The Quest of Excited‐State Intramolecular Proton Transfer via Eight‐Membered Ring π‐Conjugated Hydrogen Bonding System

Searching for eight‐membered ring π‐conjugated hydrogen bonding (8‐MR H‐bonding) systems with excited‐state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π‐conjugated molecules ( 8‐HB‐1 , 8‐HB‐L1 and 8‐HB‐2 ) potentially possessing 8‐MR H‐bonding are strategically designed, synthesized and characterized. The configurations of these three potential molecules are checked by their X‐ray structures, among which 8‐HB‐L1 (a structurally locked 8‐HB‐1 core chromophore) is proved to be an 8‐MR H‐bonding system, whereas 8‐HB‐1 and 8‐HB‐2 are too sterically hindered to form the 8‐MR intramolecular H‐bond. The ESIPT property of 8‐HB‐L1 is confirmed by the dual fluorescence consisting of normal and proton‐transfer tautomer emissions. The insight into the ESIPT process of 8‐HB‐L1 is provided by femtosecond fluorescence upconversion measurements together with computational simulation. The results demonstrate for the first time a successful synthetic route to attain the 8‐MR H‐bonding molecule 8‐HB‐L1 with ESIPT property.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Acid‐Induced Shift of Intramolecular Hydrogen Bonding Responsible for Excited‐State Intramolecular Proton Transfer

The significant progress recently achieved in designing smart acid‐responsive materials based on intramolecular charge transfer inspired us to utilize excited‐state intramolecular proton transfer (ESIPT) for developing a turn‐on acid‐responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT‐active fluorophores, 2‐(2‐hydroxyphenyl)pyridine (HPP) and 2‐(2‐hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT‐HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady‐state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn‐on was caused by the acid‐induced shift of the ESIPT‐responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid‐responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.     

Control over Excited State Intramolecular Proton Transfer and Photoinduced Tautomerization: Influence of the Hydrogen‐Bond Geometry

The influence of H‐bond geometry on the dynamics of excited state intramolecular proton transfer (ESIPT) and photoinduced tautomerization in a series of phenol‐quinoline compounds is investigated. Control over the proton donor–acceptor distance (d_DA) and dihedral angle between the proton donor–acceptor subunits is achieved by introducing methylene backbone straps of increasing lengths to link the phenol and quinoline. We demonstrate that a long d_DA correlates with a higher barrier for ESIPT, while a large dihedral angle opens highly efficient deactivation channels after ESIPT, preventing the formation of the fully relaxed tautomer photoproduct.     

Three Modes of Proton Transfer in One Chromophore: Photoinduced Tautomerization in 2‐(1H‐Pyrazol‐5‐yl)Pyridines,Their Dimers and Alcohol Complexes

Studies of 2‐(1H‐pyrazol‐5‐yl)pyridine (PPP) and its derivatives 2‐(4‐methyl‐1H‐pyrazol‐5‐yl)pyridine (MPP) and 2‐(3‐bromo‐1H‐pyrazol‐5‐yl)pyridine (BPP) by stationary and time‐resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited‐state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited‐state intermolecular double‐proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent‐assisted double‐proton transfer in hydrogen‐bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited‐state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground‐state syn–anti equilibria are detected and discussed. The fraction of the higher‐energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen‐bond donor or acceptor abilities.     

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.     

Excited‐State Proton Transfer and Intramolecular Charge Transfer in 1,3‐Diketone Molecules

The photophysical signature of the tautomeric species of the asymmetric (N,N‐dimethylanilino)‐1,3‐diketone molecule are investigated using approaches rooted in density functional theory (DFT) and time‐dependent DFT (TD‐DFT). In particular, since this molecule, in the excited state, can undergo proton transfer reactions coupled to intramolecular charge transfer events, the different radiative and nonradiative channels are investigated by making use of different density‐based indexes. The use of these tools, together with the analysis of both singlet and triplet potential energy surfaces, provide new insights into excited‐state reactivity allowing one to rationalize the experimental findings including different behavior of the molecule as a function of solvent polarity.     

Gelation‐Induced Enhanced Fluorescence Emission from Organogels of Salicylanilide‐Containing Compounds Exhibiting Excited‐State Intramolecular Proton Transfer: Synthesis and Self‐Assembly

Self‐assembly structure, stability, hydrogen‐bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by field‐emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), UV/Vis absorption and photoluminescence, as well as theoretical studies by DFT and semiempirical calculations with CI (AM1/PECI=8) methods. It was found that salicylanilides form gels in nonpolar solvents due to π‐stacking interaction complemented by the presence of both inter‐ and intramolecular hydrogen bonding. The supramolecular arrangement in these organogels predicted by XRD shows lamellar and hexagonal columnar structures for gelators 3 and 4 , respectively. Of particular interest is the observation of significant fluorescence enhancement accompanying gelation, which was ascribed to the formation of J‐aggregates and inhibition of intramolecular rotation in the gel state.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

Iron Oxide Nanoparticles Labeled with an Excited‐State Intramolecular Proton Transfer Dye

Excited‐state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica‐coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.     

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine “strap”. In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.     

A Strap Strategy for Construction of an Excited‐State Intramolecular Proton Transfer (ESIPT) System with Dual Fluorescence

An amine‐embedded flexible alkyl strap has been incorporated into an emissive boryl‐substituted dithienylpyrrole skeleton as a new entity of excited‐state intramolecular proton transfer (ESIPT) chromophores. The π‐electron system shows a dual emission, which covers a wide range of the visible region depending on the solvent polarity. The incorporation of the aminoalkyl strap as well as the terminal boryl groups efficiently stabilize the zwitterionic excited‐state species resulting from the ESIPT even in an aqueous medium.     

Influence of the Intramolecular Hydrogen Bond on the Fluorescence of 2-ortho-Aminophenyl Pyridines

The dynamic nature of excited-state intramolecular proton transfer (ESIPT) and its effect on emission spectra is an attractive strategy to create multi-emissive dyes. Here we describe the behavior of a series of hydrogen-bonded triphenylpyridines with a set of donor–acceptor combinations that allowed us to perceive the influence of each substitution on the photophysical properties of the dyes. The susceptibility of these ESIPT moieties to pH variations was also studied, elucidating that the level of protonation had a significant effect on the emission color. The assignment of each emission band was made by using DFT and td-DFT calculations that were in agreement with the experimental results. This study emphasizes the versatility of triphenylpyridines that can be synthesized effortlessly with a logical and independent C-2, C-4 and C-6 substitution in order to have the desired ESIPT modulation and subsequent multi-emission response.     

How To Reach Intense Luminescence for Compounds Capable of Excited‐State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9′,10′:4,5]imidazo[1,2‐f]phenanthridine and imidazo[1,2‐f]phenanthridine skeletons, which mediate excited‐state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five‐membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady‐state and time‐resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited‐state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited‐state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT‐capable compounds originates from their enol excited states.     

Low‐Threshold Wavelength‐Switchable Organic Nanowire Lasers Based on Excited‐State Intramolecular Proton Transfer

Coherent light signals generated at the nanoscale are crucial to the realization of photonic integrated circuits. Self‐assembled nanowires from organic dyes can provide both a gain medium and an effective resonant cavity, which have been utilized for fulfilling miniaturized lasers. Excited‐state intramolecular proton transfer (ESIPT), a classical molecular photoisomerization process, can be used to build a typical four‐level system, which is more favorable for population inversion. Low‐power driven lasing in proton‐transfer molecular nanowires with an optimized ESIPT energy‐level process has been achieved. With high gain and low loss from the ESIPT, the wires can be applied as effective FP‐type resonators, which generated single‐mode lasing with a very low threshold. The lasing wavelength can be reversibly switched based on a conformation conversion of the excited keto form in the ESIPT process.     

Anion Receptors Containing ‐NH Binding Sites: Hydrogen‐Bond Formation or Neat Proton Transfer?

When the amide‐containing receptor 1 ⁺ is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH₃COO^?, F^?, H₂PO₄^?), it undergoes deprotonation of the ‐NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl^?, Br^?, NO₃^?), 1 ⁺ establishes true hydrogen‐bond interactions of decreasing intensity. The less acidic receptor 2 ⁺ undergoes neat proton transfer with only the more basic anions CH₃COO^? and F^?, and establishes hydrogen‐bond interactions with H₂PO₄^?. An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and ¹H NMR spectra, is proposed.     

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited‐state proton transfer (ESPT) can be used to probe protein binding sites. For 2‐(2′‐hydroxy‐4′‐aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active‐site conformation. The emission properties are tuned by hydrogen‐bonding interactions of HABT with the neighboring amino acid T766 and with active‐site water. The use of ESPT tuners opens the possibility of creating two‐color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.     

Excited‐State Double Proton Transfer in Model Base Pairs: The Stepwise Reaction on the Heterodimer of 7‐Azaindole Analogues

A four fused‐ring system 11‐propyl‐6H‐indolo[2,3‐b]quinoline ( 6 HIQ ) is strategically designed and synthesized; it possesses a central moiety of 7‐azaindole ( 7AI ) and undergoes excited‐state double proton transfer (ESDPT). Despite a barrierless type of ESDPT in the 6 HIQ dimer, femtosecond dynamics and a kinetic isotope effect provide indications for a stepwise ESDPT process in the 6 HIQ/7AI heterodimer, in which 6 HIQ (deuterated 6 HIQ ) delivers the pyrrolyl proton (deuteron) to 7AI (deuterated 7AI ) in less than 150 fs, forming an intermediate with a charge‐transfer‐like ion pair, followed by the transfer of a pyrrolyl proton (deuteron) from cation‐like 7AI (deuterated 7AI ) to the pyridinyl nitrogen of the anion‐like 6 HIQ (deuterated 6 HIQ ) in ～1.5±0.3 ps (3.5±0.3 ps). The barrier of second proton transfer is estimated to be 2.86 kcal mol^?1 for the 6 HIQ/7AI heterodimer.