Intramolecular excimer formation in macromolecules. I. Energy migration and excimer formation in copolymers exhibiting nearest-neighbor excimer interactions

Energy migration and intramolecular excimer formation have been studied in a series of copolymers comprising 1-vinylnaphthalene, 2-vinylnaphthalene, and styrene with methyl methacrylate. The technique of fluorescence depolarization was used to characterize energy migration in glassy solutions of the copolymers. The extent of energy migration in these copolymers is determined by the mean sequence length of aromatic species l?_a. Assuming that excimer formation occurs as a result of nearest-neighbor interactions, the concentration of excimer sites in the macromolecule will be proportional to the fraction of links between aromatic species f_aa. It is proposed that these sites are populated via energy migration from the site of absorption. Proportionality between the ratio of excimer to monomer emission intensities and the function l?_a·f_aa was predicted. Good agreement with this relationship was obtained in each of the copolymer systems studied. Reactivity ratios of methyl methacrylate (r_m) in copolymerization at 70°C with the aromatic monomers (r_a) were determined as: 1-vinylnaphthalene—r_m = 0.43, r_a = 1.71: 2-vinylnaphthalene—r_m = 0.37, r_a = 4.46; styrene-r_m = 0.45, r_a = 0.58.     

Phospholipid-induced aggregation and anthracene excimer formation

The zinc complex of anthryl bis(dipicolylamine) (1) aggregates upon binding with long-chain aliphatic phosphates and displays anthracene excimer fluorescence, which provides a new strategy toward detection of the biologically important lysophosphatidic acid in aqueous solution.     

Sequence distribution and intramolecular excimer formation in styrene-acrylonitrile copolymers

Styrene-acrylonitrile copolymers, like many other copolymers containing styrene, exhibit both normal and excimer fluorescence. We have shown that the ratio of the excimer to monomer fluorescence intensities in random styrene-acrylonitrile copolymers is linearly dependent upon the concentration of styrene-styrene bonds in the copolymer. This observation is consistent with a photophysical model which allows the energy absorbed by styrene units to migrate freely along the copolymer chain. Some of the energy is emitted in the form of normal fluorescence; some of the energy, trapped by neighbouring styrene-styrene pairs suitably oriented to allow excimer formation, is emitted as excimer fluorescence. The fluorescence characteristics of acrylonitrile-styrene copolymers are contrasted with those of methyl methacrylate-styrene copolymers, in which the methylmethacrylate sequences are believed to present partial barriers to energy migration along the copolymer chains.     

Induced fit interanion discrimination by binding-induced excimer formation

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a 2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change, giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6, which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host complexes with a range of anions.     

Steric-controlled excimer formation in naphthalene analogues of chalcone

Naphthalene analogues of chalcone (typical vinylarenes) are well known for their intramolecular charge transfer (CT) process. The all four possible isomers NC1, NC2, NC3 and NC4 were observed to give structure-less broad CT bands, whereas, excimer formation at higher concentration was reported only for NC4 [Res. Chem. Intermed. 25 (1999) 903]. However, conventional GMMX calculation data reveals that the naphthalene portion of all the isomers are planar, hence, excimer emission is expected from all of them, i.e., if NC4 can form an excimer, then the rests are also capable of doing it. In this paper we have actually succeeded in resolving the excimer peaks for all these four isomers by optimization of concentration, though the extent of excimer formation was observed to be the maximum for NC4. These differential tendencies of excimer formation can be explained by the change in extent of intermolecular stacking interaction of the naphthalene moieties. Variation occurs here due to steric perturbation arising from the specific orientation of the near resident non-planar aroyl component with respect to the naphthalene moiety.     

表面活性剂水溶液中芘的激基缔合物形成

在离子型和非离子型表面活性剂水溶液中观察到了芘的激基缔合物荧光, 其荧光强度与单体荧光强度比值对表面活性剂浓度的关系曲线中有一峰值。指出表面活性剂单体分子在水溶液中呈绕曲状构型, 对应上述峰值的表面活性剂浓度为其临界胶束浓度。     

Intramolecular exciplex and intermolecular excimer formation of 1,8-naphthalimide-linker-phenothiazine dyads

Steady-state fluorescence spectra were measured for 1,8-naphthahlimide-linker-phenothiazine dyads (NI-L-PTZ, where L = octamethylenyl ((CH2)8) and 3,6,9-trioxaundecyl ((CH2CH2O)3C2H4)), NI-C8-PTZ and NI-O-PTZ, as well as the NI derivatives substituted on the nitrogen atom with various linker groups without PTZ as the reference NI molecule in n-hexane. Normal fluorescence peaks were observed at 367-369 nm in all NI molecules together with a broader emission around 470 nm, which is assigned to the excimer emission between the NI in the singlet excited state (1NI*) and the NI moiety of another NI molecule (1[NI/NI]*). In addition, a broad peak around 600 nm was observed only for NI-L-PTZ, which is assigned to an intramolecular exciplex emission between donor (PTZ) and acceptor (NI) moieties in the excited singlet state, 1[NI-L-NI]*. The formation of an intramolecular exciplex corresponds to the existence of a conformer with a weak face-to-face interaction between the NI and PTZ moieties in the excited state because of the long and flexible linkers. The excited-state dynamics of the NI molecules in n-hexane were established by means of time-resolved fluorescence spectroscopy.     

Fluorescence quenching by excimer formation: quenching constant approximations for excimer formation-dissociation by classical potential models

Fluorescence quenching by excimer formation is studied on the assumption that the excimer formation and dissociation can be modeled as overdamped motion in an attractive potential (classical potential models). An approach to the zeroth-order, concentration-independent quenching constants is proposed which starts from a mean reaction-time ansatz and reduces the calculation essentially to the solution of the eigenvalue problem for the Smoluchowski operator which describes the excimer equilibration. For a square-well potential model it is shown that a quenching constant expansion in terms of relaxation modes, truncated at the kinetic level, gives a satisfactory approximation of the recently obtained exact zeroth-order result under defined conditions. It is demonstrated how this two-mode approach can be applied for a quenching constant estimation if the excimer formation and dissociation are modeled by more realistic interaction potentials, as for instance, Morse- or Gaussian-type ones.     

Intramolecular excimer formation in polymers: Pyrene Labelled Polyvinylacetate

The rate constant for intramolecular excimer formation between pyrenyl side-groups, in a polyvinylacetate chain at a mean separation of 200 bonds, has been measured as a function of molecular weight, solvent viscosity and solvent thermodynamic power. Above M = 1 × 10⁵, the rate constant is 1.4 × 10⁷ sec^?1 in low viscosity bad solvents. This value is about twenty times that for the rate constant of the analogous reaction between the two terminal groups in a chain with a mean end-to-end separation of 200 bonds. Increases of the viscosity and of the thermodynamic power of the solvent depress the rate constant, in agreement with the behaviour expected for a diffusion controlled reaction.     

Remarkably enhanced excimer formation of naphthylacetate in cation-charged gamma-cyclodextrin

[reaction: see text] 6(A),6(D)-Bispyridinio-appended gamma-cyclodextrin effectively enhanced the excimer fluorescence of 2-naphthylacetate. This increase derived from the increase of formation of the 1:2 complex between the cation-charged gamma-cyclodextrin and 2-naphthylacetate by the electrostatic interaction between the host and the guest.     

Distributional effects on excimer formation in micellar surfactant solutions

An analysis is presented of excimer formation of molecules solubilized in micellar surfactant solutions. This treatment takes account of both the statistical distribution of solubilizate molecules among the micelles and the kinetic factors affecting excimer formation. By using this analysis to interpret the results of steady-state fluorescence-intensity measurements, where the excimer and monomer emissions of pyrene solubilized in a number of surfactants are measured, it is possible to gain information about micellar size and about the mobility and disposition of pyrene molecules within the micelle.     

Intramolecular excimer formation in a naphthalene-appended dinuclear iron-oxo complex

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.     

Intramolecular excimer formation for covalently linked boron dipyrromethene dyes

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.     

Direct observation of excimer formation in anthracene and 9,9′-bianthryl

The existence of anthracene excimer in fluid solution was confirmed for the first time by observing the coincidence of the rise time of excimer fluorescence with the decay time of monomer fluorescence with the use of a picosecond laser, a streak camera, and a computer. The continuum model of diffusion theory is found to be applicable to the excimer formation and the encounter distance between anthracene in the excited state and that in the ground state is calculated to be 8 ± 2 Å. The anomalously broad featureless fluorescence observed for 9,9′-bianthryl in glycerol-methanol (9:1) solution was found to be emitted from the species formed only in the excited state and its rise time was found to be coincident with the decay time of fluorescence from anthracene moiety composing 9,9′-bianthryl, α,ω-9,9′-bianthrylpropane, -butane, -hexane, and -dodecane were found not to form excimer in the excited state.     

A fluorescent pyrophosphate sensor via excimer formation in water

A new fluorescent sensor based on a pyrene/Zn(II)-dpa (dpa = bis(2-pyridylmethyl)amine) conjugate displays excimer emission selective for pyrophosphate over other anions.     

Intramolecular excimer formation and delayed fluorescence in sterically constrained pyrene dimers

The synthesis is described for a series of five molecular dyads comprising pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer. The extent of through-space communication between the pyrene units is modulated by steric interactions imposed by bulky moieties attached at the 6,8-positions of each pyrene unit. For the control compound, only hydrogen atoms occupy the 6,8 positions (DP1), whereas the remaining compounds incorporate ethynylene groups terminated with either triisopropylsilyl (DP2), 1-tert-butylbenzene (DP3), 2,6-di-tert-butylbenzene (DP4) or 1-tert-butyl-3,5-dimethylbenzene (DP5) units. Each compound shows a mixture of monomer and excimer fluorescence in fluid solution at room temperature, but only monomer emission in a glassy matrix at 77 K. The ratio of monomer to excimer fluorescence depends markedly on the molecular structure; DP1 is heavily biased in favour of the excimer and DP4 is enriched with monomer fluorescence. Photophysical properties, including laser induced and delayed fluorescence data, are reported for each compound. Delayed fluorescence occurs by both intramolecular and bimolecular steps, but these events take place on different timescales. The possibility is raised for using intramolecular triplet-triplet annihilation as a means of molecular imaging.