Recognition-Induced Enhanced Emission of Core-Fluorescent ESIPT-type Macrocycles.

Core-fluorescent cavitands based on 2-(2'-resorcinol)benzimidazole fluorophores (RBIs) merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity, show excited state intramolecular proton transfer (ESIPT) and readily undergo deprotonation to form dianionic cavitands, capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts (due to emission from anionic double keto tautomers) and cation-selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity. Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non-macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non-macrocyclic analogs are anionic double keto tautomers, which are the result of excited state intramolecular proton transfer (ESIPT) or excited state double proton transfer (ESDPT).

Long-Range Proton Transfer in 7-Hydroxy-Quinoline-Based Azomethine Dyes: A Hidden Reason for the Low Efficiency.

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans-cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data.

Blockade of persistent colored isomer formation in photochromic 3H-naphthopyrans by excited-state intramolecular proton transfer.

In photochemistry the excited-state intramolecular proton transfer process (ESIPT) is often observed as a highly efficient singlet excited state depletion pathway, which in the presence of a strong intramolecular hydrogen bond may proceed on a subpicosecond time scale. The present work describes the suppression of unwanted transoid-trans isomer formation in photochromic 3H-naphthopyran derivatives by the introduction of a 5-hydroxy substituent. According to time-resolved spectroscopy experiments and excited-state ab initio calculations, transoid-cis → transoid-trans photoisomerization is reduced by a competitive ESIPT channel in nonpolar solvent (cyclohexane). Upon specific solute-solvent interactions (methanol, acetonitrile) the intramolecular hydrogen bond in the transoid-cis form is perturbed, favoring the internal conversion S1 → S0 process as photostabilizing channel.

Mechanistic insights into photochromic 3H-naphthopyran showing strong photocoloration.

3,3-Diphenylbenzo[f]chromene (1) represents an important architectural platform for photochromic systems. Since the practical utility of such chromophores is largely dependent upon the kinetics of coloration and decoloration, elucidating the mechanistic details of these processes is of great value. Toward this end, we studied the photochromic reaction of (3-(2-methoxyphenyl)-3-phenyl-3H-benzo[f]chromene (2) by both time-resolved UV-vis and mid-IR spectroscopies. We found that irradiation of 2 at 365 nm generates long-lived colored transoid-cis isomers with lifetimes of 17.1 s and 17.5 min (at 21 °C) and even longer-lived transoid-trans isomers with a lifetime of 16 h. These experimental results were supplemented with ab initio ground-state and excited-state calculations, and the resulting theoretical interpretation may be useful for the design of new photochromic systems with optimized photofunctionality.

Control of the Photo-Isomerization Mechanism in 3H-Naphthopyrans to Prevent Formation of Unwanted Long-Lived Photoproducts.

In the photochromic reactions of 3H-naphthopyrans, two colored isomers TC (transoid-cis) and TT (transoid-trans) are formed. In terms of optimized photo-switchable materials, synthetic efforts are nowadays evolving toward developing 3H-naphthopyran derivatives that would not be able to photoproduce the long-living transoid-trans, TT, photoproduct. The substitution with a methoxy group at position 10 results in significant reduction of the TT isomer formation yield. The TC photophysics responsible for TT suppression were revealed here using a combination of multi-scale time resolved absorption UV-vis spectroscopy and ab initio calculations. The substitution changes the TC excited-state potential energy landscape, the bicycle-pedal isomerization path is favored over the rotation around a single double bond. The bicycle-pedal path is aborted in halfway to TT formation due to S1→S0 internal conversion populating back the TC species in the ground electronic state. This is validated by a shorter TC S1 state lifetime for methoxy derivative in comparison to that of the parent-unsubstituted compound (0.47 ± 0.05 ps vs. 0.87 ± 0.09 ps) in cyclohexane.

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran.

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC→TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S1 →S0 internal conversion (major), intersystem crossing S1 →T1 , pyran ring formation, photoenolization and TC→TT isomerization. Computations support the S1 →S0 and T1 →S0 channels as responsible for photostabilization of the TC form.

Photochromic reaction in 3H-naphthopyrans studied by vibrational spectroscopy and quantum chemical calculations.

Structural details on the species involved in the photochromic reaction of 3H-naphthopyrans in solution have been formerly determined using NMR spectroscopy. Herein we show that at room temperature time-resolved FT-IR spectroscopy is a simple and efficient tool for structural characterization of colored species generated upon continuous UV light irradiation of the model compound 3H-naphthopyran: 3,3-diphenyl-3H-naphtho[2,1-b]pyran. In solution and in the polymer matrix phase, a colored species transoid-cis is formed after a single-photon excitation process, while transoid-trans is a secondary long-lived photoproduct generated after two-step excitation involving two photons. Understanding the reaction mechanism leading to long-lived colored species can help with the design of new 3H-naphthopyran derivatives structurally optimized for making a photochromic reaction free from transoid-trans products, which is often important for applications. Ab initio calculations show that photoinduced ring-opening followed by isomerization occurs on a multidimensional potential-energy surface. The barriers separating the considered isomeric forms, both in the ground and in the excited state, help to interpret the step-by-step dynamics of the photoprocesses. The system is composed of a variety of ground state equilibrium forms. Each of them is characterized by fast excited-state deactivation pathways which may drive the system through different conical intersection regions.

Ultrafast dynamics of the ESIPT photoswitch N-(3-pyridinyl)-2-pyridinecarboxamide.

Molecular switches based on proton transfer that are photochromic and can be interconverted by light at different wavelengths back and forth between two thermodynamically stable tautomeric states in solution at room temperature are rare to date. We report on a study of the ultrafast conversion of the bistable proton transfer switch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) to a corresponding iminol after photoexcitation at λpump ≈ 265 nm by means of femtosecond time-resolved broad-band and single-colour transient electronic absorption spectroscopy (TEAS), transient fluorescence spectroscopy (TFLS), and transient vibrational absorption spectroscopy (TVAS) in acetonitrile solution. The interpretation of the data was accompanied by ab initio quantum chemical calculations of the excited electronic states and the vibrational frequencies of the reactant and product in their ground electronic state. The TEAS experiments provided four time constants, τ1 = 0.09 ± 0.01 ps, τ2 = 0.61 ± 0.01 ps, τ3 = 5.10 ± 0.80 ps, and τ4 = 20.0 ± 1.0 ps. The first two agree well with the measured TFLS lifetimes, τ1,TFL < 0.18 ps and τ2,TFL = 0.50 ± 0.01 ps. τ1 is related to the relaxation of the initially excited Franck-Condon (FC) state of the pyridinecarboxamide, followed by the excited-state intramolecular proton transfer (ESIPT) step to the neighbouring pyridine. The subsequent return of the molecules to the electronic ground state takes place within τ2, mediated by a conical intersection (CI) at a twisted configuration of the pyridinecarboxamide moiety. The main components in all TEAS time profiles feature a rise with τ2 and a decay with τ4 and describe subsequent molecular transformations in the electronic ground state. τ3 is ascribed to vibrational cooling of the molecules. The final iminol exhibits a permanent UV absorption at λ = 247 nm, where its absorbance is stronger than that of the carboxamide reactant. The iminol structure is unambiguously identified by the TVA spectra, which show the build-up of corresponding vibrational bands with τ4,TVA = 23 ± 2 ps after the initial bleach of the reactant vibrational bands, in excellent agreement with the TEAS data. Its lifetime is >10 ns.

Direct Arylation of Dipyrrolonaphthyridinediones Leads to Red-Emitting Dyes with Conformational Freedom.

A series of bis-aryl dipyrrolonaphthyridinediones (DPNDs) with various substitution patterns have been designed and synthesized by direct arylation. This reaction occurs regioselectively at positions 3 and 9, which gives a straightforward entry to unique dyes that absorb in the red/far-red region and emit in the far-red/NIR region. The photophysical properties (absorption and fluorescence wavelength) can be controlled by altering the steric hindrance and electronic character of the peripheral aryl group. The fluorescence quantum yields are moderate in the majority of cases and almost independent of changes in the solvent polarity. DPNDs bearing substituents at the ortho position that prevent free rotation of the phenyl ring emit stronger fluorescence relative to the unhindered para analogues and, at the same time their absorption and emission bands are blueshifted. We have used computational methods to explain why electron-donating substituents have significantly stronger effects than electron-withdrawing substituents on the optical properties by invoking the electron-accepting character of the DPND. Electrochemical measurements revealed that the HOMO and LUMO, as well as the electrochemical band gap, also depend strongly on the electronic character of the peripheral group.

Ferroelectric molecular field-switch based on double proton transfer process: Static and dynamical simulations.

In this work, we present a reversible ferroelectric molecular switch controlled by an external electric field. The studied (2Z)-1-(6-((Z)-2-hydroxy-2-phenylvinyl)pyridin-3-yl)-2-(pyridin-2(1H)-ylidene)ethanone (DSA) molecule is polarized by two uniaxial intramolecular hydrogen bonds. Two protons can be transferred along hydrogen bonds upon an electric field applied along the main molecular axis. The process results in reversion of the dipole moment of the system. Static ab initio and on-the-fly dynamical simulations of the DSA molecule placed in an external electric field give insight into the mechanism of the double proton transfer (DPT) in the system and allow for estimation of the time scale of this process. The results indicate that with increasing strength of the electric field, the step-wise mechanism of DPT changes into the downhill barrierless process in which the synchronous and asynchronous DPTs compete with each other.

Contacts for organic switches with carbon-nanotube leads.

We focus on two classes of organic switches operating due to the photo- or field-induced proton transfer (PT) process. By means of first-principles simulations, we search for the atomic contacts that strengthen diversity of the two swapped current-voltage (I-V) characteristics between two tautomers. We emphasize that the low-resistive contacts do not necessarily possess good switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states. Instead, the more resistive contacts multiply a current gear to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts: (i) in the very low-voltage regime, the direct connections to the nanotubes perform better than the popular sulfur contacts, and (ii) in the higher-voltage regime, the best are the peroxide (-O-O-) contacts. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.

Excited-state intramolecular proton transfer: photoswitching in salicylidene methylamine derivatives.

The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012, 13, 4287-4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π-electron-donating and π-electron-withdrawing substituents results in an electron-density push-and-pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization.

Effect of chemical substitutions on photo-switching properties of 3-hydroxy-picolinic acid studied by ab initio methods.

Effect of chemical substitutions to the molecular structure of 3-hydroxy-picolinic acid on photo-switching properties of the system operating on excited-state intramolecular double proton transfer (d-ESIPT) process [M. F. Rode and A. L. Sobolewski, Chem. Phys. 409, 41 (2012)] was studied with the aid of electronic structure theory methods. It was shown that simultaneous application of electron-donating and electron-withdrawing substitutions at certain positions of the molecular frame increases the height of the S0-state tautomerization barrier (ensuring thermal stability of isomers) and facilitates a barrierless access to the S1/S0 conical intersection from the Franck-Condon region of the S1 potential-energy surface. Results of study point to the conclusion that the most challenging issue for practical design of a fast molecular photoswitch based on d-ESIPT phenomenon are to ensure a selectivity of optical excitation of a given tautomeric form of the system.

7-Hydroxyquinoline-8-carbaldehydes. 1. Ground- and excited-state long-range prototropic tautomerization.

Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H and (13)C NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular "frame" (7-hydroquinolines, 7-HQs) and a proton "crane" (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, OH (7-quinolinol)) and NH (7(1H)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, HMMQC, shows only a trace of the NH form in highly polar and/or protic media). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV-vis spectroscopic properties of 7-HQCs with those of 7-HQs, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted 7-HQCs.

7-Hydroxyquinoline-8-carbaldehydes. 2. Prototropic equilibria.

Prototropic equilibria were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that in aqueous solutions may assume four main neutral and ionic structures: 7-quinolinol (OH), 7(1H)-quinolinone (NH), deprotonated anion (A), and protonated cation (C). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground and excited-state long-range tautomerization (part 1) as well as protonation and deprotonation processes. The photophysical properties of neutral and ionic forms of 7-HQCs are compared with those of 7-hydroxyquinolines (7-HQs), synthetic precursors of the former. The experimental results are corroborated by ab initio computations.

Electronic spectra and reversible photoisomerization of protonated naphthalenes in solid neon.

Alpha- and beta-protonated naphthalenes (α- and ß-HN(+)) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C(10)H(9)(+) ion beam. The absorption spectra reveal S(1)/S(2) ← S(0) transitions with onsets at 502.1 and 396.1 nm for α-HN(+), and 534.5 and 322.3 nm in the case of ß-HN(+). Wavelength-dispersed fluorescence was detected for α-HN(+), starting at 504.4 nm. Light-induced α-HN(+) → ß-HN(+) isomerization was observed upon S(2) ← S(0) excitation of α-HN(+), whereas ß-HN(+) relaxed back into the more stable alpha form either upon excitation to S(1) or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN(+) ↔ ß-HN(+) photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.

Photophysics of Schiff bases: theoretical study of salicylidene methylamine.

The proton-transfer reaction in a model aromatic Schiff base, salicylidene methylamine (SMA), in the ground and in the lowest electronically-excited singlet states, is theoretically analyzed with the aid of second-order approximate coupled-cluster model CC2, time-dependent density functional theory (TD-DFT) using the Becke, three-parameter Lee-Yang-Parr (B3LYP) functional, and complete active space perturbation theory CASPT2 electronic structure methods. Computed vertical-absorption spectra for the stable ground-state isomers of SMA fully confirm the photochromism of SMA. The potential-energy profiles of the ground and the lowest excited singlet state are calculated and four photophysically relevant isomeric forms of SMA; α, ß, γ, and δ are discussed. The calculations indicate two S(1)/S(0) conical intersections which provide non-adiabatic gates for a radiationless decay to the ground state. The photophysical scheme which emerges from the theoretical study is related to recent experimental results obtained for SMA and its derivatives in the low-temperature argon matrices (J. Grzegorzek, A. Filarowski, Z. Mielke, Phys. Chem. Chem. Phys. 2011, 13, 16596-16605). Our results suggest that aromatic Schiff bases are potential candidates for optically driven molecular switches.

Photophysics of indole-2-carboxylic acid in an aqueous environment studied by fluorescence spectroscopy in combination with ab initio calculations.

The photo-physics and -chemistry of indoles are known to be highly complex and strongly dependent on their precise molecular structure and environment. Combination of spectroscopic analysis with quantum chemical calculations should be a powerful tool to unravel precise excited state deactivation mechanisms. At the same time, combined studies are seldom and likely far from trivial. In this work we explore the feasibility of combining spectroscopic and quantum-chemical data into one consistent model. The molecule of choice is indole-2-carboxylic acid (ICA) in aqueous media. Excited state dynamics are determined by time-resolved fluorescence experiments, while excited state reaction pathways of ICA-H(2)O clusters are explored by ab initio calculations.

Effect of chemical substituents on the energetical landscape of a molecular photoswitch: an ab initio study.

The effect of chemical substitutions on the energetical landscape of an optical molecular switch (Phys. Chem. Chem. Phys. 2008, 10, 1243) was studied with the aid of ab initio electronic structure methods. Series of different chemical moieties were substituted into the molecular frame of 7-hydroxyquinoline as well as into the "molecular crane" at position 8 of the frame. It was shown that the π-electron-donating/withdrawing properties of substituents substantially modify the energetical landscape of the system in the ground as well as in the lowest excited ππ* and nπ* singlet states.

Photochemistry of the water dimer: time-dependent quantum wave-packet description of the dynamics at the S1-S0 conical intersection.

The photoinduced electron-driven proton-transfer dynamics of the water-dimer system has been investigated by time-dependent quantum wave-packet calculations. The main nuclear degrees of freedom driving the system from the Frank-Condon region to the S(0)-S(1) conical intersection are the distance between the oxygen atoms and the displacement of the hydrogen atom from the oxygen-oxygen bond center. Two important coupling modes have been investigated: Rotation of the H-donating water dangling proton and asymmetric stretching of the H-accepting water dangling protons' O(a)H bonds. Potential energy surfaces of the ground and lowest excited electronic states have been constructed on the basis of ab initio calculations. The time-dependent quantum wave-packet propagation has been employed within the (2 + 1)-dimensional systems for the description of the nonadiabatic dynamics of water dimer. The effects of the initial vibrational state of the system on the electronic population transfer and dissociation dynamics are presented. To approximate the photochemical behavior of water dimer in bulk water, we add a boundary condition into the (2 + 1)-dimensional systems to simulate the existence of water bulk. The results provide insight into the mechanisms of excited state deactivation of the water-dimer system in gas phase and in bulk water through the electron-driven proton-transfer process.