Intramolecular excimer formation with non-planar molecules: 1,3-dibiphenylpropane. Kinetic and thermodynamic results

Intramolecular excimer formation with dibiphenylpropane in several alkane solvents is reported. By a combination of fluorescence decay time and quantum yield measurements the rate constants describing the excimer formation and deactivation and the values for ΔH and ΔS of the excimer formation equilibrium could be determined. These data are compared with results obtained with di(2-naphthyl) propane and di(1-pyrenyl) propane. It is argued that the difficulty to observe excimer emission with biphenyl under photoexcitation results from a very large value of ?ΔS. A comparison between the excimer fluorescence of dibiphenylpropane and [2.2] biphenylophane points to an unusually weak transannular interaction in the phane.     

Intramolecular excimer formation in macromolecules—V. Head-to-head polystyrene,block copolymers of styrene and butadiene and regular condensation polymers of α-methylstyrene

Studies of the photophysical properties of head-to-head polystyrene, block copolymers poly(butadiene-b-styrene) and polystyrenes of various sequence length of styrene chromophore and regular copolymers of α-methylstyrene indicate that interactions between phenyl chromophores located distantly upon the macromolecule are not of major significance in determination of excimer formation in styrene polymers. It is concluded that intramolecular excimer formation in polystyrenes occurs primarily as a consequence of interactions between nearest neighbour chromophores in the polymer chain. The influence of polymer coil dimensions upon intramolecular excimer formation is demonstrated. The photophysical behaviour of poly(butadiene-b-styrene) block copolymers shows the importance of intramolecular energy migration in population of potential excimer sites.     

Investigation of the excimer dynamics in 9,10-dichloroanthracene crystals using picosecond spectroscopy

The dynamics of excimer formation, excimer migration, and excimer dissociation in β-9,10-dichloroanthracene crystals were investigated by means of picosecond time-resolved fluorescence spectroscopy. In the temperature range from T = 20 K to T = 40 K we were able to temporally resolve the relaxation into the excimer state. The excimer formation rates were determined to be k ≈ 1.8 × 10¹¹ s^?1 at T = 40 K and k ≈ 2.0 × 10¹⁰ s^?1 at T<30 K. The excimer migration was investigated by measuring the excimer annihilation rate as a function of temperature. At room temperature 20% of the excimers are dissociated. The excimer binding energy is estimated to be B = 1360 cm^?1. The experimental results are explained in terms of a kinetic scheme comprising the population and depopulation of exciton, trap, and excimer states. The nature of the trap state is identified and it is shown that thermal activation of a 25 cm^?1 librational mode induces the relaxation of the trap into the excimer state.     

Normal and second excimers in macromolecules—II: Poly(1-methoxy-4-vinylnaphthalene) in polystyrene solid solution

Normal and second excimer formation has been examined for poly(1-methoxy-4-vinylnaphthalane) (PMVN) in polystyrene solid films as a function of measurement temperature from 420 to 77 K and film casting temperature between 369 and 303 K. It became clear that the conformation of the second excimer forming site is different from that of the normal excimer forming site. The difference in energy between the most stable conformation and the excimer forming site is 2.0 kcal/mol for the normal excimer and less than 1 kcal/mol for the second excimer. Moreover the temperature dependence of the second excimer to monomer intensity ratio is found to give information about T_γ (γ dispersion temperature) of the host polystyrene films, below which the motion of phenyl rings is restricted.     

Photoluminescence of biphenyl-containing polymers

Poly(4-vinylbiphenyl) and copolymers of methyl methacrylate and 4-vinylbiphenyl show both monomeric (λ_max = 325 nm) and excimer (λ_max = 380 nm) fluorescence. The quantum yield of excimer emission increases and the monomeric emission decreases with increase in the fraction of vinylbiphenyl units in the copolymer. The decrease of the monomeric emission is closely related to a decrease in singlet lifetime. These results are interpreted in terms of a kinetic controlled excimer formation. Comparison of the emission in the homo and copolymers with that of the dimeric model compound shows that excimer formation in the polymer strongly depends upon the possibility of energy migration along sequences of vinylbiphenyl units. This conclusion is considered as of particular relevance due to the change in geometry of the biphenyl unit upon excitation.     

Solvent effect on thermal degradation of plasticized para-substituted polystyrenes

The thermal effect on stability of a series of para-substituted polystyrenes with methyl, methoxy and α-methyl substituents in various solvents was studied in the temperature range of 298-363 K. They gave a monomer fluorescence as a minor part and excimer fluorescence as a major part. Thermal heating of para-substituted polystyrenes shows a decrease in both monomer and excimer fluorescences in all used solvents. Thermal heating causes a small fluorescence quenching effect at lower temperatures in solution but becomes very dominant at higher temperatures. Added terephthalate and phthalate plasticizers to these para-substituted polystyrenes caused a quenching of both monomer and excimer fluorescences without the formation of exciplex emission. The thermal quenching processes of the plasticized polymers were accompanied by a change in the structure of the fluorescence spectra at high heating temperatures. This may indicate that thermodestruction of these polymers starts from a random chain scission. The change in solvent polarity has considerable effect on fluorescence quenching but it has a minor effect on the thermal degradation of these polymers. The binding energies for excimer formation were calculated in the used solvents.     

Transition between dilute and concentrated solution behaviour as revealed by excimer fluorescence

Excimer fluorescence measurements at 25° on polystyrene, with molecular weights 111,000, 392,000 and 900,000, in 1,2-dichloroethane over an extended concentration region are reported. The curve representing the normalized ratio of excimer to monomer fluorescence intensities shows an upward curvature at c[η] ≈ 1 for all the molecular weights. This observation is attributed to enhanced intermolecular excimer formation. Some theoretical aspects on this phenomenon are also considered.     

Intramolecular excimer of DI(1-pyrenylmethyleneoxycarbonyl) alkanes

Intramolecular excimer emission of a series of di(1-pyrenylmethyleneoxycarbonyl)alkanes (n = 0–22) has been studied under photostationary conditions. The intramolecular excimer formation rate constant decreases monotonously with increasing number of methylene units of the chains in the low temperature region. The temperature dependence of the rate constant for each sample was studied to give an apparent activation energy of ca. 5 kcal/mol.     

Molecular dynamics of polystyrene model molecules: Solvent effects on intramolecular excimer formation in 2,4-diphenylpentanes

Intramolecular excimer formation in 2,4-diphenylpentanes has been examined in a homologous series of alkanes, in ethanol and in mixtures of ethanol, ethylene glycol and glycerol. The ratio of the emission intensities of dimer and monomer (I_D/I_M) is not affected in low viscosity solvents but, above 4 cP, viscosity effects are discernible and a relationship of the form I_D/I_M = Aη^?2 is obeyed. In methylene chloride, only the dl molecule exhibits a decrease of the efficiency of excimer sampling. The temperature dependence of I_D/I_M in isooctane and methylene chloride has been interpreted in terms of the activation energy of the excimer sampling.     

Photochemical Properties of Supramolecular Dyad with Pyrenylethenylquinoline as a Photochrome

Spectral and photochemical properties of a bichromophoric dyad with two photochromes, 2-[(2-(pyren-1-yl)ethenyl]quinoline (PEQ) bridged by ten methylene groups have been investigated. Stationary spectroscopy does not show substantial interaction between the PEQ groups in the dyad, but the appearance of another, more long-living component in the f luorescence decay kinetics indicates possible excimer formation. The formation of the chemically active excimer is also confirmed by the intramolecular [2+2] photocycloaddition reaction to give tetrasubstituted cyclobutane with two vicinal pyrene substituents. This reaction competes with trans–cis photoisomerization of the dyad. It has been proposed that the π-stacking interaction of two PEQ groups with a long system of π-bonds plays an important role in the excimer formation followed by the photocycloaddition reaction.     

Fluorescence sensing of Cu and Hg by a dipyrene ligand involving an excimer-switch off mechanism

A novel dipyrene ligand 1 has been designed which shows intramolecular excimer formation in solution. Its specific interaction with Cu²⁺ or Hg²⁺ leads to the disruption of the excimer and results in a fluorescence-mediated sensing of these ions in a mixed organic-aqueous solution. Apart from steady-state studies, time-resolved fluorescence measurements also reveal that excimer-switch off caused by metal ion coordination leads to the selective detection of these ions.     

Study of non-conjugated bichromophores linked by a polyether chain. Kinetic analysis of the formation of intramolecular excimers and photocyclomerization of some bisanthracenes

Dynamic properties of polyoyethylene chains are evaluated by the determination of rate constants (k_DM) of intramolecular excimer formation for four bis(9-anthryloxy)polyoxaalkanes in benzene and ethanol. The Kinetic parameters for the photodimenzation process of these compounds have been determined.     

Characteristics of excimer formation in langmuir-blodgett assemblies of 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine and dipalmitoylphosphatidylcholine

Langmuir-Blodgett film alloys of PPDPC (1-palmitoyl-2-[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine) and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine) were assembled in varying molar proportions on quartz glass slides. The transferred layers were then characterized according to their pyrene excimer to monomer fluorescence intensity ratio i_E/i_M and fluorescence quantum yields as a function of film composition. The observed deviation from non-ideal mixing is considered to be due to the formation of regular distribution patterns of PPDPC in a DPPC lattice. The observed critical mole fractions of PPDPC evident as steps in I_E/I_M versus X_PPDPC plots can be accounted for by a model involving a trigonal distribution of pyrenedecanoyl chains in the phospholipid acyl chain lattice. The implications of the distribution of PPDPC as superlattices to the excimer formation mechanism are discussed.     

Luminescence studies in polymers—II : Temperature effect on polyvinylcarbazole fluorescence

Polyvinylcarbazole (PVCa) films and solutions emit normal and excimer fluorescence between 77 and 425 K. The absolute (I_M and I_D) and relative (I_M/I_D) intensities emitted strongly depend on temperature. The usual U-shaped curve is obtained for log I_M/I_D as a function of 1/T in the case of polymer films. In solution, two minima corresponding to two different excimers are observed. The formation and dissociation of PVCa excimers in films and solutions have been interpreted according to the usual kinetic scheme. The binding energies for the excimer in films and for the high temperature excimer in solutions are respectively 4 and 2·8 kcal mole⁻¹.     

Self-assembling behavior of binary mixture of hexa-peri-hexabenzocoronene derivatives with different molecular symmetry

The self-assembly and mesophase behavior of a series of mixtures of hexa-peri-hexabenzocoronene (HBC) derivatives carrying alkyl chains of different lengths at different positions and thus of different molecular symmetry (C₁- and D_3h-symmetry) were examined by powder X-ray diffractometry, UV–vis spectrometry, photoluminescence, polarizing optical microscopy, and differential scanning calorimetry. Significant reduction of isotropic phase transition temperature/clearing temperature was observed compare to single component symmetric analogues. The chain length modulates the mesophase behavior existed in the mixture. In particular, with long dodecyl groups, the mixture exhibited excimer/exciplex formation as well as aggregation induced emission phenomena in the non-polar solvent of hexane. Formation dynamics of intermolecular excimer/exciplex was studied by nuclear magnetic resonance and UV–vis spectroscopic method.     

Normal and second excimers in macromolecules—I: Poly(1-methoxy-4-vinylnaphthalene) in fluid solution

The emission properties of poly(1-methoxy-4-vinylnaphthalene) (PMVN) in 2-methyltetrahydrofuran solution have been investigated over the range 77–350 K. It exhibits, in addition to the monomer fluorescence (348 and 360 nm), two structureless emissions derived from two different types of excimer, viz. the normal excimer (420 nm) and the second excimer (380 nm), the latter having a partially overlapping structure of aromatic rings. The intensity of second excimer emission of PMVN increases with decreasing temperature, while the normal excimer emission shows a maximum at 215 K. Kinetic analysis of transient decay curves for the fluorescence of PMVN gave results consistent with the previously reported kinetic scheme for the dimer model, 1,3-bis(4-methoxy-1-naphthyl)propane, showing that the second excimer is not formed from or converted to the normal excimer and that two types of excimer are formed independently from the excited monomer.     

Microenvironmental effects of cyclodextrins I: Enhancement of intramolecular excimer formation of polymethylene-bisβ-naphthoate via inclusion with cyclodextrins

The effects of α-, β- and γ-cyclodextrins (CDs) on the fluorescence spectra of a series of polymethylene-bis-β-naphthoates (Bn) have been studied. It is observed that β-CD and γ-CD enhance Bn intramolecular excimer fluorescence, indicating the formation of two-to-one guest host inclusion complexes. The possible conformation of these inclusion complexes is discussed.     

Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar-methylstyrene)s in solution

Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl₄). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.