Organic chromophores for advanced applications: models and spectroscopy

This thesis presents an extensive study of charge-transfer (CT) dyes, an interesting class of organic chromophores for advanced applications in the fields of nonlinear optics, energy harvesting, LEDs and solar cells, just to cite a few. The study combines theoretical and experimental work to offer a reliable description of optical properties and spectra of CT chromophores in different environments. CT dyes constitute excellent model systems for investigating charge- and energy-transfer processes, that represent key phenomena in physics, chemistry and biology. The work starts with the description of essential-state models that are at the heart of the theoretical approach that we have developed for CT dyes. The low-energy physics of CT chromophores is governed by charge resonance between electron-donor (D) and electron-acceptor (A) groups and their optical spectra and properties are dominated by CT transitions. The behavior of CT dyes can be described in terms of comparatively simple models that just account for the few relevant degrees of freedom. These essential-state models are general and allow for the rationalization of optical spectra and properties of families of CT chromophores, and to set up reliable structure-properties relationships as required to guide the chemical synthesis. A thorough understanding of the physics of CT chromophores requires their detailed spectroscopic characterization. Essential-state models allow for the calculation of linear and nonlinear spectra of polar and multipolar chromophores accounting for molecular vibrations and for polar solvation, making possible a detailed analysis of the spectral position, intensities and bandshapes. In this thesis, we develop the calculation of electroabsorption spectra, and we discuss, both at the experimental and theoretical level, fluorescence thermochromism and low-temperature fluorescence anisotropy spectra of different families of CT dyes. The models developed for CT dyes in solution naturally lent themselves to be extended to the description of the same dyes in more complex and challenging environments. Specifically, collective and cooperative phenomena are expected in aggregates, films, crystals and more generally in clusters where different chromophores interact via electrostatic interactions. Two families of systems are studied: the first one composed by a zwitterionic DA-chromophore, its covalent dimer and its self-assembled film on gold; the second one composed by a quadrupolar CT chromophore and its covalent dimers. These examples allow to demonstrate the power of the bottom-up approach in this context, and to fully exploit the advantages of essential-state models. The same electrostatic interactions that are responsible for cooperative and collective phenomena in multichromophoric assemblies drive the phenomenon of energy transfer in heterochromophoric macromolecules. We are able to prove that the extended-dipole approximation for interchromophore interactions leads to qualitatively different results than the more commonly adopted point-dipole approximation. Specifically, the extended-dipole approximation opens new channels for energy transfer through optically dark states, that, in the point-dipole approximation are strictly forbidden

Supramolecular chirality: a caveat in assigning the handedness of chiral aggregates

The handedness of a supramolecular chiral aggregate is often assigned based on the sign of circular dichroism spectra, adopting the exciton chirality method. However, the method does not properly account for the nature of intermolecular interactions. We introduce a generalized picture on the use of the sign of chiral signals in determining the helicity of chiral aggregates, rooted in the exciton model, supported by TD-DFT results

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability

When designing molecular functional materials, the properties of the active specie, the dye, must be optimized fully accounting for the presence of a surrounding medium (a solvent, a polymeric matrix, etc) that may largely alter the dye behavior. Here we present an effective model to account for the spectroscopic effects of the medium electronic polarizability on the properties of charge-transfer dyes. Different classes of molecules are considered and the proposed antiadiabatic approach to solvation is contrasted with the adiabatic approach, currently adopted in all quantum chemical approaches to solvation. Transition frequencies and band-shapes are addressed, and the role of the medium polarizability on symmetry-breaking phenomena is also discussed

Aggregates of Cyanine Dyes: When Molecular Vibrations and Electrostatic Screening Make the Difference

Aggregates of cyanine dyes are currently investigated as promising materials for advanced electronic and photonic applications. The spectral properties of aggregates of cyanine dyes can be tuned by altering the supramolecular packing, which is affected by the length of the dye, the presence of alkyl chains, or the nature of the counterions. In this work, we present a joint experimental and theoretical study of a family of cyanine dyes forming aggregates of different types according to the length of the polymethinic chain. Linear and nonlinear optical spectra of aggregates are rationalized here in terms of an essential-state model accounting for intermolecular interactions together with the molecular polarizability and vibronic coupling. A strategy is implemented to properly account for screening effects, distinguishing between electrostatic intermolecular interactions relevant to the ground state (mean-field effect) and the interactions relevant to the excited states (excitonic effects). To the best of our knowledge, this is the first attempt to simulate nonlinear spectral properties of aggregates of symmetric dyes accounting for molecular vibrations

Thermally activated delayed fluorescence: A critical assessment of environmental effects on the singlet–triplet energy gap

The effective design of dyes optimized for thermally activated delayed fluorescence (TADF) requires the precise control of two tiny energies: the singlet–triplet gap, which has to be maintained within thermal energy, and the strength of spin–orbit coupling. A subtle interplay among low-energy excited states having dominant charge-transfer and local character then governs TADF efficiency, making models for environmental effects both crucial and challenging. The main message of this paper is a warning to the community of chemists, physicists, and material scientists working in the field: the adiabatic approximation implicitly imposed to the treatment of fast environmental degrees of freedom in quantum–classical and continuum solvation models leads to uncontrolled results. Several approximation schemes were proposed to mitigate the issue, but we underline that the adiabatic approximation to fast solvation is inadequate and cannot be improved; rather, it must be abandoned in favor of an antiadiabatic approach

Understanding TADF: a joint experimental and theoretical study of DMAC-TRZ

Thermally-activated delayed fluorescence (TADF) is a promising strategy to harvest triplets in OLED towards improved efficiency, but several issues must be addressed to fully exploit its potential, including the nature of involved excited singlet and triplet states and their response to the local environment in order to concurrently optimize the dye inside the matrix. Towards this ambitious aim, we present an extensive spectroscopic study of a typical TADF dye in liquid and glassy solvents. TD-DFT results for the same molecule in gas-phase and under an applied electric field are exploited to build a reliable model for the dye, rigorously validated against experiment. The model, accounting for charge transfer and local singlet and triplet states, spin-orbit coupling, conformational and vibrational degrees of freedom, sets the basis for a sound understanding of the photophysics of TADF dyes in different environments. The charge-transfer nature of the fluorescent state and of the almost degenerate phosphorescent state is unambiguously demonstrated. The concurrent role played by conformational degrees of freedom and the matrix polarizability in governing TADF is addressed

Antiadiabatic View of Fast Environmental Effects on Optical Spectra

An antiadiabatic approach is proposed to model how the refractive index of the surrounding medium affects optical spectra of molecular systems in condensed phases. The approach solves some of the issues affecting current implementations of continuum solvation models and more generally of effective models where a classical description is adopted for the molecular environment

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

Ultrabright Föster Resonance Energy Transfer Nanovesicles:The Role of Dye Diffusion

The development of contrast agents based on fluorescent nanoparticles with high brightness and stability is a key factor to improve the resolution and signal-to-noise ratio of current fluorescence imaging techniques. However, the design of bright fluorescent nanoparticles remains challenging due to fluorescence self-quenching at high concentrations. Developing bright nanoparticles showing FRET emission adds several advantages to the system, including an amplified Stokes shift, the possibility of ratiometric measurements, and of verifying the nanoparticle stability. Herein, we have developed Förster resonance energy transfer (FRET)-based nanovesicles at different dye loadings and investigated them through complementary experimental techniques, including conventional fluorescence spectroscopy and super-resolution microscopy supported by molecular dynamics calculations. We show that the optical properties can be modulated by dye loading at the nanoscopic level due to the dye's molecular diffusion in fluid-like membranes. This work shows the first proof of a FRET pair dye's dynamism in liquid-like membranes, resulting in optimized nanoprobes that are 120-fold brighter than QDot 605 and exhibit >80% FRET efficiency with vesicle-to-vesicle variations that are mostly below 10%.J.M.-F. gratefully thanks the financial support received by the 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. We acknowledge the European Commission (EC) FP7-PEOPLE-2013-Initial Training Networks (ITN) “NANO2FUN” project no. 607721 for being the spark that initiates this work and EC project MSCA-RISE-2020 "MICRO4NANO" project no.101007804. This work was also financially supported by Generalitat de Catalunya (grant no. 2017-SGR-918), the Ministry of Economy, Industry, and Competitiveness (Spain), through the “MOTHER” project (MAT2016-80826-R), the Ministry of Science and Innovation of Spain through the grant PID2019-105622RB-I00 (Mol4Bio). ICMAB-CSIC also acknowledges support from the MINECO through the Severo Ochoa Programme FUNFUTURE (SEV-2015-0496 and CEX2019-000917-S). K.D.B. acknowledges the National Science Foundation (CBET-1517273 and CHE-1726345). C.S. and A.P. benefited from the equipment and framework of the COMP-HUB Initiative, funded by the “Departments of Excellence” program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022). We thank the CESGA Supercomputing Center for technical support and the use of computational resources. The contribution of S.I.-T. has been done under the Materials Science PhD program in the Barcelona Autonomous University (UAB). Characterizations of nanovesicles were made at the ICTS “NANBIOSIS”, more specifically by the U6 unit of CIBER-BBN. The authors would like also to thank the collaboration of Hamamatsu Photonics for the quantum yield determinations using the Quantaurus-QY Plus UV–NIR absolute PL quantum yield spectrometer.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Dye-Loaded Quatsomes Exhibiting FRET as Nanoprobes for Bioimaging

Fluorescent organic nanoparticles (FONs) are emerging as an attractive alternative to the well-established fluorescent inorganic nanoparticles or small organic dyes. Their proper design allows one to obtain biocompatible probes with superior brightness and high photostability, although usually affected by low colloidal stability. Herein, we present a type of FONs with outstanding photophysical and physicochemical properties in-line with the stringent requirements for biomedical applications. These FONs are based on quatsome (QS) nanovesicles containing a pair of fluorescent carbocyanine molecules that give rise to Förster resonance energy transfer (FRET). Structural homogeneity, high brightness, photostability, and high FRET efficiency make these FONs a promising class of optical bioprobes. Loaded QSs have been used for in vitro bioimaging, demonstrating the nanovesicle membrane integrity after cell internalization, and the possibility to monitor the intracellular vesicle fate. Taken together, the proposed QSs loaded with a FRET pair constitute a promising platform for bioimaging and theranostics.J.M.F. gratefully thank the financial support received by the 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 (TECSPR17-1-0035). This work was also financially supported by the Ministry of Economy, Industry, and Competitiveness, Spain, through the “MOTHER” project (MAT2016-80826-R) and the “FLOWERS” project (FUNMAT-FIP-2016) funded by the Severo Ochoa (SEV-2015-0496) awarded to ICMAB. Instituto de Salud Carlos III, through “Acciones CIBER”, also supported this work. Characterization of nanovesicles was made at the ICTS “NANBIOSIS”, more specifically by the U6 unit of CIBER-BBN. The authors acknowledge the European Commission (EC) FP7-PEOPLE-2013-Initial Training Networks (ITN) “NANO2FUN” project no. 607721 for being the spark that initiated this work. K.B.D. acknowledges support from the National Science Foundation (CBET-1517273 and CHE-1726345). C.S. and A.P. benefited from the equipment and framework of the COMP-HUB Initiative, funded by the “Departments of Excellence” program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022).Peer reviewe