Dual and Panchromatic Emission from N-Aryl-phenanthridinones: Extension of the Seesaw Photophysical Model with a Slight Twist

White-light emission from a single organic molecule, known as a single white-light emitter, is a rare phenomenon and desirable property for a class of materials with potential future applications in white lighting. Since N-aryl-naphthalimides (NANs) have been shown to follow excited state behavior and unique dual or panchromatic emission through a substituent pattern prescribed via a seesaw photophysical model, this study investigates the substituent effects on the fluorescence emission of structurally related N-aryl-phenanthridinones (NAPs) dyes. Following a similar placement prescription of an electron-releasing group (ERG) and electron-withdrawing group (EWG) at the phenanthridinone core and N-aryl moiety, we discovered from time-dependent density functional theory (TD-DFT) results that NAPs show a substitution pattern opposite to NANs in order to promote S2 and higher excited states. Interestingly, 2-methoxy-5-[4-nitro-3(trifluoromethyl)phenyl]phenanthridin-6(5H)-one 6e displayed a pronounced dual and panchromatic fluorescence dye depending on the solvent. For the six dyes included in the study, full spectral information in a variety of solvents, as well as fluorescence quantum yield and lifetime are reported. TD-DFT calculations support the predicted optical behavior via mixing of S2 and S6 excited states via anti-Kasha type of emission behavior.

Inter-Network Charge-Transfer Excited State Formation Within a Two-fold Catenated Metal-Organic Framework

Charge-transfer excited state (CTES) defines the ability to split photon energy into work producing redox equivalents suitable for photocatalysis. Here, we report inter-net CTES formation within a two-fold catenated crystalline metal-organic framework (MOF), constructed with two linkers, N,N'-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) and 2,6-dicarboxynaphthalene (NDC). The structural flexibility puts two complementary linkers from two nets in a proximal position to interact strongly. Supported by the electrochemical and steady-state electronic spectroscopic data, this ground-state interaction facilitates forming CTES that can be populated by direct excitation. We map the dynamics of the CTES which persists over a few nanoseconds and highlight the utilities of such relatively long-lived CTES as enhanced conductivity of the MOF under light over that measured in dark and as a proof-of-the-principle test, photo-reduction of methyl viologen under white light.

Boosting Sulfides Photooxidation by Fusing Naphthalimide and Flavin together

Efficient and selective photocatalytic conversion of chemicals with visible light and naturally abundant resources has long been desired, but this requires finely designed sensitizers that are capable to convert light...

An efficient implementation of spin-orbit coupling within the framework of semiempirical orthogonalization-corrected methods for ultrafast intersystem crossing dynamics

We implement spin-orbit coupling (SOC) within the framework of semiempirical orthogonalization-corrected methods (OMx). The excited-state wavefunction is generated from configuration interaction with single excitations (CIS). The SOC Hamiltonian in terms of the one-electron Breit-Pauli operator with effective nuclear charges is adopted in this work. Benchmark calculations show that SOCs evaluated using the OMx/CIS method agree very well with those obtained from time-dependent density functional theory. As a particularly attractive application, we incorporate SOCs between singlet and triplet states into Tully's fewest switches surface hopping algorithm to enable excited-state nonadiabatic dynamics simulations, treating internal conversion and intersystem crossing on an equal footing. This semiempirical dynamics simulation approach is applied to investigate ultrafast intersystem crossing processes in core-substituted naphthalenediimides.

Photochemical Reactivity of Naphthol-Naphthalimide Conjugates and Their Biological Activity

Quinone methide precursors 1a–e, with different alkyl linkers between the naphthol and the naphthalimide chromophore, were synthesized. Their photophysical properties and photochemical reactivity were investigated and connected with biological activity. Upon excitation of the naphthol, Förster resonance energy transfer (FRET) to the naphthalimide takes place and the quantum yields of fluorescence are low (Φ_F ≈ 10⁻²). Due to FRET, photodehydration of naphthols to QMs takes place inefficiently (Φ_R ≈ 10⁻⁵). However, the formation of QMs can also be initiated upon excitation of naphthalimide, the lower energy chromophore, in a process that involves photoinduced electron transfer (PET) from the naphthol to the naphthalimide. Fluorescence titrations revealed that 1a and 1e form complexes with ct-DNA with moderate association constants K_a ≈ 10⁵–10⁶ M⁻¹, as well as with bovine serum albumin (BSA) K_a ≈ 10⁵ M⁻¹ (1:1 complex). The irradiation of the complex 1e@BSA resulted in the alkylation of the protein, probably via QM. The antiproliferative activity of 1a–e against two human cancer cell lines (H460 and MCF 7) was investigated with the cells kept in the dark or irradiated at 350 nm, whereupon cytotoxicity increased, particularly for 1e (>100 times). Although the enhancement of this activity upon UV irradiation has no imminent therapeutic application, the results presented have importance in the rational design of new generations of anticancer phototherapeutics that absorb visible light.

Intersystem Crossing and Electron Spin Selectivity in Anthracene-Naphthalimide Compact Electron Donor-Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (Φ_Δ =92 %, τ_T =438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (¹ CT→S₀ , takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (Φ_Δ =40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the ³ An and ³ NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet-triplet annihilation upconversion (TTA UC). High UC quantum yield (Φ_UC =12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.

Long-lived triplet charge-separated state in naphthalenediimide based donor-acceptor systems

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor-acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π-π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet-triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor-acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Persistent Organic Room-Temperature Phosphorescence in Cyclohexane-trans-1,2-Bisphthalimide Derivatives: The Dramatic Impact of Heterochiral vs Homochiral interactions

Persistent metal-free room-temperature phosphorescence (RTP) materials attract significant interest owing to the production of long-lived triplet excited states. Although several organic designs show RTP, the impact of intermolecular interactions on the triplet excitons stabilization and migrations remains hardly understood because obtaining different ordered intermolecular interactions while conserving identical molecular electronic properties is very challenging. We propose here a new strategy to circumvent this problem by taking advantage of the distinct molecular packing that can be found between enantiomer and racemic forms of a chiral molecule. Structural, photophysical, and chiroptical investigations of chiral cyclohexane bisphthalimide derivatives showed that heterochiral and homochiral dimer interactions play a crucial role on the triplet excited state stabilization, resulting in higher RTP efficiency for enantiopure systems than for racemic one. This study paves the way to the use of molecular chirality to rationalize supramolecular properties arising from subtle intermolecular interactions.

Base-Assisted Imidization: A Synthetic Method for the Introduction of Bulky Imide Substituents to Control Packing and Optical Properties of Naphthalene and Perylene Imides

We report the direct imidization of naphthalene and perylene dicarboxylic anhydrides/esters with bulky ortho,ortho‐diaryl‐ and ortho,ortho‐dialkynylaniline derivatives. This imidization method uses n‐butyllithium as a strong base to increase the reactivity of bulky amine derivatives, proceeds under mild reaction conditions, requires only stoichiometric amounts of reactants and gives straightforward access to new sterically crowded rylene dicarboximides. Mechanistic investigations suggest an isoimide as intermediary product, which was converted to the corresponding imide upon addition of an aqueous base. Single‐crystal X‐ray diffraction analyses reveal dimeric packing motifs for monoimides, while two‐side shielded bisimides crystallize in isolated molecules without close π–π‐interactions. Spectroscopic investigations disclose the influence of the bulky substituents on the optical properties in the solid state.

Continuous-Flow Photochemical Transformations of 1,4-Naphthoquinones and Phthalimides in a Concentrating Solar Trough Reactor

A series of photochemical transformations has been successfully conducted under continuous-flow conditions in a concentrating solar trough reactor. Photoacylations and [2+2]-photocycloadditions involving 1,4-naphthoquinones gave the corresponding photoproducts in moderate to high yields with residence times of 70min. Likewise, acetone-sensitized photodecarboxylations involving phthalimides furnished the corresponding benzylated hydroxy phthalimidines in good to excellent yields and purity with residence times of 40min. Compared with corresponding exposures to direct sunlight conducted in a solar float, flow operation generally gave superior conversions and subsequent yields.

Photophysical properties and electron transfer photochemical reactivity of substituted phthalimides

Substituents on phthalimide affect its photophysics and photochemical reactivity. Electron donors generally result in low quantum yields of intersystem crossing and reactivity from singlet excited states.

Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes

The interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron-donor and electron-acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers. To this aim, the photoinduced electron transfer processes involving a 1,8-naphthalimide chromophore, embedded in several rotaxane-based dyads, were investigated by steady-state and time-resolved absorption and luminescence spectroscopic experiments in the 300 fs-10 ns time window. Different rotaxanes built around the dialkylammonium/ dibenzo[24]crown-8 ether supramolecular motif were designed and synthesized to decipher the relevance of key structural factors, such as the chemical deactivation of the ammonium-crown ether recognition, the presence of a secondary site for the ring along the axle, and the covalent functionalization of the macrocycle with a phenothiazine electron donor. Indeed, the conformational freedom of these compounds gives rise to a rich dynamic behavior induced by light and may provide opportunities for investigating and understanding phenomena that take place in complex (bio)molecular architectures.

On the large apparent Stokes shift of phthalimides

The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy. In all solvents the compound exhibits an unusually large Stokes shift of ∼10 000 cm-1. It is attributed to an ultrafast (<100 fs) depletion of the initially excited state, which results in the population of a weakly emitting state. Quantum chemical computations (DFT-MRCI) support this. They identify two energetically low-lying singlet ππ* excitations of different oscillator strength. Whereas the Stokes shift and thereby the ultrafast depletion of the initial excitation are hardly affected by the solvent later processes respond strongly. The fluorescence lifetime varies from ∼10 ps (cyclohexane) to ∼3 ns (water). This is attributed to a varying energetic accessibility of nπ* excitations.

Strong ion pair charge transfer interaction of 1,8-naphthalimide–bipyridinium conjugates with basic anions – towards the development of a new type of turn-on fluorescent anion sensors

Fluoride, acetate and benzoate anions reduce naphthalimide–viologen dyads into radicals, which is indicated by a strong fluorescence enhancement.

Photocyclization of Tetra- and Pentapeptides Containing Adamantylphthalimide and Phenylalanines: Reaction Efficiency and Diastereoselectivity

A series of tetrapeptides and pentapeptides was synthesized bearing a phthalimide chromophore at the N-terminus. The C-terminus of the peptides was strategically substituted with an amino acid, Phe, Phe(OMe), or Phe(OMe)₂ characterized by different oxidation potentials. The photochemical reactivity of the peptides was investigated by preparative irradiation and isolation of photoproducts, as well as with laser flash photolysis. Upon photoexcitation, the peptides undergo photoinduced electron transfer (PET) and decarboxylation, followed by diastereoselective cyclization with the retention of configuration for tetrapeptides or inversion of configuration for pentapeptides. Molecular dynamics (MD) simulations and NOE experiments enabled assignment of the stereochemistry of the cyclic peptides. MD simulations of the linear peptides disclosed conformational reasons for the observed diastereoselectivity, being due to the peptide backbone spatial orientation imposed by the Phe amino acids. The photochemical efficiency for the decarboxylation and cyclization is not dependent on the peptide length, but it depends on the oxidation potential of the amino acid at the C-terminus. The results described herein are particularly important for the rational design of efficient photochemical reactions for the preparation of cyclic peptides with the desired selectivity.

On the theoretical prediction of fluorescence rates from first principles using the path integral approach

In this work, we present and implement the theory for calculating fluorescence rates and absorption and emission spectra from first principles, using the path integral approach. We discuss some approximations and modifications to the full set of equations that improve speed and numerical stability for the case when a large number of modes are considered. New methods to approximate the excited state potential energy surface are also discussed and it is shown that for most purposes, these can be used instead of a full geometry optimization to obtain the rates mentioned above. A few examples are presented and the overall performance of the method is discussed. It is shown that the rates and spectra computed in this way are well within the acceptable range of errors and can be used in future predictions, particularly for screening purposes, with the only limitation on size being that of the electronic structure calculation itself.

Recent Advances in Photodecarboxylations Involving Phthalimides

Owing to their favourable photophysical and electrochemical properties, phthalimides undergo a variety of highly efficient photodecarboxylation reactions. These transformations have been applied to the synthesis of macrocyclic compounds as well as bioactive addition adducts. N-Acetoxyphthalimides are versatile precursors to imidyl and alkyl radicals through photodecarboxylation and have subsequently been used for a variety of coupling reactions. The generally mild reaction conditions make these reactions attractive for green chemical applications. The process protocols were successfully transferred to novel photoreactor devices, among these falling film or continuous flow reactors.

Experimental evidence of TICT state in 4-piperidinyl-1,8-naphthalimide – a kinetic and mechanistic study

The excited state processes in N-propyl-4-piperidinyl-1,8-naphthalimide have been studied by measuring its fluorescence spectra and decay curves in solvents of different polarity and viscosity and also in a frozen solvent glass.

Transient two-dimensional vibrational spectroscopy of an operating molecular machine

Synthetic molecular machines are promising building blocks for future nanoscopic devices. However, the details of their mechanical behaviour are in many cases still largely unknown. A deeper understanding of mechanics at the molecular level is essential for the design and construction of complex nanodevices. Here, we show that transient two-dimensional infrared (T2DIR) spectroscopy makes it possible to monitor the conformational changes of a translational molecular machine during its operation. Translation of a macrocyclic ring from one station to another on a molecular thread is initiated by a UV pulse. The arrival of the shuttling macrocycle at the final station is visible from a newly appearing cross peak between these two moieties. To eliminate spectral congestion in the T2DIR spectra, we use a subtraction method applicable to many other complex molecular systems. The T2DIR spectra indicate that the macrocycle adopts a boat-like conformation at the final station, which contrasts with the chair-like conformation at the initial station.

A deeper understanding of the mechanics of molecular machines is limited by the fast motions which are in the nanosecond or picosecond timescale. Here the authors present a real-time observation of structural changes in a rotaxane-based molecular shuttle by transient two-dimensional infrared spectroscopy.

Thermodynamic Insights on a Bistable Acid-Base Switchable Molecular Shuttle with Strongly Shifted Co-conformational Equilibria

Bistable [2]rotaxanes in which the affinities of the two stations can be reversed form the basis of molecular shuttles. Gaining quantitative information on such rotaxanes in which the ring distribution between the two stations is largely nonsymmetric has proven to be very challenging. Herein, we report on two independent experimental methodologies, based on luminescence lifetime measurements and acid-base titrations, to determine the relative populations of the two co-conformations of a [2]rotaxane. The assays yield convergent results and are sensitive enough to measure an equilibrium constant (K≈4000) out of reach for NMR spectroscopy. We also estimate the ring distribution constant in the switched (deprotonated) state (K'<10^-4 ), and report the highest positional efficiency for stimuli-induced shuttling to date (>99.92 %). Finally, our results show that the pK_a of the pH-responsive station depends on the ring affinity of the pH-insensitive station, an observation that paves the way for the design of new artificial allosteric systems.

Studies in organic and physical photochemistry - an interdisciplinary approach

Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical chemistry. The aim of this review is to illustrate this very fruitful interdisciplinary approach and cooperation. A profound understanding of the photochemical reactivity and reaction mechanisms is particularly helpful for optimization and application of these reactions. Some typical reactions and particular aspects are reported such as the Norrish-Type II reaction and the Yang cyclization and related transformations, the [2 + 2] photocycloadditions, particularly the Paternò-Büchi reaction, photochemical electron transfer induced transformations, different kinds of catalytic reactions such as photoredox catalysis for organic synthesis and photooxygenation are discussed. Particular aspects such as the structure and reactivity of aryl cations, photochemical reactions in the crystalline state, chiral memory, different mechanisms of hydrogen transfer in photochemical reactions or fundamental aspects of stereoselectivity are discussed. Photochemical reactions are also investigated in the context of chemical engineering. Particularly, continuous flow reactors are of interest. Novel reactor systems are developed and modeling of photochemical transformations and different reactors play a key role in such studies. This research domain builds a bridge between fundamental studies of organic photochemical reactions and their industrial application.

Two-Dimensional Perovskite Activation with an Organic Luminophore

A great advantage of the hybrid organic-inorganic perovskites is the chemical flexibility and the possibility of a molecular engineering of each part of the material (the inorganic part and the organic part respectively) in order to improve or add some functionalities. An adequately chosen organic luminophore has been introduced inside a lead bromide type organic-inorganic perovskite, while respecting the two-dimensional perovskite structure. A substantial increase of the brilliance of the perovskite is obtained. This activation of the perovskite luminescence by the adequate engineering of the organic part is an original approach, and is particularly interesting in the framework of the light-emitting devices such as organic light-emitting diodes (OLEDs) or lasers.

Benchmarking the Bethe-Salpeter Formalism on a Standard Organic Molecular Set

We perform benchmark calculations of the Bethe-Salpeter vertical excitation energies for the set of 28 molecules constituting the well-known Thiel’s set, complemented by a series of small molecules representative of the dye chemistry field. We show that Bethe-Salpeter calculations based on a molecular orbital energy spectrum obtained with non-self-consistent G0W0 calculations starting from semilocal DFT functionals dramatically underestimate the transition energies. Starting from the popular PBE0 hybrid functional significantly improves the results even though this leads to an average -0.59 eV redshift compared to reference calculations for Thiel’s set. It is shown, however, that a simple self-consistent scheme at the GW level, with an update of the quasiparticle energies, not only leads to a much better agreement with reference values, but also significantly reduces the impact of the starting DFT functional. On average, the Bethe-Salpeter scheme based on self-consistent GW calculations comes close to the best time-dependent DFT calculations with the PBE0 functional with a 0.98 correlation coefficient and a 0.18 (0.25) eV mean absolute deviation compared to TD-PBE0 (theoretical best estimates) with a tendency to be red-shifted. We also observe that TD-DFT and the standard adiabatic Bethe-Salpeter implementation may differ significantly for states implying a large multiple excitation character.

Ultrafast Intersystem-Crossing Dynamics and Breakdown of the Kasha-Vavilov's Rule of Naphthalenediimides

The fluorescence quantum yield of a red naphthalenediimide dye (rNDI) with amino and Br core substituents has been found to decrease by a factor of almost 2 by going from S1 ← S0 to S2 ← S0 excitation. Time-resolved spectroscopic measurements reveal that this deviation from the Kasha-Vavilov's rule is due to an ultrafast, < 200 fs, intersystem-crossing (ISC) from the S2 state to the triplet manifold, due to the ππ* → nπ* character of the transition and to the presence of the heavy Br atom. In non-core substituted naphthalenediimide (pNDI), ISC is slower, ∼2 ps, and was found to be reversible on a time scale shorter than that of vibrational cooling. The fluorescence and triplet quantum yields of rNDI, thus, can be substantially changed by a simple variation of the excitation wavelength.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Photochemistry of N-(selenoalkyl)-phthalimides. Formation of N, Se-heterocyclic systems

A number of selenium heterocyclic derivatives are obtained upon direct or acetone-sensitized irradiation of a variety ofN-(selenomethyl)alkyl-phthalimides. The reaction proceeds by photoinduced intramolecular electron transfer between the Se atom and the phthalimide moiety.

Full cLR-PCM calculations of the solvatochromic effects on emission energies

Are cLR-PCM excited-state geometries providing more accurate solvatochromic shifts?

Excimer and exciplex emissions of 1,8-naphthalimides caused by aggregation in extremely polar or nonpolar solvents

1,8-Naphtalimdes showed aggregation induced emission in both extremely polar or nonpolar solvents, which is attributed to the intermolecular-excimer and intramolecular-exciplex formation.

Effects of side chains on DNA binding, cell permeability, nuclear localization and cytotoxicity of 4-aminonaphthalimides

The guanidinoethyl group increases DNA binding, and decreases the cell permeability and cytotoxity; the dimethylaminopropyl group enhances the cell permeability and cytotoxity.