Theoretical study of the excited‐state double proton transfer in the (3‐methyl‐7‐azaindole)‐(7‐azaindole) heterodimer

Excited‐state double proton transfer (ESDPT) in the (3‐methyl‐7‐azaindole)‐(7‐azaindole) heterodimer is theoretically investigated by the long‐range corrected time‐dependent density functional theory method and the complete‐active‐space second‐order perturbation theory method. The calculated potential energy profiles exhibit a lower barrier for the concerted mechanism in the locally excited state than for the stepwise mechanism through the charge‐transfer state. This result suggests that the ESDPT in the isolated heterodimer is likely to follow the former mechanism, as has been exhibited for the ESDPT in the homodimer of 7‐azaindole. © 2012 Wiley Periodicals, Inc.     

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.     

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.     

The Cyclic Hydrogen‐Bonded 6‐Azaindole Trimer and its Prominent Excited‐State Triple‐Proton‐Transfer Reaction

The compound 6‐azaindole undergoes self‐assembly by formation of N(1)?H???N(6) hydrogen bonds (H bonds), forming a cyclic, triply H‐bonded trimer. The formation phenomenon is visualized by scanning tunneling microscopy. Remarkably, the H‐bonded trimer undergoes excited‐state triple proton transfer (ESTPT), resulting in a proton‐transfer tautomer emission maximized at 435 nm (325 nm of the normal emission) in cyclohexane. Computational approaches affirm the thermodynamically favorable H‐bonded trimer formation and the associated ESTPT reaction. Thus, nearly half a century after Michael Kasha discovered the double H‐bonded dimer of 7‐azaindole and its associated excited‐state double‐proton‐transfer reaction, the triply H‐bonded trimer formation of 6‐azaindole and its ESTPT reaction are demonstrated.     

Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

The first excited‐state proton transfer (ESPT ) in 7AI ‐H₂O complex and its derivatives, in which the hydrogen atom at the C₂ position in pyrrole ring was replaced by halogen atom X (X = F, Cl, Br), were studied at the TD ‐M06‐2X/6‐31 + G(d, p) level. The double proton transfer took place in a concerted but asynchronous protolysis pathway. The vibrational‐mode selectivity of excited‐state double proton transfer in the model system was confirmed. The specific vibrational‐mode could shorten the reaction path and accelerate the reaction rate. The substituent effects on the excited‐state proton transfer process were discussed. When the H atom at C₂ position in 7AI ‐H₂O complex was replaced by halogen atom, some geometrical parameters changed obviously, the barrier height of ESDPT reduced, and the asynchronicity of proton transfer enlarged. The above changes correlated with the Pauling electronegativity of halogen atom.     

N−H‐Type Excited‐State Proton Transfer in Compounds Possessing a Seven‐Membered‐Ring Intramolecular Hydrogen Bond

A series of compounds containing 5‐(2‐aminobenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one ( o ‐ABDI ) as the core chromophore with a seven‐membered‐ring N?H‐type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N?H proton and thus the hydrogen‐bond strength can be fine‐tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron‐withdrawing strength of R, that is, in the order H<COCH₃<COPh<Tosyl<COCF₃. The tosyl and trifluoroacetyl derivatives undergo ultrafast, irreversible excited‐state intramolecular proton transfer (ESIPT) that results in proton‐transfer emission solely in the red region. Reversible ESIPT, and hence dual emission, involving the normal and proton‐transfer tautomers was resolved for the acetyl‐ and benzyl‐substituted counterparts. For o ‐ABDI , which has the weakest acidity, ESIPT is prohibited due to its highly endergonic reaction. The results clearly demonstrate the harnessing of ESIPT by modifying the proton acidity and hydrogen‐bonding strength in a seven‐membered‐ring intramolecular hydrogen‐bonding system. For all the compounds studied, the emission quantum yields are weak (ca. 10^?3) in dichloromethane, but strong in the solid form, ranging from 3.2 to 47.4 %.     

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.     

Restriction of Photoinduced Twisted Intramolecular Charge Transfer

An intensive investigation of structure–property relationships in the aggregation‐induced enhanced emission (AIEE) of luminescent compounds is essential for the rational design of highly emissive solid‐state materials. In the AIEE‐active compounds N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]isophthalamide and N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]‐5‐tert‐butylisophthalamide, fast photoinduced twisted intramolecular charge transfer (TICT) of the enol excited state is found to be mainly responsible for the weak emission of their dilute solutions. The photoinduced TICT enol excited state is formed with a greatly distorted configuration, due to the large rotation about the C? N single bond. This facilitates nonradiative TICT decay from the normal enol excited state to the highly twisted enol excited state, rather than proton‐transfer decay to the keto excited state. In aggregates, photoinduced nonradiative deactivation of TICT is strongly prohibited, so that excited‐state intramolecular proton transfer (ESIPT) becomes the dominant decay, and hence contributes greatly to the subsequent emission enhancement of the keto form. Molecular design and investigation of analogous single‐armed compounds further verifies this kind of AIEE mechanism.     

Excited‐State Multiple Proton Transfer Depending on the Acidity and Basicity of Mediating Alcohols in 7‐Azaindole–(ROH)2 (R=H,CH3) Complexes: A Theoretical Study

Long‐range proton transfer plays an important role in many chemical and biological phenomena. It has recently been reported that the rate of excited‐state multiple proton transfer depends on the acidity and basicity of mediating alcohols in the H‐bonded wire. The excited‐state triple proton transfer in 7‐azaindole complexes through cyclic H‐bonded wires was theoretically studied to investigate rates depending on the mediating alcohols. This study showed that the acidity and basicity of alcohols collectively functioned to assist proton transfers depending on the paths; the proton transfers of protolytic and solvolytic paths were assisted by the pull‐behind effect and the push‐ahead effect, respectively. Both proton‐donating and accepting abilities of alcohols in the H‐bonded wire can accumulate to help proton transfer, and the strong acidity and basicity of the alcohols with relatively small structural changes in the wire have larger impacts on reducing the activation energies than those of alcohols that trigger proton transfer.     

Excited state tautomerization of azaindole

Fluorescent tryptophan analogs, like azatryptophan, offer an advantage for exploring protein and peptide structure and dynamics. The chromophoric moieties, azaindole, of the azatryptophan analogs are investigated for their potential as fluorescent probes. The photophysical properties of 4-azaindole (4AI) and 5-azaindole (5AI) and their tautomers are characterized through computational and experimental methods. Both 4AI and 5AI undergo excited state tautomerization in the presence of 1 M NaOH. The protonated forms of 4AI and 5AI have a fluorescence emission of 415 and 410 nm, respectively, while the tautomers of 4AI and 5AI have a fluorescent emission of 480 and 450 nm, respectively. Gas phase computations (B3LYP/6-31+G**) show that the N1H azaindole tautomer is lower in energy in the ground state by as much as 12.5 kcal mol(-1), while the N(n)H azaindole tautomer is lower in energy in the excited state by as much as 18.1 kcal mol(-1). Solvent effects on the tautomer energy differences were computed using the isodensity polarized continuum model (IPCM). The polarity of the solvent helps to reduce the energy difference between the tautomers in the ground state by as much as 5.8 kcal mol(-1), but not enough to reverse the ground state tautomer preference.     

Photophysics,Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

The photophysical properties of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S₀–S_i transitions were investigated using the ab initio methods [MP2, CIS(D), EOM‐CCSD] with the correlation‐consistent basis sets. Also the time‐dependent density functional theory (TD‐DFT) has been employed. The lowest singlet excited states of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close‐lying π, π* and n, π* transitions. Experimental steady‐state and time‐resolved spectral studies indicate formation of an isoalloxazinic excited state via excited‐state double‐proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5‐deazaalloxazine molecule. Solvatochromism of both 5‐deazaalloxazine and its 1,3‐dimethyl substituted derivative was analyzed using the Kamlet–Taft scale and four‐parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute–solvent hydrogen‐bonding interactions in their excited state.     

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.     

Electron–Proton Decoupling in Excited‐State Hydrogen Atom Transfer in the Gas Phase

Hydrogen‐release by photoexcitation, excited‐state‐hydrogen‐transfer (ESHT), is one of the important photochemical processes that occur in aromatic acids and is responsible for photoprotection of biomolecules. The mechanism is described by conversion of the initial state to a charge‐separated state along the O(N)‐H bond elongation, leading to dissociation. Thus ESHT is not a simple H‐atom transfer in which a proton and a 1s electron move together. Here we show that the electron‐transfer and the proton‐motion are decoupled in gas‐phase ESHT. We monitor electron and proton transfer independently by picosecond time‐resolved near‐infrared and infrared spectroscopy for isolated phenol–(ammonia)₅, a benchmark molecular cluster. Electron transfer from phenol to ammonia occurred in less than 3 picoseconds, while the overall H‐atom transfer took 15 picoseconds. The observed electron‐proton decoupling will allow for a deeper understanding and control of of photochemistry in biomolecules.     

Spectroscopic studies of charge transfer complexes of meso‐tetra‐p‐tolylporphyrin and its zinc complex with some aromatic nitro acceptors in different organic solvents

The charge transfer complex (CTC) formation of 5,10,15,20‐tetra(p‐tolyl)porphyrin (TTP) and zinc 5,10,15,20‐tetra(p‐tolyl)porphyrin with some aromatic nitro acceptors such as 2,4,6‐trinitrophenol (picric acid), 3,5‐dinitrosalicylic acid, 3,5‐dinitrobenzoic acid (DNB) and 2,4‐dinitrophenol (DNP) was studied spectrophotometrically in different organic solvents at different temperatures. The spectrophotometric titration, Job's and straight line methods indicated the formation of 1:1 CTCs. The values of the equilibrium constant (K_CT) and molar extinction coefficient (ε_CT) were calculated for each complex. The ionization potential of the donors and the dissociation energy of the charge transfer excited state for the CTC in different solvents was also determined and was found to be constant. The spectroscopic and thermodynamic properties were observed to be sensitive to the electron affinity of the acceptors and the nature of the solvent. No CT band was observed between Zn‐TTP as donor and DNP or DNB as acceptors in various organic solvents at different temperature. Bimolecular reactions between singlet excited TTP (¹TTP*) and the acceptors were investigated in solvents with various polarities. A new emission band was observed. The fluorescence intensity of the donor band decreased with increasing the concentration of the acceptor accompanied by an increase in the intensity of the new emission. The new emission of the CTCs can be interpreted as a CT excited complex (exciplex). Copyright © 2007 John Wiley & Sons, Ltd.     

Spiroconjugated Intramolecular Charge‐Transfer Emission in Non‐Typical Spiroconjugated Molecules: The Effect of Molecular Structure upon the Excited‐State Configuration

A set of terfluorenes and terfluorene‐like molecules with different pendant substitutions or side groups were designed and synthesized, their photophysical properties and the excited‐state geometries were studied. Dual fluorescence emissions were observed in compounds with rigid pendant groups bearing electron‐donating N atoms. According to our earlier studies, in this set of terfluorenes, the blue emission is from the local π–π* transition, while the long‐wavelength emission is attributed to a spiroconjugation‐like through‐space charge‐transfer process. Herein, we probe further into how the molecular structures (referring to the side groups, the type of linkage between central fluorene and the 2,2′‐azanediyldiethanol units, and—most importantly—the amount of pendant groups), as well as the excited‐state geometries, affect the charge‐transfer process of these terfluorenes or terfluorene‐like compounds. 9‐(9,9,9′′,9′′‐tetrahexyl‐9H,9′H,9′′H‐[2,2′:7′,2′′‐terfluoren]‐9′‐yl)‐1,2,3,5,6,7‐hexahydropyrido[3,2,1‐ij]quinolone (TFPJH), with only one julolidine pendant group, was particularly synthesized, which exhibits complete “perpendicular” conformation between julolidine and the central fluorene unit in the excited state, thus typical spiroconjugation could be achieved. Notably, its photophysical behaviors resemble those of TFPJ with two pendant julolidines. This study proves that spiroconjugation does happen in these terfluorene derivatives, although their structures are not in line with the typical orthogonal π fragments. The spiroconjugation charge‐transfer emission closely relates to the electron‐donating N atoms on the pendant groups, and to the rigid connection between the central fluorene and the N atoms, whereas the amount of pendant groups and the nature of the side chromophores have little effect. These findings may shed light on the understanding of the through‐space charge‐transfer properties and the emission color tuning of fluorene derivatives.     

Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase. 2. Cooperative nature of double-proton transfer revealed by H/D kinetic isotopic effects

The dispersed fluorescence (DF) spectra of the 7-azaindole dimer (7AI2) and deuterated dimers 7AI2-hd and 7AI2-dd, where hd and dd indicate the deuteration of an imino proton and two imino protons, have been measured in a supersonic free jet expansion. The undeuterated 7AI2-hh dimer exhibits only the tautomer fluorescence, but both the normal and tautomer fluorescence have been detected by exciting the origins of 7AI2-h*d, 7AI2-hd* and 7AI2-dd in the S1-S0 region, where h* and d* indicate the localization of the excitation on 7AI-h or 7AI-d moiety. The DF spectra indicate that 7AI2-h*d and 7AI2-hd* undergo excited-state proton/deuteron transfer (ESPDT), while excited-state double-deuteron transfer (ESDDT) occurs in 7AI2-dd. The H/D kinetic isotopic effects on ESDPT have been investigated by measuring the intensity ratios of the normal fluorescence to the tautomer fluorescence. The ESPDT rate is about 1/60th of the ESDPT rate, and the ESDDT rate is about 1/12th of the ESPDT rate, where ESPDT rate is an average of the rates for 7AI2-h*d and 7AI2-hd*. The observed H/D kinetic isotope effects imply that the ESDPT reaction of 7AI2 has a "cooperative" nature; i.e., the motion of the two moving protons strongly couples each other through the electron motions. The difference in the estimated ESPDT reaction rates, 9.8 x 10(9) and 6.9 x 109 s(-1) for 7AI2-h*d and 7AI2-hd*, respectively, is consistent with the concerted mechanism rather than the stepwise mechanism.     

Theoretical insights into the excited‐state process of 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol

In this paper, we theoretically explore the motivation and behaviors of the excited‐state intramolecular proton transfer (ESIPT) reaction for a novel white organic light‐emitting diode (WOLED) material 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol (t‐MTTH). The “atoms in molecules” (AIM) method is adopted to verify the formation and existence of the hydrogen bond O? H···N. By analyzing the excited‐state hydrogen bonding behaviors via changes in the chemical bonding and infrared (IR) vibrational spectra, we confirm that the intramolecular hydrogen bond O? H···N should be getting strengthened in the first excited state in four kinds of solvents, thus revealing the tendency of ESIPT reaction. Further, the role of charge‐transfer interaction is addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. Also, the electron distribution confirms the ESIPT tendency once again. The scanned and optimized potential energy curves according to variational O? H coordinate in the solvents demonstrate that the proton transfer reaction should occur in the S₁ state, and the potential energy barriers along with ESIPT direction support this reaction. Based on the excited‐state behaviors reported in this work, the experimental spectral phenomenon has been reasonably explained.     

A detailed theoretical study on the excited‐state hydrogen‐bonding dynamics and the proton transfer mechanism for a novel white‐light fluorophore

In this work, density functional theory (DFT) and time‐dependent DFT (TDDFT) methods were used to investigate the excited‐state dynamics of the excited‐state hydrogen‐bonding variations and proton transfer mechanism for a novel white‐light fluorophore 2‐(4‐[dimethylamino]phenyl)‐7‐hyroxy‐6‐(3‐phenylpropanoyl)‐4H‐chromen‐4‐one ( 1 ). The methods we adopted could successfully reproduce the experimental electronic spectra, which shows the appropriateness of the theoretical level in this work. Using molecular electrostatic potential (MEP) as well as the reduced density gradient (RDG) versus the product of the sign of the second largest eigenvalue of the electron density Hessian matrix and electron density (sign[λ₂]ρ), we demonstrate that an intramolecular hydrogen bond O1–H2···O3 should be formed spontaneously in the S₀ state. By analyzing the chemical structures, infrared vibrational spectra, and hydrogen‐bonding energies, we confirm that O1–H2·O3 should be strengthened in the S₁ state, which reveals the possibility of an excited‐state intramolecular proton transfer (ESIPT) process. On investigating the excitation process, we find the S₀ → S₁ transition corresponding to the charge transfer, which provides the driving force for ESIPT. By constructing the potential energy curves, we show that the ESIPT reaction results in a dynamic equilibrium in the S₁ state between the forward and backward processes, which facilitates the emission of white light.     

Steady‐State Spectroscopy of the 2‐(N‐methylacetimidoyl)‐1‐naphthol Molecule

The steady‐state spectroscopy of 2‐(N‐methylacetimidoyl)‐1‐naphthol (MAN) reveals composite absorption and emission spectra from 298 to 193 K in hexane. The ground electronic state (So) absorption can be assigned to the sum of three molecular structures: the OH normal tautomer, and two NH proton transfer tautomers. The NH‐structures are the most stable ones in equilibrium with the OH tautomer for the S₀ state. On photoexcitation of the OH tautomer the excited state intramolecular proton transfer is undergone, and the corresponding NH emission is monitored at 470 nm. On photoexcitation of the NH tautomers the previous emission is monitored in addition to another emission at 600 nm, which is ascribed to intramolecular hydrogen‐bonded (IHB) nonplanar NH structures generated from the IHB planar NH tautomers. A Jab?oński diagram is introduced which gathers all the experimental evidence as well as the theoretical calculations executed at the DFT‐B3LYP and TD‐DFT levels. The MAN molecule is compared with other analogs such as 1‐hydroxy‐2‐acetonaphthone (HAN), 2‐(1?‐hydroxy‐2?‐naphthyl)benzimidazole and methyl 1‐hydroxy‐2‐naphthoate to validate the theoretical calculations. Photoexcitation of MAN generates two emission bands at longer wavelengths than that of the emission band of HAN. The MAN molecule exhibits a great photostability in hydrocarbon solution which depends on the photophysics of the NH tautomers (keto forms).     

Synthesis and light‐emitting properties of carbazole‐based copolymers bearing cyano‐substituted arylenevinylene chromophore

Two arylenevinylene compounds bearing the cyano group at α‐position ( 6 ) and β‐position ( 9 ) from the dialkoxylphenylene unit were synthesized, in which the molecular termini were functionalized with 3‐bromocarbazole. The Suzuki coupling copolymerization of these compounds with 1,4‐bis[(3′‐bromocarbazole‐9′‐yl)methylene]‐2,5‐didecyloxybenzene and 9,9‐dihexylfluorene‐2,7‐bis(boronic acid) was carried out to obtain copolymers ( cp67 and cp97 ) containing the cyano‐substituted arylenevinylene fluorophore of 7 mol %. Model compounds ( 6 ′ and 9 ′) corresponding to the arylenevinylene fluorophore were also prepared. The UV spectra of copolymers resembled that of homopolymer hp with no arylenevinylene segment in both CHCl₃ solution and thin film. The emission maxima of copolymers in CHCl₃ (394 nm) agreed with that of homopolymer indicating that the emission bands originated from the carbazole‐fluorene‐carbazole segment. The emission maximum wavelength of copolymer cp67 in thin film (477 nm) indicated fluorescence from the cyano‐substituted arylenevinylene fluorophore because of the occurrence of fluorescence resonance electron transfer. In contrast, copolymer cp97 showed fluorescence at 528 nm to suggest the formation of a new emissive species such as a charge‐transfer complex (exciplex). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 91–98, 2010