Adèle D. Laurent

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.

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.

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.

Is the Tamm-Dancoff Approximation Reliable for the Calculation of Absorption and Fluorescence Band Shapes?

The reliability of the Tamm-Dancoff approximation (TDA) for predicting vibrationally resolved absorption and emission spectra of several prototypical conjugated molecules has been addressed by performing a series of extensive theoretical calculations. To this end, we have systematically compared the TDA results with the full Time-Dependent Density Functional Theory (TDDFT), the Random Phase Approximation (RPA), as well as the Configuration Interaction Singles (CIS) methods that are routinely employed for the prediction of optical spectra of large molecules. Comparisons have been made with experimental results for both the band shapes and 0-0 energies. They revealed that TDA is generally able to reproduce the experimental band shapes along with the positions of the absorption and emission peaks. With respect to TDDFT, TDA leads to an underestimation of the relative intensities for most cases but does not alter any other feature of the spectra. For the case of 0-0 energies, it leads to a better agreement between theory and experiment compared to TDDFT for the majority of the molecules studied, at least when combined with the popular B3LYP functional.

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.

Exploring Structural and Optical Properties of Fluorescent Proteins by Squeezing: Modeling High-Pressure Effects on the mStrawberry and mCherry Red Fluorescent Proteins

Molecular dynamics calculations of pressure effects on mStrawberry and mCherry fluorescent proteins are reported. The simulations reveal that mStrawberry has much floppier structure at atmospheric pressure, as evidenced by larger backbone fluctuations and the coexistence of two conformers that differ by Ser146 orientation. Consequently, pressure increase has a larger effect on mStrawberry, making its structure more rigid and reducing the population of one of the conformers. The most significant effect of pressure increase is in the hydrogen-bonding network between the chromophore and the nearby residues. The quantum-mechanics/molecular mechanics calculations of excitation energies in mStrawberry explain the observed blue shift and identify Lys70 as the residue that has the most pronounced effect on the spectra. The results suggest that pressure increase causes an initial increase of fluorescence yield only for relatively floppy fluorescent proteins, whereas the fluorescent proteins that have more rigid structures have quantum yields close to their maximum. The results suggest that a low quantum yield in fluorescent proteins is dynamic in nature and depends on the range of thermal motions of the chromophore and fluctuations in the H-bonding network rather than on their average structure.

Theoretical Investigation of the Geometries and UV−vis Spectra of Poly(l-glutamic acid) Featuring a Photochromic Azobenzene Side Chain

The geometries and UV-vis spectra of azobenzene dyes grafted as a side chain on poly(l-glutamic acid) have been investigated using a combination of quantum mechanics/molecular mechanics (QM/MM) and time-dependent density functional theory (TD-DFT) methods at the TD-PBE0/6-311+G(d,p)//B3LYP/6-311G(d,p):Amber ff99 level of theory. The influence of the secondary structure of the polypeptide on the electronic properties of both the trans and cis conformations of azobenzene dyes has been studied. It turns out that the grafted dyes exhibit a red-shift of the π → π* absorption energies mainly due to the auxochromic shift induced by the peptidic group used to link the chromophoric unit to the polypeptide and that specific interactions between the glutamic side chain and the azobenzene moiety lead to a large blue-shift of the n → π* transition.

Effect of the Enhanced Cyan Fluorescent Protein framework on the UV/visible absorption spectra of some chromophores

Mutation on Enhanced Cyan Fluorescent Protein (ECFP) has been investigated by means of a hybrid quantum mechanics and molecular mechanics (QM/MM) approach. A simple model, which represents the electronic response of the surroundings (ERS) by a polarizable continuum characterized by the relative dielectric constant extrapolated to infinite frequency, is used. For all mutations, this method (QM/MM+ERS) reproduces the experimental trend (shift) and is in correct agreement with experimental maximum absorption wavelength (between −10 and +10 nm). The effect of the ECFP on the optical properties is analyzed and decomposed into three major contributions (geometric deformation, electrostatic polarization and electronic response). It is shown that these three contributions have similar magnitude and one cannot neglect one with respect to the others. In addition, these contributions differ greatly from one chromophore to another showing that the same protein framework acts differently on different chromophores.

Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far‐Red (Polarized) Luminescence

The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pKR+ =20.4, Ered1/2 =-0.72 V) compared to its azaoxa 2 (pKR+ =15.2, Ered1/2 =-0.45 V) and dioxa 1 (pKR+ =8.8, Ered1/2 =-0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4-15). The electronic absorption is modulated from the orange to the far-red spectral range (560-731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.

Using Time‐Dependent Density Functional Theory to Probe the Nature of Donor–Acceptor Stenhouse Adduct Photochromes

We present the first theoretical investigation of a recently proposed class of photochromes, namely donor-acceptor Stenhouse adduct (DASA) switches [J. Am. Chem. Soc. 2014, 136, 8169-8172]. By using density functional theory and its time-dependent counterpart, we investigate the ground- and excited-state structures, electronic transition energies, and several properties of the two isomeric forms. In addition to demonstrating that the selected level of theory is able to reproduce the main experimental facts, we show that 1) the two forms of the DASA photochromes are close to isoenergetic; 2) the two isomers possess similar total dipole moments, in spite of their very different sizes; 3) both isomers have a zwitterionic nature; 4) the nature of the dipole-allowed electronic excited state is vastly different in the two forms; and 5) the specific band shape of the extended DASA can be reproduced by vibronic calculations.