Aromatic oxadiazole‐based conjugated polymers with excited‐state intramolecular proton transfer: Their synthesis and sensing ability for explosive nitroaromatic compounds

Aromatic polyoxadiazole derivatives containing 9,9′‐dioctylfluorene were successfully synthesized via the Suzuki coupling reaction. The oxadiazole moiety in the polymer backbone was linked with the bis(hydroxyphenyl) group in its 2‐position to exhibit a large Stokes shift in the emission spectrum due to the excited‐state intramolecular proton transfer. To prepare the polymer via the Suzuki cross‐coupling reaction, the hydroxyl group in the monomer was protected with the t‐butoxycarbonyl group before polymerization and removed after polymerization to a desirable extent. The polymer with the free hydroxyl group showed a considerable sensitivity for nitroaromatic compounds, exhibiting fluorescence quenching in a chloroform solution. The interaction between the electron‐donating OH group and electron‐deficient nitroaromatic compounds seemed to play a decisive role in fluorescence quenching. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2059–2068, 2006     

Synthesis and characterization of nitrogen‐linked carbazole‐containing fluorescent polymers

We have prepared four light‐emitting polymers bearing a chromophore composed of carbazole and fluorene by the Suzuki coupling polycondensation. Two nonconjugated polymers (P3CzBFXy and P2CzFXy) had a chromophore tethered by the p‐xylylene spacer, whose connection point between carbazole and fluorene in addition to the number of fluorene unit was systematically changed to investigate the emission wavelength and intensity. The red‐shifted absorption and emission maximum wavelengths together with the improved fluorescence quantum yield of polymers P3CzBFXy and P2CzFXy indicate that the increment of the number of para‐connected benzene rings included in the chromophore effectively extends the conjugation length. The fact that polymer P3CzBFXy has longer wavelength absorption and emission spectra also indicates the interaction of the carbazole nitrogen lone pair with the oligophenylene moiety. Other two polymers P3CzFPy and P3CzFPym having the heterocycle directly bound to the carbazole nitrogen were prepared to know the character of the carbazole nitrogen lone pair and their influence on the fluorescence behavior. The fluorescence spectra of polymer P3CzFPym bearing the pyrimidine ring gradually red‐shifted in conjunction with the decrease of fluorescence quantum yield on going from toluene solution to CHCl₃ solution because of the intramolecular charge transfer at the excited state. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3729–3735, 2010     

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.     

生色团连接的苯骈三氮唑衍生物的激发态分子内质子转移

用从头算和密度泛函理论研究了对硝基二苯乙烯作为生色团连接的2-(2-羟基-苯基)-苯骈三氮唑的衍生物2-羟基-5-[对硝基-二苯乙烯基-氧亚甲基]-苯基-(2H-苯骈三氮唑)(C1)和4′-硝基-3,4-二[2-羟基-(2H-苯骈三氮唑)-苄氧基]-二苯乙烯(C2)发生激发态分子内质子转移(ESIPT)的可能性.系统研究了C1和C2发生ESIPT的互变异构体的基态与激发态的性质变化,包括相关的键长、键角等结构参数,Mulliken电荷和偶极矩,前线轨道以及势能曲线.计算结果表明,对于C1来讲,酮式(keto)的基态(K)不存在稳定结构,因此发生基态分子内质子转移(GSIPT)可能性很小.酮式的激发态(K*)的氢键强度要远强于烯醇式(enol)的激发态(E*)的氢键强度.分子在光致激发后,质子供体所带负电荷减小而质子受体所带负电荷增加.在K*,HOMO→LUMO的电子跃迁导致电子密度从"酚环"向质子化杂环转移.E*→K*跃迁只需要克服较小的能垒(约41 kJ.mol-1).计算结果表明C1发生ESIPT的可能性很大.C2由于具有高能量,其具有基态的单质子转移特征的异构体EK(同时含烯醇E与酮K结构)、具有基态的双质子转移特征的异构体2K(含有双酮结构),以及具有双酮结构特征的激发态2K*均无法获得它们的稳定结构,因此,基态分子内单或双质子转移和激发态分子内双重质子转移发生的可能性极小.然而,由于双烯醇式的激发态(2E*)和EK的激发态(EK*)存在稳定结构,且2E*→EK*跃迁具有低能垒,因此C2有可能发生激发态分子内单重质子转移.本文进一步计算了两个分子的紫外-可见吸收光谱与荧光发射光谱,获得了具有较大斯托克位移的ESIPT的荧光发射峰.     

Synthesis of imidazole‐containing conjugated polymers bearing phenol unit as side group and excited state intramolecular proton transfer‐mediated fluorescence

Dibromobenzimidazole and dibromoimidazole bearing hydroxyl group‐protected phenol unit ( 1 and 2 ) were prepared and they showed an intramolecular hydrogen bonding between ether oxygen and amino proton of imidazole. The palladium‐catalyzed Suzuki coupling polymerization of 1 and 2 with benzene bis(boronic acid) derivatives gave soluble polymers ( 3 and 4 ), where the molecular weights were limited probably due to the coordination ability of imidazole to palladium metal. The phenol hydroxyl groups were subsequently deprotected using BBr₃ to obtain 3 ′ and 4 ′. From the ¹H NMR spectra, the complete conversion to the hydroxyl group and the formation of another type of intramolecular hydrogen bonding between hydroxyl proton and imine nitrogen were confirmed. In the UV and PL spectra of 3 ′ and 4 ′, the excited state intramolecular proton transfer (ESIPT) occurred to shift the emission spectra toward lower energy region compared to 3 and 4 . Especially, the PL spectrum of 3 ′ demonstrated large stokes shift (145 nm) in THF solution. The ESIPT‐mediated fluorescence was influenced by the addition of methanol and trifluoroacetic acid, which inhibited the formation of intramolecular hydrogen bonding. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4822–4829, 2009     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

Spectroscopic studies of the conformational properties of naphthoyl chitosan in dilute solutions

A naphthoyl chitosan derivative was prepared, and its conformations in dilute solutions were characterized with spectroscopic methods, including circular dichroism (CD) spectroscopy and fluorescence emission spectroscopy. The CD spectrum of this polymer showed a negative band at about 295 nm in dimethyl sulfoxide (DMSO), indicating that the polymer adopted a helical secondary structure. A helix reversion occurred at concentrations greater than 1 mg/mL. The intensity of the CD signal decreased with the addition of water to the solution, and this suggested a change from a helical conformation to a looser one as a result of the collapse of intramolecular hydrogen bonds. In the fluorescence emission experiments, two kinds of excimer emission bands were detected at 375 and 425 nm, and they were assigned to a partially overlapped dimer with a twisted geometry and a fully overlapped dimer with a sandwichlike geometry, respectively. Adding water to a solution of naphthoyl chitosan in DMSO resulted in a gradual reduction of the emission intensity at 375 nm, and this implied that the twisted arrangement of the chromophore was destroyed by the presence of water. The relative intensity (i.e., the ratio of the intensity of the excimer emission at 425 nm to that of the excimer emission at 375 nm) depended on the solvent (DMSO, N,N‐dimethylformamide, N,N‐dimethylacetamide, and 1‐methyl‐2‐pyrrolidinone), and this indicated that the conformation of naphthoyl chitosan was solvent‐dependent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2747–2758, 2004     

Vinyl monomers bearing chromophore moieties and their polymers. XI. Synthesis and photochemical behavior of carbazole‐containing methacrylic monomers and their polymers

Three carbazole‐containing methacrylic monomers, 2‐(N‐carbazolyl)ethyl methacrylate(CzEMA), 6‐(N‐carbazolyl)hexyl methacrylate(CzHMA), and 11‐(N‐carbazolyl)undecyl methacrylate (CzUMA), and their saturated model compounds, 2‐(N‐carbazolyl)ethyl isobutyrate, 6‐(N‐carbazolyl)hexyl isobutyrate, and 11‐(N‐carbazolyl)undecyl isobutyrate, were synthesized and polymerized. UV absorption spectra showed that there was either negligible or no interaction between the carbon–carbon double bond of the methacrylic group and the carbazolyl chromophore moiety in the ground state for these monomers. Fluorescence spectra of the monomers, their model compounds, and the polymers were recorded in the solvents with different polarities. CzEMA exhibited the fluorescence structural self‐quenching effect (SSQE), but CzHMA and CzUMA did not. In addition, the SSQE of CzEMA depended strongly on the polarity of the solvents. That is, the stronger the polarity of a solvent was, the more obvious the SSQE was. Therefore, the SSQE of CzEMA mainly was caused by the intramolecular charge‐transfer interaction between the excited electron‐donating carbazolyl chromophore moiety and the electron‐accepting carbon–carbon double bond of the methacrylic group. This was confirmed by the fluorescence‐decay curves and the fluorescence lifetimes of the monomers, their model compounds, and the polymers. The monomers, their model compounds, and the polymers initiated the photopolymerization of methyl methacrylate (MMA) upon UV irradiation. CzEMA showed greater initiation ability than the other two monomers and their model compounds; this was ascribed to the photoinduced intramolecular charge‐transfer interaction. The higher initiation efficiency of the homopolymers compared to that of the copolymers with MMA was interpreted as the result of singlet energy migration of the excited carbazolyl chromophores along the polymer chains. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 679–688, 2000     

Perylene Bisimide and Naphthyl‐Based Molecular Dyads: Hydrogen Bonds Driving Co‐planarization and Anomalous Temperature‐Response Fluorescence

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho‐methoxy group (NM) at its bay positions (PBI‐2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol^?1) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co‐planarization of the different aromatic units, resulting in delocalization of the π‐electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co‐planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H‐bond‐driven co‐planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.     

Synthesis and self‐organization properties of copolyurethanes based on perylenediimide and naphthalenediimide units

A series of perylene and naphthalene diimide‐containing random copolyurethanes with different ratios of perylene/naphthalene diimide content was synthesized and characterized. Copolymerization improved the solubility of these rigid aromatic diimides, and the copolymers were soluble in common organic solvents like chloroform, tetrahydrofuran, and so forth. The absorption spectra of perylene‐based copolymers showed a red‐shifted peak at a wavelength of 557 nm corresponding to J‐type aggregates. For naphthalene copolymers, the quenching of fluorescence at higher naphthalene incorporation suggested the presence of aggregates because of the extensive π‐π stacking of the aromatic core. FTIR spectroscopic analysis showed that the hydrogen bonding tendency of the polymer decreased with increase in perylene/naphthalene incorporation. The fluorescence spectra of the perylene polymers were exactly a mirror image of the absorption spectra. The fluorescence spectra of the naphthalene polymers at higher naphthalene incorporation showed a red‐shifted excimer like emission peak, which was assigned as static excimers based on their excitation spectra. These polymers could exhibit two types of secondary interaction modes, namely, hydrogen bonding (via urethane linkage) and π‐stacking (via aromatic perylene or naphthalene units) thus highlighting the importance of polymer design in inducing self‐organization at both low and high incorporation of the rigid bisimide moieties. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1224–1235, 2009     

Syntheses and nickel coordination properties of cyclen substituted with mixed stilbene/poly(ethylene glycol) dendritic arms

We synthesized a series of cyclens substituted with mixed stilbene and poly(ethylene glycol) dendritic arms. All dendrimers terminated with different peripheral groups had good solubility in common organic solvents, and dendrimers terminated with ? CO₂H groups (CO₂H‐dendrimers) were also soluble in alkaline solutions. The nickel coordination properties of these dendrimers were investigated in organic solvents. Dendrimers terminated with ? CN groups (CN‐dendrimers) and the second‐generation CO₂H‐dendrimer [(CO₂H)₈L2] could produce pentacoordinated nickel complexes; the third‐generation CO₂H‐dendrimer [(CO₂H)₁₆L3] could form tetra‐ and pentacoordinated nickel complexes, and the nickel complex of the fourth‐generation CO₂H‐dendrimer [(CO₂H)₃₂L4] could not be obtained. This result was due to the fact that the globular surface of (CO₂H)₁₆L3 formed a hydrogen‐bond network that selectively penetrated cations and inhibited the access of anions to the core. The formation of the hydrogen‐bond network was confirmed by Fourier transform infrared, ¹H NMR, and fluorescence data. The CN‐dendrimers could not form hydrogen bonds on the surface, and the first‐ and second‐generation CO₂H‐dendrimers could not form intramolecular hydrogen‐bond networks. Therefore, they had no selectivity for positive nickel ions and negative chloride ions. (CO₂H)₃₂L4 could not produce a nickel complex because it had a crammed backbone structure that could not penetrate nickel and chloride ions. Therefore, it was possible to control the ion access of cations and anions with the hydrogen‐bond network of (CO₂H)₁₆L3. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5414–5428, 2005     

Synthesis and photophysical properties of polyfluorenes bearing silicon‐based functional groups

Five polyfluorenes bearing bulky trimethylsilyl (PTMS1 and PTMS2), tris(trimethylsilyl)silyl (PTTMS1), and silsesquioxane groups (PPOSS1 and PPOSS2) were synthesized through palladium‐catalyzed Suzuki coupling reactions. In the solution state, every polymer showed comparable ultraviolet–visible spectra, and they emitted blue light with high quantum efficiency. In the solid state, however, three trimethylsilyl‐functionalized polyfluorenes indicated redshifts of the fluorescence peak. In particular, PTMS1 and PTTMS1, having a hydrogen at the C‐9 position of fluorene, also showed green‐light emissions. After the annealing of the spin‐coated films, the blue‐emissive peak decreased and the green‐emissive peak became stronger in the photoluminescence spectra of three trimethylsilyl‐functionalized polyfluorenes. In contrast, PPOSS2 showed a pure blue‐light emission in the film state and even after the thermal treatment, which could be accomplished by the encapsulation of the polymer chains by the large polyhedral oligomeric silsesquioxane molecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2119–2127, 2005     

Luminescence studies of polymer matrices. III. Characterization and evaluation of acrylic acid‐based polymers as hosts for a phosphorescent coding system

The phosphorescence characteristics of naphthyl labelled poly(acrylic acid) film samples have been studied as a function of temperature, with a view to investigating the effect of matrix control on the level of triplet emission observed. Two relaxations, which serve to deactivate the excited triplet states have been detected from phosphorescence lifetime measurements: the α (or glass) and the βtransition (which can be associated with the onset of rotation of the carboxylic acid group). Investigation of the emission from 2‐benzoyl naphthalene dispersed within both an acrylic acid‐methyl methacrylate copolymer and a PAA film, respectively, has revealed that the more intense, longer‐lived phosphorescence results from the modified polymer. This is considered to reflect the existence of (i) hydrogen‐bonding interactions induced by the presence of carboxylic acid groups which serve to form a rigid matrix and (ii) intramolecular aggregates of methyl methacrylate units which create hydrophobic‐rich pockets, capable of sustaining intense phosphorescence at room temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2127–2136, 1999     

Oligonucleotide‐mediated aggregation of a cationic conjugated polymer for fluorescent detection of mercury ions in an aqueous medium

A cationically charged conjugated polymer ( P2 ) functionalized with quaternary ammonium salt was newly synthesized via Suzuki cross‐coupling polymerization. The functionalized P2 features different fluorescence colors according to its phases (blue emission in solution and green emission as asolid) which is caused by intramolecular and intermolecular exciton migration, respectively. The use of P2 as a novel fluorescent sensing platform is demonstrated for mercury ion detection. The detection of mercury ions is accomplished in two steps: (1) the cationic, blue‐emitting P2 absorbs an anionic oligonucleotide, polythymidine ( PT ) via electrostatic interaction to form a complex with green emission due to aggregation between the two species; (2) the addition of mercury ions to the complex produces a new complex of PT ‐Hg²⁺ via more favorable specific interaction, resulting in the isolation of P2 and the consequent recovery of blue fluorescence of P2 . It suggests that this detection system has high selectivity and sensitivity down to the ～10^?7 M level, even in mixtures of metal ions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2393–2400     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

The investigation of proton transfer and fluorescence‐sensing mechanisms of [2‐(2‐hydroxy‐phenyl)‐1H‐benzoimidazol‐5‐yl]‐phenyl‐methanone

Given the tremendous potential of fluorescence sensors in recent years, in this present work, we theoretically explore a novel fluorescence chemosensor [2‐(2‐Hydroxy‐phenyl)‐1H‐benzoimidazol‐5‐yl]‐phenyl‐methanone (HBPM) about its excited state behaviors and probe‐response mechanism. Using density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we explore the S₀‐state and S₁‐state hydrogen bond dynamical behaviors and confirm that the strengthening intramolecular hydrogen bond in the S₁ state may promote the excited state intramolecular proton transfer (ESIPT) reaction. In view of the photoexcitation process, we find that the charge redistribution around the hydroxyl moiety plays important roles in providing driving force for ESIPT. And the constructed potential energy curves further verify that the ESIPT process of HBPM should be ultrafast. That is the reason why the normal HBPM fluorescence cannot be detected in previous experiment. Furthermore, with the addition of fluoride anions, the exothermal deprotonation process occurs spontaneously along with the intermolecular hydrogen bond O–H?F. It reveals the uniqueness of detecting fluoride anions using HBPM molecules. As a whole, the fluoride anions inhibit the initial ESIPT process of HBPM, which results in different fluorescence behaviors. This work presents the clear ESIPT process and fluoride anion‐sensing mechanism of a novel HBPM chemosensor.     

Time‐dependent density functional theory study on the hydrogen bonding‐induced twisted intramolecular charge‐transfer excited states of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine

In this work, the time‐dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen‐bonded intramolecular charge‐transfer excited state of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine (DMAPIP) in methanol (MeOH) solvent. All the geometric conformations of the ground state and locally excited (LE) state and the twisted intramolecular charge‐transfer (TICT) state for isolated DMAPIP and its hydrogen‐bonded complexes have been optimized. At the same time, the absorption and fluorescence spectra of DMAPIP and the hydrogen‐bonded complexes in different electronic states are also calculated. We theoretically demonstrated for the first time that the intermolecular hydrogen bond formed between DMAPIP and MeOH can induce the formation of the TICT state for DMAPIP in MeOH solvent. Therefore, the two components at 414 and 506 nm observed in the fluorescence spectra of DMAPIP in MeOH solvent were reassigned in this work. The fluorescence peak at 414 nm is confirmed to be the LE state. Furthermore, the red‐shifted shoulder at 506 nm should be originated from the hydrogen‐bonded TICT excited state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Synthesis and characterization of fluorescence end‐labeled polystyrene via reversible addition‐fragmentation chain transfer (RAFT) polymerization

Fluorescence end‐labeled polystyrene (PS) with heteroaromatic carbazole or indole group were prepared conveniently via reversible addition‐fragmentation chain transfer (RAFT) polymerization using dithiocarbamates, ethyl 2‐(9H‐carbazole‐9‐carbonothioylthio)propanoate (ECCP) and benzyl 2‐phenyl‐1H‐indole‐1‐carbodithioate (BPIC) as RAFT agents. The end functionality of obtained PS with different molecular weights was high. The steady‐state and the time‐resolved fluorescence techniques had been used to study the fluorescence behaviors of obtained end‐labeled PS. The fluorescence of dithiocarbamates resulting PS in solid powder cannot be monitored; however, they exhibited structured absorptions and emissions in solvent DMF and the fluorescence lifetimes of PS had no obvious change with molecular weights increasing. These observations suggested that the polymer chains were possibly stretched adequately in DMF, that is, the fluorescence end group was exposed into solvent molecules and little quenching of excited state occurred upon incorporation into polymer chain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6198–6205, 2008     

Synthesis and photoluminescence behaviors of anthracene‐layered polymers

Novel anthracene‐layered polymers containing fluorescence quenchers such as ferrocene and nitrobenzene units at the polymer termini were designed and synthesized. Their optical properties were investigated in detail. The photoluminescence spectra of the polymers were featureless without vibrational structures, indicating that the anthracenes are effectively interacting each other in a single polymer chain in the excited state. Fluorescence emission from the layered anthracene units was effectively quenched by the aromatic units at the polymer termini owing to energy and electron transfer through a single polymer chain. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2815–2821     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.