescence, though the average errors are significantly larger for the latter phenomena. No single functional stands out as the most accurate for all aspects, but B3LYP yields the smallest mean absolute deviation. On the other hand, M06-2X could be a valuable compromise for excited-states as it reproduces the 0-0 energies and also gives reasonable band shapes. The typical mean absolute deviations between the relative positions of the experimental and theoretical peaks in the vibrationally resolved spectra are ca. 100 cm(-1) for absorption and 250 cm(-1) for emission. In the same time, the relative intensities of the different maxima are reproduced by TD-DFT with a ca. 10-15% accuracy.

Revisiting the optical signatures of BODIPY with ab initio tools

BODIPY dyes constitute one of the most efficient class of fluorescent molecules, yet their absorption and emission signatures are hardly predictable with theoretical tools. Here, we use a robust Time-Dependent Density Functional Theory approach that simultaneously accounts for solvent and vibrational effects, in order to simulate the optical properties of a large panel of BODIPY derivatives. In particular, this contribution is focussed on the accurate determination of both the 0–0 energies and vibronic shapes, that allow meaningful comparisons between experimental measurements and theoretical simulations. It turns out that Truhlars M06-2X functional is well suited for modelling the variations of the 0–0 energies induced by side groups, modifications of the skeleton, stiffening or extension of the π-path. Indeed, while the absolute mean deviation remains quite sizeable, the determination coefficient between experimental and theoretical energies is exceptionally large (R2 = 0.98), highlighting the robustness of the proposed approach. In addition, for most BODIPYs, theory is able to accurately reproduce vibrationally resolved bands. The developed protocol was successfully applied to provide insights for both pH and ion sensors. It also allowed the understanding of the optical behaviours of a series of BODIPY dimers and NIR dyes. This constitutes an unprecedented investigation of several BODIPY dyes both in terms of the number of treated molecules (more than sixty) and of the reliability of the predictions.

On the Computation of Adiabatic Energies in Aza-Boron-Dipyrromethene Dyes

We have simulated the optical properties of Aza-Boron-dipyrromethene (Aza-BODIPY) dyes and, more precisely, the 0-0 energies as well as the shape of both absorption and fluorescence bands, thanks to the computation of vibronic couplings. To this end, time-dependent density functional theory (TD-DFT) calculations have been carried out with a systematic account of both vibrational and solvent effects. In a first step, we assessed different atomic basis sets, a panel of global and range-separated hybrid functionals as well as different solvent models (linear-response, corrected linear-response, and state-specific). In this way, we have defined an accurate yet efficient protocol for these dyes. In a second stage, several simulations have been carried out to investigate acidochromic and complexation effects, as well as the impact of side groups on the topology of the optical bands. In each case, theory is able to accurately reproduce experimental results and the proposed protocol is consequently useful to design new dyes featuring improved properties.

Boranil and Related NBO Dyes: Insights From Theory

The simulations of excited-state properties, that is, the 0-0 energies and vibronic shapes, of a large panel of fluorophores presenting a NBO atomic sequence have been achieved with a Time-Dependent Density Functional Theory (TD-DFT) approach. We have combined eight hybrid exchange-correlation functionals (B3LYP, PBE0, M06, BMK, M06-2X, CAM-B3LYP, ωB97X-D, and ωB97) to the linear-response (LR) and the state specific (SS) Polarizable Continuum Model (PCM) methods in both their equilibrium (eq) and nonequilibrium (neq) limits. We show that the combination of the SS-PCM scheme to a functional incorporating a low amount of exact exchange can yield unphysical values for molecules presenting large increase of their dipole moments upon excitation. We therefore apply a functional possessing a large exact exchange ratio to simulate the properties of NBO dyes, including large dyads.

Optical Signatures of OBO Fluorophores: A Theoretical Analysis.

Dioxaborines dyes, based on the OBO atomic sequence, constitute one promising series of molecules for both organic electronics and bioimaging applications. Using Time-Dependent Density Functional Theory, we have simulated the optical signatures of these fluoroborates. In particular, we have computed the 0-0 energies and shapes of both the absorption and the emission bands. To assess the importance of solvent effects three polarization schemes have been applied within the Polarizable Continuum Model: the linear-response (LR), the corrected linear-response (cLR), and the state-specific (SS). We show that the SS approach is unable to yield consistent chemical trends for these challenging compounds that combine charge-transfer and cyanine characters. On the contrary, LR and cLR are more effective in reproducing chemical trends in OBO dyes. We have applied our computational protocol not only to analyze the signatures of existing dyes but also to design structures with red-shifted absorption and emission bands.

Expanding the polymethine paradigm: evidence for the contribution of a bis-dipolar electronic structure.

Although it has been reported in a few instances that the spectroscopic properties of cyanine dyes were strongly dependent on the nature of the chemical substitution of their central carbon atom, there has not been to date any systematic study specifically aimed at rationalizing this behavior. In this article, such a systematic study is carried out on an extended family of 17 polymethine dyes carrying different substituents on their central carbon, some of those being specifically synthesized for this study, some of those similar to previously reported compounds, for the sake of comparison. Their absorption properties, which spread over the whole visible to near-infrared spectral range, are seen to be dramatically dependent on the electron-donating character of this central substituent. By correlating this behavior to NMR spectroscopy and (vibronic) TD-DFT calculations, we show that it results from a profound modification of the ground state electronic configuration, namely, a progressive localization of the cationic charge on the central carbon as the electron-donating nature of the central substituent is increased.

Excited-states of BODIPY–cyanines: ultimate TD-DFT challenges?

We have investigated with first principle approaches the optical signatures of derivatives combining a BODIPY core and cyanine-like side chains. More precisely, we computed the 0–0 energies with a Time-Dependent Density Functional Theory (TD-DFT) procedure systematically including both vibrational and continuum solvent effects. However, despite its refinement, this protocol yields large deviations compared to experimental references. For this reason, we turned towards a mixed protocol where the potential energy surfaces of both the ground and the first electronically excited states are evaluated with TD-DFT whereas the vertical transition energies (both absorption and emission) are determined with the CIS(D)/SOS-CIS(D) approaches, that include a perturbative correction for the double excitations. The pros and cons of such a mixed method are discussed in the framework of these challenging dyes.

Electronic Band Shapes Calculated with Optimally Tuned Range-Separated Hybrid Functionals

Using a set of 20 organic molecules, we assess the accuracy of both the absorption and emission band shapes obtained by two optimally tuned range-separated hybrid functionals possessing 0% (LC-PBE*) and 25% (LC-PBE0*) of short-range exact exchange as well as by four other hybrid functionals including or not dispersion and long-range corrections (APF-D, PBE0-1/3, SOGGA11-X, and ωB97X-D). The band topologies are compared to experimental data and to previous time-dependent density functional theory calculations. It turns out that both optimally tuned functionals vastly improve the vibronic band shapes obtained with the non-tuned LC-PBE approach but, statistically, do not yield more accurate topologies than standard hybrid functionals. In other words, optimal tuning allows to obtain more accurate excited-state energies without degrading the description of band shapes. In addition, the LC-PBE0* 0-0 energies have been determined for a set of 40 compounds, and it is shown that the results are, on average, less accurate than those obtained by LC-PBE* for the same panel of molecules. The correlation between the optimal range-separation parameters determined for LC-PBE* and LC-PBE0* is discussed as well.

Perylene-derived triplet acceptors with optimized excited state energy levels for triplet-triplet annihilation assisted upconversion.

A series of perylene derivatives are prepared as triplet energy acceptors for triplet-triplet annihilation (TTA) assisted upconversion. The aim is to optimize the energy levels of the T1 and S1 states of the triplet acceptors, so that the prerequisite for TTA (2E(T1) > E(S1)) can be better satisfied, and eventually to increase the upconversion efficiency. Tuning of the energy levels of the excited states of the triplet acceptors is realized either by attaching aryl groups to perylene (via single or triple carbon-carbon bonds), or by assembling a perylene-BODIPY dyad, in which the components present complementary S1 and T1 state energy levels. The S1 state energy levels of the perylene derivatives are generally decreased compared to perylene. The anti-Stokes shift, TTA, and upconversion efficiencies of the new triplet acceptors are improved with respect to the perylene hallmark. For the perylene-BODIPY dyads, the fluorescence emission was substantially quenched in polar solvents. Moreover, we found that extension of the π-conjugation of BODIPY energy donor significantly reduces the energy level of the S1 state. Low S1 state energy level and high T1 state energy level are beneficial for triplet photosensitizers.

Fluorescence in rhoda- and iridacyclopentadienes neglecting the spin-orbit coupling of the heavy atom: the ligand dominates.

We present a detailed photophysical study and theoretical analysis of 2,5-bis(arylethynyl)rhodacyclopenta-2,4-dienes (1a–c and 2a–c) and a 2,5-bis(arylethynyl)iridacyclopenta-2,4-diene (3). Despite the presence of heavy atoms, these systems display unusually intense fluorescence from the S1 excited state and no phosphorescence from T1. The S1 → T1 intersystem crossing (ISC) is remarkably slow with a rate constant of 108 s–1 (i.e., on the nanosecond time scale). Traditionally, for organometallic systems bearing 4d or 5d metals, ISC is 2–3 orders of magnitude faster. Emission lifetime measurements suggest that the title compounds undergo S1 → T1 interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be ΔHISC = 28 kJ mol–1 (2340 cm–1) for 1a, which is supported by density functional theory (DFT) and time-dependent DFT calculations [ΔHISC(calc.) = 11 kJ mol–1 (920 cm–1) for 1a-H]. However, below 233 K a second, temperature-independent ISC process via spin–orbit coupling occurs. The calculated lifetime for this S1 → T1 ISC process is 1.1 s, indicating that although this is the main path for triplet state formation upon photoexcitation in common organometallic luminophores, it plays a minor role in our Rh compounds. Thus, the organic π-chromophore ligand seems to neglect the presence of the heavy rhodium or iridium atom, winning control over the excited-state photophysical behavior. This is attributed to a large energy separation of the ligand-centered highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) from the metal-centered orbitals. The lowest excited states S1 and T1 arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T1 state associated with a large S1–T1 energy separation favor an “organic-like” photophysical behavior.