Amplified Two‐Photon Absorption in Trans‐A2B2‐Porphyrins Bearing Nitrophenylethynyl Substituents

We show that peripheral nitro groups enhance the maximum two‐photon absorption cross‐section of trans‐A₂B₂‐porphyrins bearing two phenylethynyl substituents by more than one order of magnitude. Maximum values as high as 1000 GM result from realization of suitable conditions for effective resonance enhancement along with a lowering of the energy and intensification of the two‐photon allowed transitions in the Soret region.     

Porphyrin‐Ryleneimide Hybrids: Low‐Bandgap Acceptors in Energy‐Transfer Cassettes

Energy‐transfer cassettes consisting of naphthaleneimide‐fused metalloporphyrin acceptors (M=Zn and Pd) and BODIPY donors have been designed and synthesized. These systems have rigid pseudo‐tetrahedral structures with a donor‐acceptor separation of ca. 17.5 Å. Spectroscopic investigations, including femtosecond transient absorption measurements, showed efficient excitation energy transfer (EET) occurring according to the Förster mechanism. Strong fluorescence of the donor units and significant spectral overlap of the donor and acceptor subunits are prerequisites for the efficient EET in these systems.     

On the reactivity of isoindolo[2,1-a]quinazoline-5-ones

Base- and acid-catalyzed nucleophilic addition of 11H-isoindolo[2,1-a]quinazoline-5-one to aromatic aldehydes and maleimides was investigated. The aldol adducts and condensation products were obtained stereoselectively. Main diastereomers of the Michael adducts were isolated in 74-89% yield, and converted by N-methylation to new stable α-substituted isoindole derivatives, for which 6-methylisoindolo[2,1-a]]quinazoline-5-one stands as the unsubstituted reference. The stability of the latter was monitored in moist aerated CDCl₃ solution, and one of the oxidative hydrolysis product was characterized by X-ray diffraction analysis as the corresponding N-arylphthalimide. The reactivity of the unsubstituted 6-methylisoindolo[2,1-a]]quinazoline-5-one was also investigated with acetylenic Michael acceptors. Fully conjugated isoindole derivatives possessing an original pull-push-pull structure were obtained. The conformations and molecular orbitals of the dibenzoylacetylene adduct were studied at the DFT level of theory. Its static quadratic hyperpolarizabilty β₀ was also calculated at the ZINDO level.     

The Role of Ion Pairs in the Second‐Order NLO Response of 4‐X‐1‐Methylpiridinium Salts

A series of 4‐X‐1‐methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)‐CH?CHC₆H₅; (E)‐CH?CHC₆H₄‐4′‐C(CH₃)₃; (E)‐CH?CHC₆H₄‐4′‐N(CH₃)₂; (E)‐CH?CHC₆H₄‐4′‐N(C₄H₉)₂; (E,E)‐(CH?CH)₂C₆H₄‐4′‐N(CH₃)₂) with various organic (CF₃SO₃^?, p‐CH₃C₆H₄SO₃^?), inorganic (I^?, ClO₄^?, SCN^?, [Hg₂I₆]^2?) and organometallic (cis‐[Ir(CO)₂I₂]^?) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second‐order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second‐order NLO response, investigated by the electric‐field‐induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10^?3 M , and their dipole moments were determined. Generally, below 5×10^?4 M , ion pairs start to dissociate into ions with parallel increase of the second‐order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10^?3 M , some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second‐order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self‐aggregate.     

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

3′,5′‐Dimethoxybenzoin (DMB) is a bichromophoric system that has widespread application as a highly efficient photoremovable protecting group (PRPG) for the release of diverse functional groups. The photodeprotection of DMB phototriggers is remarkably clean, and is accompanied by the formation of a biologically benign cyclization product, 3′,5′‐dimethoxybenzofuran (DMBF). The underlying mechanism of the DMB deprotection and cyclization has, however, until now remained unclear. Femtosecond transient absorption (fs‐TA) spectroscopy and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopy were employed to detect the transient species directly, and examine the dynamic transformations involved in the primary photoreactions for DMB diethyl phosphate (DMBDP) in acetonitrile (CH₃CN). To assess the electronic character and the role played by the individual sub‐chromophore, that is, the benzoyl, and the di‐meta‐methoxybenzylic moieties, for the DMBDP deprotection, comparative fs‐TA measurements were also carried out for the reference compounds diethyl phosphate acetophenone (DPAP), and 3′,5′‐dimethoxybenzylic diethyl phosphate (DMBnDP) in the same solvent. Comparison of the fs‐TA spectra reveals that the photoexcited DMBDP exhibits distinctly different spectral character and dynamic evolution from those of the reference compounds. This fact, combined with the related steady‐state spectral and density functional theoretical results, strongly suggests the presence in DMBDP of a significant interaction between the two sub‐chromophores, and that this interaction plays a governing role in determining the nature of the photoexcitation and the reaction channel of the subsequent photophysical and photochemical transformations. The ns‐TR³ results and their correlation with the fs‐TA spectra and dynamics provide evidence for a novel concerted deprotection–cyclization mechanism for DMBDP in CH₃CN. By monitoring the direct generation of the transient DMBF product, the cyclization time constant was determined unequivocally to be ≈1 ns. This indicates that there is little relevance for the long‐lived intermediates (>10 ns) in giving the DMBF product, and excludes the stepwise mechanism proposed in the literature as the major pathway for the DMB cyclization reaction. This work provides important new insights into the origin of the 3′,5′‐dimethoxy substitution effect for the DMB photodeprotection. It also helps to clarify the many different views presented in previous mechanistic studies of the DMB PRPGs. In addition to this, our fs‐TA results on the reference compound DMBnDP in CH₃CN provide the first direct observation (to the best of our knowledge) showing the predominance of a prompt (≈2 ps) heterolytic bond cleavage after photoexcitation of meta‐methoxybenzylic compounds. This provides insight into the long‐term controversies about the photoinitiated dissociation mode of related substituted benzylic compounds.     

The Role of Hydrogen Bonds and Electrostatic Interactions in Enhancing Two-Photon Absorption in Green and Yellow Fluorescent Proteins

The spectral properties of fluorescent proteins (FPs) depend on the protein environment of the chromophore (CRO). A deeper understanding of the CRO – environment interactions in terms of FPs’ spectral characteristics will allow for a rational design of novel markers with desired properties. Here, we are taking a step towards achieving this important goal. With the time-dependent density functional theory (TDDFT), we calculate one- and two-photon absorption (OPA and TPA) spectra for 5 green FPs (GFPs) and 3 yellow FPs (YFPs) differing in amino acid sequence. The goal is to reveal the roles of: (i) electrostatic interactions, (ii) hydrogen-bonds (h-bonds) and (iii) h-bonds together with distant electrostatic field in absorption spectra tuning. Our results point to design hypothesis towards FPs optimised for TPA-based applications. Both h-bonds and electrostatic interactions co-operate in enhancing TPA cross-section ( ) for the transition in GFPs. Furthermore, it seems that details of h-bonds network in the CRO's vicinity influences response to CRO – environment electrostatic interactions in YFPs. We postulate that engineering FPs with more hydrophilic CRO's environment can lead to greater . We also find that removing h-bonds formed with the CRO's phenolate leads to TPA enhancement for transition to higher excited states than S₁. Particularly Y145 and T203 residues are important in this regard.     

Deciphering the Multifarious Charge-Transport Behaviour of Crystalline Propeller-Shaped Triphenylamine Analogues

A collection of para-substituted propeller-shaped triphenylamine (TPA) derivatives have been computationally investigated for charge-transport characteristics exhibited by the derivatives by using the Marcus–Hush formalism. The various substituents chosen herein, with features that range from electron withdrawing to electron donating in nature, play a key role in defining the reorganisation energy and electronic coupling properties of the TPA derivatives. The TPA moiety is expected to possess weak electronic coupling on the basis of poor orbital overlap upon aggregation, owing to the restriction imposed by the propeller shape of the TPA core. However, the substituent groups attached to the TPA core can significantly dictate the crystal-packing motif of the TPA derivatives, wherein the variety of noncovalent intermolecular interactions subsequently generated drive the packing arrangement and influence electronic coupling between the neighbouring orbitals. Intermolecular interactions in the crystalline architecture of TPA derivatives were probed by using Hirshfeld and quantum theory of atoms-in-molecules techniques. Furthermore, symmetry-adapted perturbation theory analysis of the TPA analogues has revealed that a periodic arrangement of energetically stable dimers with significant electronic coupling is essential to contribute high charge-carrier mobility to the overall crystal.     

Resolving Electronic Transitions in Synthetic Fluorescent Protein Chromophores by Magnetic Circular Dichroism

The detailed electronic structures of fluorescent chromophores are important for their use in imaging of living cells. A series of green fluorescent protein chromophore derivatives is examined by magnetic circular dichroism (MCD) spectroscopy, which allows the resolution of more bands than plain absorption and fluorescence. Observed spectral patterns are rationalized with the aid of time‐dependent density functional theory (TDDFT) computations and the sum‐over‐state (SOS) formalism, which also reveals a significant dependence of MCD intensities on chromophore conformation. The combination of organic and theoretical chemistry with spectroscopic techniques also appears useful in the rational design of fluorescence labels and understanding of the chromophore's properties. For example, the absorption threshold can be heavily affected by substitution on the phenyl ring but not much on the five‐member ring, and methoxy groups can be used to further tune the electronic levels.     

Intramolecular Charge Transfer and Solvation of Photoactive Molecules with Conjugated Push–Pull Structures

A comparative investigation on the photophysical properties and solvation‐related ICT dynamics of three push–pull compounds containing different donors including carbazole, triphenylamine and phenothiazine, was performed. The steady‐state spectra and theoretical calculations show the charge transfers from the central donors to the acceptors at each side. The characterization of the extent of charge transfer was determined by various means, including estimation of the dipole moment, the electron density distribution of HOMO and LUMO, CDD and change in Gibb's free energy, which show the charge transfer strength to be in the order PDHP > BDHT > PDHC. This suggests that the electron‐donating ability of the donor groups plays a crucial role in the charge transfer in these compounds. The TA data show the excited‐state relaxation dynamics follow a sequential model: FC→ICT→ICT′→S₀, and are affected by the solvent polarity. The results presented here demonstrate that the compound with a higher degree of ICT characteristic interacts more strongly with stronger polar solvent molecules, which can accelerate the solvation and spectral evolution to lower energy levels. The A–π‐D –π‐A architectures with prominent ICT characteristics based on carbazole, triphenylamine and phenothiazine might be potential scaffolds for light‐harvesting and photovoltaic devices. These results are of value for understanding structure–property relationships and the rational design of functional materials for photoelectric applications.