Synthesis of a [3.3]biphenylophane, and its photophysical and photochemical properties studied by emission and transient absorption measurements

[3.3](4,4′)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at 295 and 77 K, and phosphorescence from the locally-excited (LE) triplet state at 77 K. Based on the transient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with the LE triplet at 295 and 77 K. The triplet excimer of BPP is shown to be formed via intersystem crossing from the singlet excimer state, and concluded to be non-phosphorescent.     

A THEORETICAL STUDY OF THE CYTOSINE EXCIMER STATE: THE ROLE OF GEOMETRY OPTIMIZATION

Abstract A study of the lowest excited dimer (excimer) singlet state of the DNA base cytosine was performed with fully optimized geometry (near the cis-syn form) using a slightly modified version of MOPAC6 (AM1 hamiltonian) and 112 excited configurations involving an active space consisting of the four highest occupied and the two lowest empty MO of the dimer. A binding energy of 3 kcal/mol for the excimer state is demonstrated, and the excimer fluorescence is predicted to be shifted 150 nm to longer wavelengths relative to that of the monomer fluorescence. The excimer geometry has the planes nonparallel (40–45° interplanar angle) and distorted, with the C₅-C₆ bonds showing quite close contact (2.4 Å). Excitation is found to be more localized in the region of the C₅-C₆ bond than expected from the monomer wavefunction. No stable excimer was found if the planes were constrained to be planar, nor if the triplet state was selected. The results suggest that the excimer geometry found in this study may be a precursor to the cis-syn cyclobutane photodimer.     

Observation of intramolecular singlet and triplet excimers of tethered naphthalene moieties under the geometric constraints imposed by the host framework of zeolites

This paper describes the emission spectroscopic investigation of singlet and triplet excimers of tethered bisnaphthalene species at room temperature. 1,1'-Dinaphthylethane (DNE) and 1,1'-dinaphthylpropane (DNP) were incorporated into a 3-D network of a 1.3 nm diameter pseudo-spherical supercage of zeolites Y (with an entry aperture of 0.74 nm), and a 1-D channel of zeolite L with a 0.71 nm aperture. In the straight channel of zeolite L, DNP adopts a parallel-sandwich conformation as suggested by the observation of entirely excimeric fluorescence, while DNE can only enter the channel in an open form. As a result, incorporation of DNP into zeolite L exchanged with Tl(+) ions, which induce enhanced intersystem-crossing, allowed the observation of a broad and featureless phosphorescence spectrum that is ascribable to the triplet excimer of DNP. In contrast, an entirely monomeric phosphorescence is observed for DNE. Furthermore, we observed a monomeric phosphorescence spectrum for DNP in Tl(+)-exchanged zeolite Y, in which DNP was shown to assume its open conformation. Thus the parallel-sandwich structure is responsible for the triplet excimer of the bisnaphthalene species as well as for the singlet counterpart. The parallel-sandwich conformation of the triplet excimer of DNP immobilized by the host framework is significantly different from the previously proposed L-shaped configuration in solution. However, it is very similar to that of the triplet excimer of triple-tethered carbazolophane, as revealed quite recently by Ohkita's group (J. Phys. Chem. B, 2007, 111, 10905). Thus the present study helps tackle the long-term issue of triplet excimer through the use of zeolites.     

Fluorescence quenching of pyrene monomer and excimer by CH3I

The fluorescence quenching rate constants of pyrene monomer and excimer by CH₃I were obtained at several temperatures in methylcyclohexane. Both quenching processes are kinetically controlled, allowing insight on the mechanism of quenching. The rate constants have both temperature-independent and temperature-dependent components. The temperature-independent component for both monomer and excimer fluorescence is consistent with quenching due to enhanced intersystem crossing to a lower energy triplet state. The monomer temperature-dependent component comes from the enhancement of the intersystem crossing to a higher energy triplet state. The thermally activated excimer quenching is associated with the excimer dissociation step to give a pyrene in a second triplet state plus a ground state pyrene molecule.     

Stabilization Studies on Divalent Five‐Membered Rings C4H4C,C4H4Si,C4H4Ge,C4H4Sn,and C4H4Pb

Energy differences, ΔX_S‐t (X = E, H and G) (ΔX_S‐t = X_(singlet)‐X_(triplet)) between singlet (s) and triplet (t) states are calculated at B3LYP/6‐311++G (3df,2p). The DFT calculations show that the triplet state of C₄H₄C is a ground state with planar conformer respect to its corresponding nonplanar singlet state. Both singlet and triplet states of C₄H₄M (M = Si, Ge, Sn and Pb) have a planar conformer with the singlet ground state. Four isodesmic reactions are presented for determining the stability energies, SE. NICS calculations are carried out for C₄H₄M to determine the aromatic character.     

Photophysical Properties of Benzoylgermane and para‐Substituted Derivatives: Substituent Effects on Electronic Transitions

In the present study, a selection of basic substitution patterns on benzoyl(trimethyl)germane was investigated using time‐dependent density‐functional theory (TDDFT) to explore the influence on the stability and on the relative order of the lowest excited electronic states. The theoretical results are in agreement with absorption and fluorescence measurements. We show that electron‐withdrawing groups decrease the energetic level of the lowest singlet and triplet state relative to the electron‐pushing systems resulting in red‐shifted radiative transitions (fluorescence). In the first triplet state electron‐withdrawing groups lead to an increased dissociation barrier and a close approach with the singlet ground state before the transition state in the triplet state is reached, favoring radiationless ground‐state recovery. The results are also in good agreement with empirical concepts of organic chemistry, therefore providing simple rules for synthetic strategies towards tuning the excited‐state properties of benzoylgermanes.     

Intramolecular singlet and triplet excimers of triply bridged [3.3.N](3,6,9)carbazolophanes

Intermoiety electronic interactions in the singlet and triplet excimer states of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n=3-6) were studied by emission and transient absorption measurements. In these [3.3.n]Cz molecules, the dihedral angle and the separation distance r between fully overlapped two carbazole rings change systematically from nearly parallel (n=3, r=3.35 A) to oblique (n=6, r=4.03 A). In rigid glass at 77 K, [3.3.n]Cz (n=3, 4) (r<4 A) exhibited red-shifted and structureless excimer fluorescence and phosphorescence while [3.3.n]Cz (n=5, 6) (r>4 A) exhibited monomer-like vibrational fluorescence and phosphorescence. In solution at 130 K, all [3.3.n]Cz molecules exhibited an excimeric fluorescence band while [3.3.5]Cz still exhibited monomer-like phosphorescence. Transient absorption spectra measured at 294 K exhibited local excitation and charge-transfer bands for all [3.3.n]Cz molecules in the excited singlet and triplet states, suggesting that not only singlet but also triplet excimers of carbazole are formed at room temperature. Furthermore, the singlet-triplet energy gap decreased with the decrease in n, suggesting that electrons are effectively delocalized over the two carbazole moieties. These findings showed that both singlet and triplet excimers of carbazole are formed with a separation distance shorter than about 4 A and are most stable in the parallel-sandwich structure and that the configurational mixing between exciton resonance and charge resonance states plays an essential role in the formation of singlet and triplet excimers of carbazole.     

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S₁ state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.     

The triplet state of anthracene photodimers and the wavelength dependence of the photodissociation process

Photodimers of anthracene and 9-methylanthracene photodissociate around 300 K mainly via the excited singlet state, around 77 K exlusively via the triplet state. As an intermediate an electronically excited “tight complex” (X_s) is proposed, of which only a small fraction rearranges to an excimer. The quantum yield of excimer and triplet formation is wavelength dependent.     

有机分子三重激发态的调控与应用

对近期有机分子三重激发态调控的研究进展进行了总结评述。控制分子的三重激发态性质,可以制备多种具有新颖性质的分子,如用于可激活光动力治疗(PDT)的光敏剂、磷光分子探针与生物标识试剂,以及可控的三重态湮灭上转换等。但目前对三重态控制方面的研究相对较少,其中的规律也很不明确。近期有文献陆续报道了使用超分子方法和共价修饰法进行的三重态调控,利用的光物理过程有单重态能量转移、三重态能量转移、电子转移等等。现有研究结果表明,三重态的调控规律与单重态的调控规律有所不同,例如:发色团的单重激发态(荧光)往往可以被光诱导电子转移(PET)所猝灭,但是在多个例子中已发现,相同发色团的三重态并不能被PET所猝灭。本文总结的研究结果及所作的分析,将对该领域的分子结构设计及后续研究起到一定的促进作用。     

Singlet and triplet molecular interaction in excimers

A perturbative approach is developed to determine, term by term, the contributions of the various forces to the excimer potentials of the singlet and triplet excimers. The results show that the singlet excimer of naphthalene is more stable than the corresponding triplet excimer primarily due to large contributions of the exciton-resonance and the dispersion energy terms. The variation of the various terms with the conformations of the excimers suggests that the singlet and triplet excimers of naphthalene cannot have identical structure.     

Excited singlet yield in T-T annihilation, a comparative study of naphthalene and anthracene in solution

A method based on the study of both triplet absorption and delayed fluorescence (DF) decays after flash excitation has been developed to determine DF yields. Values of the excited singlet yield in the T-T annihilation process have been obtained for naphthalene and anthracene in solution at room temperature by application of this method. Triplet extinction coefficients and annihilation rate constants for these compounds were also determined. Excited singlet generation is the predominant annihilation channel for naphthalene whereas it is only of minor importance for anthracene. The difference in DF yields is ascribed to differences between these aromatics with respect to the efficiencies of the annihilation channels leading respectively to excited singlet and triplet states. The higher rate of excited singlet formation for naphthalene is qualitatively accounted for by considering the possibility of radiationless transitions from the bimolecular states formed in T-T interaction towards an upper excimer singlet state.     

Upconverted emission from pyrene and di-tert-butylpyrene using Ir(ppy)3 as triplet sensitizer

Metal-to-ligand charge-transfer sensitized upconverted fluorescence in noncovalent triplet energy transfer assemblies is investigated using Ir(ppy)3 as the sensitizer (ppy=2-phenylpyridine) and pyrene or 3,8-di-tert-butylpyrene as the triplet acceptor/annihilator. Upconverted singlet fluorescence from pyrene or 3,8-di-tert-butylpyrene resulting from triplet-triplet annihilation (TTA) is observed following selective excitation of Ir(ppy)3 in deaerated dichloromethane solutions using 450-nm laser pulses. In both systems, the TTA process is confirmed by the near quadratic dependence of the upconverted fluorescence intensity on incident light power, measured by integrating the upconverted delayed fluorescence kinetic traces as a function of incident excitation power. At the relatively high concentrations of pyrene that were utilized, pyrene excimer formation was detected by its characteristic broad emission centered near 470 nm. In essence, selective excitation of Ir(ppy)3 ultimately resulted in the simultaneous sensitization of both singlet pyrene and pyrene excimers, and the latter degrades the energy stored in the pyrene singlet excited state. Furthermore, in the case of di-tert-butylpyrene/Ir(ppy)3, the formation of excimers is successfully blocked because of the presence of the sterically hindering tert-butyl groups. The current work demonstrates that sensitized TTA is indeed accessible to chromophore systems beyond those previously reported, suggesting the generality of the approach.     

Photophysics of aromatic molecules with low-lying pi sigma* states: excitation-energy dependence of fluorescence in jet-cooled aromatic nitriles

Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state.     

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.     

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S₁ and T₁) and a locally excited (LE) triplet state (T₃), can be associated with the (Me → N) conformer, the other two CT-type states (S₂ and T₂) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T₃ state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S₁ and T₃ states (likewise S₂ and T₃) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

DFT/MRCI quantum chemical studies suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO.     

Charge transfer states in the photophysics and photochemistry of polyimides and related phthalimides

Polyimides such as 6F-6F and 6F-ODA and model N-arylphthalimides are stabilized against photooxidative degradation by their electron donor (D) – acceptor (A) character. We have investigated the precise origin(s) of this effect using D and A substituents on the N-aryl groups of these compounds. The lowest excited singlet state (S₁) of N-arylphthalimides is an intramolecular charge transfer (ICT) state. A nominally twisted compound, N-(2-t-butylphenyl)phthalimide, shows greatly diminished CT absorption and blue-shifted fluorescence with reduced quantum yield when compared to the 4-t-butyl isomer with an identical N-aryl donor group. It therefore seems unnecessary to claim that the ICT state of phthalimides is a so-called TICT state. Quantum yield and fluorescence lifetime measurements lead to the conclusion that enhanced internal conversion from the ICT state (S₁) to the ground state makes a significant contribution to photostabilization of these compounds by suppressing formation of the reactive triplet state. Further stabilization of polymer films may be afforded by triplet state self-quenching which is enhanced for 6F-ODA in increasingly poor solvents. N-alkylarylphthalimides in which the aryl and phthalimide groups are not formally conjugated but, rather, joined by flexible methylene ‘spacers’, exhibit a different kind of fluorescent intramolecular CT singlet state whose formation can also stabilize these compounds by suppressing triplet state formation.     

SINGLET AND TRIPLET ENERGY TRANSFER IN α-DIKETONE DERIVATIVES OF URACIL and THYMINE

Irradiation of 1-(3,4-dioxopentyl)uracil (UPD) and 1-(3.4-dioxopentyl)thymine (TPD) in acetonitrile solution at 25°C, at the wavelength (280 nm) where only the pyrimidine absorbs the light, sensitizes both fluorescence and phosphorescence of the diketone chromophore in the sidechain. From comparison of the intensity in the corrected excitation spectra with the absorption spectra in acetonitrile solution, it was estimated that the yield of singlet energy transfer in UPD was 0.17 and in TPD was 0.44. It was also observed that the ratio of phosphorescence to fluorescence was greater in the sensitized emission than in that from direct excitation of the diketone chromophore. The yield of triplet energy transfer thus measured corresponds to minimum values for the yields of intersystem crossing from singlet excited state to triplet excited state of 0.075 in the uracil chromophore of UPD and of 0.14 in the thymine chromophore of TPD. These are in agreement with other recent values for these quantities. The value of this type of system as an intramolecular triplet counter is discussed.     

Spectral and photophysical studies on cruciform oligothiophenes in solution and the solid state

The photophysical and spectroscopic properties of a new class of oligothiophene derivatives, designated as cruciform oligomers, have been investigated in solution (room and low temperature) and in the solid state (as thin films in Zeonex matrixes). The study comprises absorption, emission, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allow the determination of the rate constants for all decay processes. From these, several conclusions are drawn. First, in solution, the main deactivation channels for the compounds are the radiationless processes: S(1) --> S(0) internal conversion and S(1) --> T(1) intersystem crossing. Second, in general, in the solid state, the fluorescence quantum yields decrease relative to solution. A comparison is made with the analogous linear alpha-oligothiophenes, revealing a lower fluorescence quantum efficiency and, in contrast to the normal oligothiophenes, that internal conversion is an important channel for the deactivation of the singlet excited state. Replacement of thiophene by 1,4-phenylene units in the longer-sized cruciform oligomer increases the fluorescence efficiency. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (S(Delta) approximately 1) provides support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds.     

THE EFFECTS OF AGGREGATION ON THE FLUORESCENCE and THE TRIPLET STATE YIELD OF HEMATOPORPHYRIN

Abstract— The fluorescence and triplet state yields of hematoporphyrin have been measured in wuter/methanol mixtures. There is a closely linked response of these yields to aggregation of the hematoporphyrin molecule. The monomer, which is the principal species present at high methanol content, has a triplet state yield of 0.91 and a fluorescence yield of 0.09. By contrast, hematoporphyrin solutions with a high water content containing aggregates have lower fluorescence and triplet state yields, e.g. 0.018 and 0.56, respectively, in water. Static, singlet state quenching in some of the aggregates is responsible for the reduced fluorescence yield. The results also show that in addition to these aggregates there are other types of aggregates where there is an increased singlet to ground state radiative transition probability, resulting from the interaction between transition dipoles in adjacent molecules.