Abstract Background Biodistribution of photosensitizer (PS) in photodynamic therapy (PDT) can be assessed by fluorescence imaging that visualizes the accumulation of PS in malignant tissue prior to PDT. At the same time, excitation of the PS during an assessment of its biodistribution results in premature photobleaching and can cause toxicity to healthy tissues. Combination of PS with a separate fluorescent moiety, which can be excited apart from PS activation, provides a possibility for fluorescence imaging (FI) guided delivery of PS to cancer site, followed by PDT. Results In this work, we report nanoformulations (NFs) of core–shell polymeric nanoparticles (NPs) co-loaded with PS [2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a, HPPH] and near infrared fluorescent organic dyes (NIRFDs) that can be excited in the first or second near-infrared windows of tissue optical transparency (NIR-I, ~ 700–950 nm and NIR-II, ~ 1000–1350 nm), where HPPH does not absorb and emit. After addition to nanoparticle suspensions, PS and NIRFDs are entrapped by the nanoparticle shell of co-polymer of N-isopropylacrylamide and acrylamide [poly(NIPAM-co-AA)], while do not bind with the polystyrene (polySt) core alone. Loading of the NIRFD and PS to the NPs shell precludes aggregation of these hydrophobic molecules in water, preventing fluorescence quenching and reduction of singlet oxygen generation. Moreover, shift of the absorption of NIRFD to longer wavelengths was found to strongly reduce an efficiency of the electronic excitation energy transfer between PS and NIRFD, increasing the efficacy of PDT with PS-NIRFD combination. As a result, use of the NFs of PS and NIR-II NIRFD enables fluorescence imaging guided PDT, as it was shown by confocal microscopy and PDT of the cancer cells in vitro. In vivo studies with subcutaneously tumored mice demonstrated a possibility to image biodistribution of tumor targeted NFs both using HPPH fluorescence with conventional imaging camera sensitive in visible and NIR-I ranges (~ 400–750 nm) and imaging camera for short-wave infrared (SWIR) region (~ 1000–1700 nm), which was recently shown to be beneficial for in vivo optical imaging. Conclusions A combination of PS with fluorescence in visible and NIR-I spectral ranges and, NIR-II fluorescent dye allowed us to obtain PS nanoformulation promising for see-and-treat PDT guided with visible-NIR-SWIR fluorescence imaging.

The features of the electronic structures of a series of new styryl dye bases were investigated and characterized by their steady-state and time-resolved spectral properties, including femtosecond transient absorption spepctroscopy and DFT quantum chemical calculations. The steady-state absorption and fluorescence spectra, fluorescence quantum yield, and lifetimes in solvents of different polarity at room temperature revealed specific redistribution of the electronic density and rearrangements in molecular geometry after electronic excitation influenced by the dimethylamino end substituents. Fast relaxation processes in the electronic structures of new styryl dye bases and the nature of their time-resolved excited state absorption spectra were investigated with femtosecond temporal resolution, and the role of twisted intramolecular charge transfer (TICT) effects was shown. Quantum chemical calculations of the electronic structure of the new styryl dye bases were performed using non-empirical Time Dependent Density Functional Theory level, and were in good agreement with experimental data.

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J-aggregates are fascinating fluorescent nanomaterials formed by highly ordered assembly of organic dyes with the spectroscopic properties dramatically different from that of single or disorderly assembled dye molecules. They demonstrate very narrow red-shifted absorption and emission bands, strongly increased absorbance together with the decrease of radiative lifetime, highly polarized emission and other valuable features. The mechanisms of their electronic transitions are understood by formation of delocalized excitons already on the level of several coupled monomers. Cyanine dyes are unique in forming J-aggregates over the broad spectral range, from blue to near-IR. With the aim to inspire further developments, this review is focused on the optical characteristics of J-aggregates in connection with the dye structures and on their diverse already realized and emerging applications.

Organic materials exhibit exceptional room temperature light emitting characteristics and enormous exciton oscillator strength, however, their low charge carrier mobility prevent their use in high-performance applications such as electrically pumped lasers. In this context, ultralow threshold polariton lasers, whose operation relies on Bose-Einstein condensation of polaritons – part-light part-matter quasiparticles, are highly advantageous since the requirement for high carrier injection no longer holds. Polariton lasers have been successfully implemented using inorganic materials owing to their excellent electrical properties, however, in most cases their relatively small exciton binding energies limit their operation temperature. It has been suggested that combining organic and inorganic semiconductors in a hybrid microcavity, exploiting resonant interactions between these materials would permit to dramatically enhance optical nonlinearities and operation temperature. Here, we obtain cavity mediated hybridization of GaAs and J-aggregate excitons in the strong coupling regime under electrical injection of carriers as well as polariton lasing up to 200 K under non-resonant optical pumping. Our demonstration paves the way towards realization of hybrid organic-inorganic microcavities which utilise the organic component for sustaining high temperature polariton condensation and efficient electrical injection through inorganic structure.

G.G. Paschos1,2*, N. Somaschi1,3, S.I. Tsintzos1, D. Coles4, J.L. Bricks5, Z. Hatzopoulos1, D.G. Lidzey4, P.G. 2 Lagoudakis3,6, P.G. Savvidis1,2,7† 3 1FORTH, Institute of Electronic Structure and Laser, 71110 Heraklion, Crete, Greece 4 2Department of Materials Science and Technology, University of Crete 71003 Heraklion, Crete, Greece 5 3Department of Physics and Astronomy, University of Southampton, United Kingdom 6 4Department of Physics and Astronomy, University of Sheffield, United Kingdom 7 5Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanskayaul. 5, Kiev 02094, 8 Ukraine 9 6Skolkovo Institute of Science and Technology Novaya St., 100, Skolkovo 143025, Russian Federation 10 7ITMO University, St. Petersburg 197101, Russian Federation 11 12 *gianpaschos@materials.uoc.gr, psav@materials.uoc.gr 13

A broadband upconverted emission combined with a broadband infrared absorption takes place in films composed of the same infrared dye molecules.

The article summarizes a number of quantum-chemical and spectral studies of the electron structure and the nature of electron transitions in neutral cyanine bases, compared to their parent cationic cyanines. The authors consider series of symmetrical indocyanines, unsymmetrical pyrido-thiacyanines and thia-styryls with donor and acceptor substituents, all of them with corresponding bases. A difference in the electron structure and spectral properties between the bases and their parents is established despite their both having closed pi-electron shells. Quantum-chemical calculations show that the bond lengths in the polymethine chains of bases are alternated similarly to polyenes, whereas the bond lengths in cationic cyanines are equalized, even in unsymmetrical dyes. The characteristic alternation of atomic charges in cationic cyanines decreases upon going to cyanine bases. Therefore, the latter are typical donor-acceptor linear conjugated systems. Going from the cationic dye to the base, electron levels are notably shifted and the energy gap is increased. The long-wavelength band in the absorption spectrum shifts hypsochromically, becomes wider and more nuanced; this effect is caused by the vibrational transitions, but not by the second electron transition. A specific n-MO also appears at the two-coordinated atom in the acceptor residue of the base, along with a n -> pi* electron transition, not present in cations. The calculated properties of cyanine bases are in a good agreement with the experimental NMR and absorption spectrum data. Overall, both quantum-chemical and spectral studies show that the observed features of cyanine bases are largely determined by the electronic and geometrical structure of these compounds as polyenic conjugated molecules.

Combined quantum-chemical and spectral investigation of cyanine bases derivatives of thiastyryls as well as their analogues with dimethylamino, metoxy and trifluorine-methyl substituents has been fulfilled. The calculations have shown that going from cationic styryl/metoxystyryl to the corresponding neutral bases is accompanied by substantial change of the equilibrium molecular geometry and charge distribution at atoms, while the experimental absorption band undergoes the essential hypsochromic shift. It is established that introducing on the donor groups in the bases causes negligible change of the carbon–carbon bond and atomic charges in the main chromophore, in contrast to the substantial changes of the magnitude and direction state dipole moments in both ground and excited states. It is found that the bases with the donor groups in benzthiazole moiety and with acceptor CF3 substituent demonstrate the inversion of the direction of the dipole moment. Based on the spectral and quantum-chemical study, one has proposed that some widening of the spectral bands is connected with the vibronic interaction, not with the second electron transition.

The steady-state and time-resolved linear spectral properties, two-photon absorption spectra and fast relaxation processes in the excited states of styryl base-type derivatives were investigated. The nature of linear absorption, fluorescence and excitation anisotropy spectra were analyzed in solvents of different polarity at room temperature and specific dependence of the solvatochromic behavior on the donor-acceptor strength of the terminal substituents was shown. Two-photon absorption (2PA) efficiency of styryl dye bases was determined in a broad spectral range using two-photon induced fluorescence technique, and cross-sections maxima of ~ 100 GM were found. The excited state absorption (ESA) and fast relaxation processes in the molecular structures were investigated by transient absorption femtosecond pump-probe methodology. The role of twisted intramolecular charge transfer (TICT) effect in the excited state of styryl dye base with dimethylamino substituent was shown. The experimental spectroscopic data were also verified by quantum chemical calculations at the Time Dependent Density Functional Theory level, combined with a polarizable continuum model.

The recent advances in molecular design of polymethine dyes absorbing and emitting light in the near-infrared region are reviewed. In contrast to many publications, which concern mostly experimental results, our work describes mainly the theoretical approaches developed to connect the spectral characteristics with the chemical constitution of polymethine dyes. Considering such approaches, molecular design can be based on modern theoretical concepts. This enables the syntheses of new perspective dye molecules with specified spectral properties.

Thin films of new sulphur-terminated organic compounds were deposited by evaporation in vacuum onto the glass, silicone, gold and polytetrafluoroethylene substrates.The influence of compound chemical structure and substrate type on morphology of the condensed solid was studied using atomic force microscopy.The significant difference in morphology of the same dye but deposited on various substrates was found.

A comprehensive investigation of the electronic structure and fast relaxation processes in the excited states of new styryl base-type derivatives was performed using steady-state, pico-, and femtosecond time-resolved spectroscopic techniques. Linear photophysical parameters of new compounds, including steady-state absorption, fluorescence, and excitation anisotropy spectra, were obtained in a number of organic solvents at room temperature. A detailed analysis of the fluorescence lifetimes and ultrafast relaxation processes in the electronically excited state of the styryl bases revealed an important role of solvate dynamics and donor-acceptor strength of the molecular structures in the formation of their excited state absorption spectra. Experimental data were in good agreement with quantum chemical calculations at the time dependent density functional theory level, combined with a polarizable continuum model.

Conjugated donor–acceptor compounds are investigated by electronic absorption and Raman spectroscopy with and without the electric field of a nonconjugated positive charge. The shifts observed can be rationalized by considering these compounds as unsymmetrical cyanine dyes. The cyanine model allows to rationalize the observation that the vibrational and optical absorption energy are linked to each other. This model can also be applied to the visual chromophore inside the protein pocket which is placed in the unsymmetric electrostatic field of the surrounding charged amino acids.

A comprehensive quantum chemical analysis along with spectral-luminescence measurements has been performed for a new series of 2-azaazulene dyes with different conjugation lengths (n) to better understand the nature of their electronic transitions. The remarkably large red shift of their main absorption bands at relatively small n is connected with the existence of totally delocalized HOMO and LUMO. Symmetry breaking is observed experimentally at n = 3 in polar solvents and theoretically at n = 5 in vacuum. Analysis shows the existence of two types of molecular orbitals (MOs): local with the charge mainly localized within the terminal groups, and delocalized with the charge distributed throughout the molecule. Correspondingly, three types of electronic transitions are present: between delocalized MOs; between one local and one delocalized MO, and between local MOs only, which is important for predicting the positions of electronic transitions to the chain length.

Polymethine dyes (PDs) with absorption bands in the near-infrared region undergo symmetry breaking in polar solvents. To investigate how symmetry breaking affects nonlinear optical responses of PDs, an extensive and challenging experimental characterization of a cationic 2-azaazulene polymethine dye, including linear absorption, fluorescence, two-photon absorption and excited-state absorption, has been performed in two solvents with different polarity. Based on this extensive set of experimental data, a three-electronic-state model, accounting for the coupling of electronic degrees of freedom to molecular vibrations and polar solvation, has been reliably parameterized and validated for this dye, fully rationalizing optical spectra in terms of spectral position, intensities and bandshapes. In low-polarity solvents where the dye is mainly in its symmetric form, a nominally forbidden two-photon absorption band is observed, due to a vibronic activation mechanism. Inhomogeneous broadening plays a major role in polar solvents: absorption spectra represent the weighted sum of contributions from states with a variable amount of symmetry breaking, leading to a complex evolution of linear and nonlinear optical spectra with solvent polarity. In more polar solvents, the dominant role of the asymmetric form leads to the activation of two-photon absorption as a result of the symmetry lowering. The subtle interplay between the two mechanisms for two-photon absorption activation, vibronic coupling and polar solvation, can be fully accounted for within the proposed microscopic model allowing a detailed interpretation of the optical spectra of PDs.

This paper presents the results of a quantum-chemical study of the molecular geometry and electron structure as well as spectral measurements of absorption and 13C NMR spectra of dimethylaminostyryls, methoxystyryls, and methylstyryls bearing 2-azaazlenium (AA) moiety in comparison with analogous dyes containing 2-benzimidazolim and 4-pyrylium residues. Based on the analysis of both calculations and experimental data, it was concluded that these types of extremely unsymmetrical cyanines differ between each other only slightly in the ground state with respect to the charge distribution and molecular geometry while their spectral properties reveal a considerable difference between dimethylaminostyryls and methoxystyryls due to the lowest disposition of the lone electron pair level of the oxygen atom and hence, limiting basicity of the p-methoxyphenylene residue. It was shown that appearance of a high-positioned local level generated by AA terminal group caused the inversion of the delocalized and quasi-local electron transitions in AA-methoxystyryl and hence drastic changes in its absorption spectrum.

A systematic investigation of the conformational structure was performed for the series of symmetrical and unsymmetrical mono-, tri-, pentamethine cyanines, and styryl dyes bearing 2-azaazulenium terminal group. The rotation energy barriers of terminal groups were determined via the dynamic variable temperature NMR experiments. The conformational transformation energy was calculated by quantum chemical methods (B3LYP and M05-2X) both for the cases of considering the solvent influence and not tacking it into account. Based on the comparison of theoretical and experimental data, relative electron-donating abilities and geometrical features of the heterocyclic terminal groups in 2-azaazulenium dyes were estimated. The arrangement of certain heterocyclic nuclei in order of basicity by considering the results of the dynamic NMR investigations was proposed. Influence of the conjugated chain length and the solvent nature on the conformational lability of the investigated dye molecules was discussed.

NIR polymethines are studied in two solvents of dissimilar polarity to investigate predictions of 2PA enhancements from ground-state symmetry-breaking (SB). However, electron-vibrational coupling appears to predominate SB.

We report here the synthesis of a series of symmetrical and unsymmetrical trimethine cyanine dyes derived from 2-azaazulene, combined spectral and quantum-chemical investigations of their molecular geometry and electron structure, as well as the nature of the lowest electron transitions. Based on the analysis of both calculations and experimental data obtained from absorption and (13)C NMR spectra, it was concluded that the 2-azaazulene residue can be treated as a weakly basic terminal group; its donor properties are provided with the participation of the HOMO-1, in contrast to the typical Brooker's terminal residues with their donor HOMOs. The new classification of the terminal groups of cyanine dyes, and hence the classification of types of unsymmetrical cyanines, is proposed. It is shown that the nature of the higher electron transitions (delocalized or local) in the cyanine dyes depends on their type. In the unsymmetrical trimethine cyanine of the mixed type, negative deviations are observed in their absorption spectra.