Siwar Chibani

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.

Improving the Accuracy of Excited-State Simulations of BODIPY and Aza-BODIPY Dyes with a Joint SOS-CIS(D) and TD-DFT Approach

BODIPY and aza-BODIPY dyes constitute two key families of organic dyes with applications in both materials science and biology. Previous attempts aiming to obtain accurate theoretical estimates of their optical properties, and in particular of their 0-0 energies, have failed. Here, using time-dependent density functional theory (TD-DFT), configuration interaction singles with a double correction [CIS(D)], and its scaled-opposite-spin variant [SOS-CIS(D)], we have determined the 0-0 energies as well as the vibronic shapes of both the absorption and emission bands of a large set of fluoroborates. Indeed, we have selected 47 BODIPY and 4 aza-BODIPY dyes presenting diverse chemical structures. TD-DFT yields a rather large mean signed error between the experimental and theoretical 0-0 energies with a systematic overshooting of the transition energies (by ca. 0.4 eV). This error is reduced to ca. 0.2 [0.1] eV when the TD-DFT 0-0 energies are corrected with vertical CIS(D) [SOS-CIS(D)] energies. For BODIPY and aza-BODIPY dyes, both CIS(D) and SOS-CIS(D) clearly outperform TD-DFT. The present computational protocol allows accurate data to be obtained for the most relevant properties, that is, 0-0 energies and optical band shapes.

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.

TD-DFT Assessment of the Excited State Intramolecular Proton Transfer in Hydroxyphenylbenzimidazole (HBI) Dyes

Dyes undergoing excited state intramolecular proton transfer (ESIPT) received increasing attention during the last decades. If their unusual large Stokes shifts and sometimes dual-fluorescence signatures have paved the way toward new applications, the rapidity of ESIPT often prevents its investigation with sole experimental approaches, and theoretical simulations are often welcome, if necessary, to obtain a full rationalization of the observations. In the present paper, we evaluate both the absorption and the fluorescence spectra of, respectively, the enol and keto forms of a series of hydroxyphenylbenzimidazole (HBI) using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT). Optical spectra were obtained accounting for both vibronic and environmental effects. The aim of this work is therefore not to evaluate the radiationless pathway going through the twisted ESIPT structures, though excited-state reaction paths between enol and keto forms have been rationalized. First we have compared three dyes differing by the strength of the donor groups, and we have quantified the impact of the flexible butyl chain substituting the imidazole side. In accordance with experiments, we show that the presence of a dialkylamino auxochrome allows to tune the excited-state potential energy surface leading to a weaker tendency to ESIPT. This trend is rationalized in terms of both structural and electronic effects. Next, larger hydroxyphenyl-phenanthroimidazole (HPI) were considered to assess the impact of a stronger π-delocalization. 0-0 energies and vibrationally resolved spectra of the corresponding fluoroborate derivatives were studied as well. The dialkylamino auxochrome significantly decreases the 0-0 energies due to the presence of an important charge transfer character, while the addition of a BODIPY moiety induces a change of the emission signature now localized on the BODIPY side rather than on the NBO core.

Solvent Effects on Cyanine Derivatives: A PCM Investigation

In this work, we present time-dependent density functional theory calculations of the excited-state geometries and electronic properties of both model cyanines and BODIPY derivatives, which are particularly challenging dyes for theoretical chemistry. In particular, we focus on environmental effects, using a panel of approaches derived from the polarizable continuum model, including full corrected linear response (cLR) values determined through a very recently developed approach. It turns out that in idealized quasi-linear cyanines, all approaches provide very similar excited-state geometries though linear response (LR), and cLR models yield very different transition energies. For the fluoroborate derivatives, LR apparently overestimates the planarity of the excited-state geometries, and cLR optimizations yield slightly smaller fluorescence energies than LR, making these values closer to experimental references. The computed corrections are however too small to explain (taken alone) the significant theory/experiment discrepancies.

Excited-State Geometries of Solvated Molecules: Going Beyond the Linear-Response Polarizable Continuum Model

The theoretical determination of excited-state structures remains an active field of research, as these data are hardly accessible by experimental approaches. In this contribution, we investigate excited-state geometries obtained with Time-Dependent Density Functional Theory, using both linear-response and, for the first time, corrected linear-response approaches of the Polarizable Continuum Model. Several chromophores representative of key dye families are used. In most cases, the corrected linear-response approach provides bond distances in between the gas and linear-response data, the latter model providing larger medium-induced structural changes than the corrected linear-response model. However, in a few cases, the solvation effects predicted by the two continuum approaches present opposite directions compared to the gas phase reference.