Optical spectra of molecular aggregates and crystals: testing approximation schemes

The interplay between exciton delocalization and molecular vibrations profoundly affects optical spectra of molecular aggregates and crystals. The exciton motion occurs on a similar timescale as molecular vibrations, leading to a complex and intrinsically non-adiabatic problem that has been handled over the years introducing several approximation schemes. Here we discuss systems where intermolecular distances are large enough so that only electrostatic intermolecular interactions enter into play and can be treated in the dipolar approximation. Moreover, we only account for interactions between transition dipole moments, as relevant to symmetric molecules, with negligible permanent (multi)polar moments in the ground and low-lying excited states. Translational symmetry is fully exploited to obtain numerically exact solutions of the relevant Hamiltonian for systems of comparatively large size. This offers a unique opportunity to assess the reliability of different approximation schemes. The so-called Heitler–London approximation, only accounting for the effects of intermolecular interactions among degenerate electronic states, leads to the celebrated exciton model, widely adopted to describe optical spectra of molecular aggregates and crystals. We demonstrate that, mainly due to a cancellation of errors, the exciton model approximates well the position of exciton bands and reasonably well the bandshapes, but it fails to predict spectral intensities, leading to underestimated intensities in J-aggregates and overestimated intensities in H-aggregates. This general result is validated against an exact sum-rule. Finally, we address the validity of several approximation schemes adopted to reduce the dimension of the vibrational basis

Understanding Förster Energy Transfer through the Lens of Molecular Dynamics

A multiscale approach to the dynamics of resonant energy transfer (RET) is presented, combining DFT and TD-DFT results on the energy donor (D) and acceptor (A) moieties with an extensive equilibrium and non-equilibrium molecular dynamics (MD) analysis of a bound D–A pair in solution to build a coarse-grained kinetic model. We demonstrate that a thorough MD study is needed to properly address RET: the enormous configuration space visited by the system cannot be reliably sampled accounting only for a few representative configurations. Moreover, the conformational motion of the RET pair, occurring in a similar time scale as the RET process itself, leads to a sizable increase of the overall process efficiency

Aggregates of polar dyes: beyond the exciton model

The physics of aggregates of polar and polarizable donor–acceptor dyes is discussed, extending a previous model to account for the coupling of electronic and vibrational degrees of freedom. Fully exploiting transla- tional symmetry, exact absorption and fluorescence spectra are calculated for aggregates with up to 6 mole- cules. A two-step procedure is presented: in the first step, a mean-field solution of the problem is proposed to define the excitonic basis via a rotation of the electronic basis. The rotation is also accompanied by a Lang–Firsov transformation of the vibrational basis. In the second step, the aggregate Hamiltonian, written on the exciton basis, is diagonalized towards exact results. The procedure leads to a reduction of the dimension of the problem, since, at least for weak coupling, only states with up to 3 excitons are needed to obtain reliable results. More interestingly, the mean-field solution represents the proper reference state to discuss excitonic and ultraexcitonic effects. The emerging picture demonstrates that the exciton model offers a reliable description of aggregates of polar and polarizable dyes in the weak coupling regime, while ultraexcitonic effects are important in the medium-strong coupling regimes, and particularly so for J-aggregates where ultraexcitonic effects show up most clearly with multistability and multiexciton generation

Fluorenyl-Loaded Quatsome Nanostructured Fluorescent Probes

Delivery of hydrophobic materials in biological systems, for example, contrast agents or drugs, is an obdurate challenge, severely restricting the use of materials with otherwise advantageous properties. The synthesis and characterization of a highly stable and water-soluble nanovesicle, referred to as a quatsome (QS, vesicle prepared from cholesterol and amphiphilic quaternary amines), that allowed the nanostructuration of a nonwater soluble fluorene-based probe are reported. Photophysical properties of fluorenyl–quatsome nanovesicles were investigated via ultraviolet–visible absorption and fluorescence spectroscopy in various solvents. Colloidal stability and morphology of the nanostructured fluorescent probes were studied via cryogenic transmission electronic microscopy, revealing a “patchy” quatsome vascular morphology. As an example of the utility of these fluorescent nanoprobes, examination of cellular distribution was evaluated in HCT 116 (an epithelial colorectal carcinoma cell line) and COS-7 (an African green monkey kidney cell line) cell lines, demonstrating the selective localization of C-QS and M-QS vesicles in lysosomes with high Pearson’s colocalization coefficient, where C-QS and M-QS refer to quatsomes prepared with hexadecyltrimethylammonium bromide or tetradecyldimethylbenzylammonium chloride, respectively. Further experiments demonstrated their use in time-dependent lysosomal tracking.This work was supported by the DGI, Spain, “Grants BEWELL CTQ2013-40480-R” and “Mother MAT 2016-80826- R”, by AGAUR, Generalitat de Catalunya, “Grant 2014-SGR17”, the Networking Research Center on Bioengineering, Biomaterials, and Nanomedicine (CIBER-BBN), and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) through FIP Flowers project. Characterizations of nanovesicles were made at the ICTS “NANBIOSIS”, more specifically by the U6 unit of CIBERBBN. The research leading to these results received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/ 2007−2013 under REA grant agreement no. 607721 (Nano2- Fun) A.A. is enrolled in the Materials Science Ph.D. program of UAB. K.D.B. and X.L. acknowledge support from the National Science Foundation (CBET-1517273).Peer reviewe

A Marvel of Chiral Squaraine Aggregates: Chiroptical Spectra beyond the Exciton Model

Squaraines are quadrupolar molecular dyes forming aggregates with remarkable structure-correlated excitonic properties within the visible to near-infrared spectral range. Upon chiral functionalization a chiroptical response such as circular dichroism is an additional spectroscopic feature. We provide a combined experimental and theoretical survey on chiral aggregates dispersed in solution of proline- derived anilino squaraines (ProSQs) with varying terminal alkyl chain length (C3 to C12 and C16) directing the aggregation. Different aggregation scenarios with characteristic spectroscopic features appear, intricately depending on the alkyl chain length: Concomitant blue- and red-shifted spectro- scopic signature both within the linear and circular absorption for intermediate chain length, and a scenario with dominating blue-shifted signatures for shorter and longer alkyl chain length. Molecular Dynamic (MD) simulations on the aggregate structure return the opposite handedness suggesting a kinetic control for the experiments. Two modified essential state models (ESM) are applied to calculate the optical spectra with prescribed geometric parameters: A model accounting for just elec- trostatic intermolecular interactions (ESM-ES) suggests two concomitant aggregates to explain the simultaneous blue- and red-shifted spectral signatures while a model including intermolecular charge transfer (ESM-CT) returns both features for a single aggregate. Due to the complexity of calculation this is the first time an explicit expression for calculating circular dichroism including intermolcular charge transfer is rendered. However, the ESM-CT model is yet limited to tetramers so that finite size effects such as disorder are not fully captured. This could be the reason why the ESM-CT does not reproduce the scenario with dominant blue-shifted spectral signatures. Clearly, a dimeric model is sufficient to describe linear absorption but fails for chiroptical properties. Furthermore, a superlinear amplification of circular dichroism intensity with increasing aggregate size is noted for both models. This adds value to the application potential for chiral squaraine aggregate

Increasing resonance energy transfer upon dilution: A counterintuitive observation in CTAB micelles

We present a comprehensive study of two indocarbocyanines, DiI (1,1′-dioctadecyl-3,3,3′3′-tetramethylindocarbocyanine perchlorate) and DiD (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate), in water suspensions in the presence of a CTAB surfactant both above and below the critical micellar concentration. The very good affinity of the two dyes with CTAB allows them to be brought into aqueous solutions minimizing aggregation phenomena. When the dyes are loaded inside the micelles, stable fluorescent nanostructures are formed that can be exploited for fundamental studies and for imaging applications. Of special interest are micelles loaded with both dyes: indeed, the large local dye concentration inside the micelles allows observing resonance energy transfer in systems where the global dye concentration is maintained at a low level, so that detailed and robust spectroscopic characterization is possible. Quite impressively, the efficiency of resonance energy transfer is boosted when diluting the sample below the critical micellar concentration. This counterintuitive result is explained in terms of the very large affinity between the dyes and CTAB which favors the dynamical formation of small molecular clusters containing both dyes. Fluorescent micelles are widely used in bioimaging and pharmacokinetic applications. More specifically, the observation of resonant energy transfer in micelles or more generally in nanostructures loaded with two dyes is routinely exploited as a way to assess the nanostructure integrity. In this context, our results demonstrate the importance of robust spectroscopic characterization of the relevant systems in different environments in order to safely assess the viability of the integrity test of nanoparticles based on resonant energy transfer.This work is dedicated to Concepció Rovira and Jaume Veciana, inspiring scientists in the field of functional molecular materials. Working with them is a prized privilege and the basis for a solid friendship. AD, MA, FB, CS and AP benefited from the equipment and support of the COMP-HUB Initiative, funded by the “Departments of Excellence” program of the Italian Ministry for Education, University and Research (MIUR, 2018–2022) and acknowledge the support from the high performance computing center at Parma University. JMF, GVN, MK and NV acknowledge support from MINECO through the Severo Ochoa Programme FUNFUTURE (SEV-2015-0496 and CEX2019-000917-S), and the Ministry of Science and Innovation of Spain through the grant PID2019-105622RB-I00 (Mol4Bio), and gratefully acknowledge the financial support received from European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Fluorenyl-Loaded Quatsome Nanostructured Fluorescent Probes

Delivery of hydrophobic materials in biological systems, for example, contrast agents or drugs, is an obdurate challenge, severely restricting the use of materials with otherwise advantageous properties. The synthesis and characterization of a highly stable and water-soluble nanovesicle, referred to as a quatsome (QS, vesicle prepared from cholesterol and amphiphilic quaternary amines), that allowed the nanostructuration of a nonwater soluble fluorene-based probe are reported. Photophysical properties of fluorenyl–quatsome nanovesicles were investigated via ultraviolet–visible absorption and fluorescence spectroscopy in various solvents. Colloidal stability and morphology of the nanostructured fluorescent probes were studied via cryogenic transmission electronic microscopy, revealing a “patchy” quatsome vascular morphology. As an example of the utility of these fluorescent nanoprobes, examination of cellular distribution was evaluated in HCT 116 (an epithelial colorectal carcinoma cell line) and COS-7 (an African green monkey kidney cell line) cell lines, demonstrating the selective localization of <b>C-QS</b> and <b>M-QS</b> vesicles in lysosomes with high Pearson’s colocalization coefficient, where <b>C-QS</b> and <b>M-QS</b> refer to quatsomes prepared with hexadecyltrimethylammonium bromide or tetradecyldimethylbenzylammonium chloride, respectively. Further experiments demonstrated their use in time-dependent lysosomal tracking