S1 Absorption Band Research Articles

Excitation-Wavelength-Dependent Photophysics of a Torsionally Disordered Push-Pull Dye.

The torsional disorder of conjugated dyes in the electronic ground state can lead to inhomogeneous broadening of the S1 ←S0 absorption band, allowing for the selective photoexcitation of molecules with different amounts of distortion. Here, we investigate how this affects electronic transitions to upper excited states. We show that torsion of a core-alkynylated push-pull dye can have opposite effects on the oscillator strength of its lowest-energy transitions. Consequently, photoselection of planar and twisted molecules can be achieved by exciting in distinct absorption bands. Whereas this has limited effect in liquids due to fast planarization of the excited molecules, it strongly affects the overall photophysics in a polymeric environment, where torsional motion is hindered, allowing for the photoselection of molecules with different fluorescence quantum yields and intersystem-crossing dynamics.

Excited-State Symmetry Breaking in Solvent Mixtures.

A large number of multipolar dyes undergo excited-state symmetry breaking (ESSB) in polar media. During this process, electronic excitation, initially distributed evenly over the molecule, localizes, at least partially, on one donor-acceptor branch. To resolve its initial stage, ESSB is investigated with a donor-acceptor-donor dye in binary mixtures of nonpolar and polar solvents using time-resolved infrared absorption spectroscopy. The presence of a few polar molecules around the dye is sufficient to initiate ESSB. Although the extent of asymmetry in a mixture is close to that in a pure solvent of similar polarity, the dynamics are slower and involve translational diffusion. However, preferential solvation in the mixtures leads to a larger local polarity. Furthermore, inhomogeneous broadening of the S1 ← S0 absorption band of the dye is observed in the mixtures, allowing for a photoselection of solutes with different local environments and ESSB dynamics.

Photoinduced Electron Transfer between Dipolar Reactants: Solvent and Excitation Wavelength Effects.

Electron transfer (ET) quenching in nonpolar media is not as well understood as in polar environments. Here, we investigate the effect of dipole-dipole interactions between the reactants using ultrafast broadband electronic spectroscopy combined with molecular dynamics simulations. We find that the quenching of the S1 state of two polar dyes, coumarin 152a and Nile red, by the polar N,N-dimethylaniline (DMA) in cyclohexane is faster by a factor up to 3 when exciting on the red edge rather than at the maximum of their S1 ← S0 absorption band. This originates from the inhomogeneous broadening of the band due to a distribution of the number of quencher molecules around the dyes. As a consequence, red-edge excitation photoselects dyes in a DMA-rich environment. Such broadening is not present in acetonitrile, and no excitation wavelength dependence of the ET dynamics is observed. The quenching of both dyes is markedly faster in nonpolar than polar solvents, independently of the excitation wavelength. According to molecular dynamics simulations, this is due to the preferential solvation of the dyes by DMA in cyclohexane. The opposite preferential solvation is predicted in acetonitrile. Consequently, close contact between the reactants in acetonitrile requires partial desolvation. By contrast, the recombination of the quenching product is slower in nonpolar than in polar solvents and exhibits much smaller dependence, if any, on the excitation wavelength.

Visible Light-Induced Direct S0 → Tn Transition of Benzophenone Promotes C(sp3)-H Alkynylation of Ethers and Amides.

Benzophenone has an S0 → S1 absorption band at 365 nm. However, the rarely reported S0 → Tn transition occurs upon irradiation at longer wavelengths. Herein, we employed benzophenone as a catalyst and exploited its S0 → Tn transition in C(sp3)-H alkynylations with hypervalent iodine reagents. The selective benzophenone excitation prevented alkynylating reagent decomposition, enabling the reaction to proceed under mild conditions. The reaction mechanism was investigated by spectroscopic and computational studies.

Ground-State Structural Disorder and Excited-State Symmetry Breaking in a Quadrupolar Molecule.

The influence of torsional disorder around the ethynyl π-bridges of a linear D-π-A-π-D molecule on the nature of its S1 excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S1 ← S0 absorption band, subpopulations with different extents of asymmetry could be excited. In nonpolar solvents, the equilibrated S1 state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is followed by planarization and symmetrization of the S1 state. Excited-state symmetry breaking is only observed in polar environments, where the equilibrated S1 state has a strong dipolar character. However, neither the extent nor the rate of symmetry breaking are enhanced in an initially distorted molecule. They are only determined by the polarity and the dynamic properties of the solvent.

Confinement-Induced Electronic Excitation Limitation of Anthracene: The Restriction of Intramolecular Vibrations

The spectroscopic behavior of anthracene encapsulated in porous zeolitic zinc imidazolate framework ZIF-8 was studied. Nanoparticles of ZIF-8 with high crystallinity and large specific surface areas were prepared in one-pot synthesis. Monitoring single anthracene encapsulation in a ZIF-8 cell was done by careful control of the synthesis of the hybrid. The molecular electronic spectra of anthracene adsorbed on the external surface of ZIF-8 were compared with those of encapsulated anthracene. It was demonstrated that the encapsulation of anthracene induced unexpected confinement effects including the restriction of intramolecular vibrations in its absorption spectrum. It was shown that vibronic components in the high-energy part of the S0 → S1 absorption band totally collapse upon encapsulation. Our study highlights the mechanism of confinement-induced electronic excitation limitation related to the restriction of intramolecular vibrations, which evidences the role of host–guest interaction in infrared abso...

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO2, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO2. In free PCA, the pyrene's forbidden S0→S1 transition has increased intensity, exhibits a long progression in 1400cm−1 Ag mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO2 causes a broadening and red shift of the S0→S1 absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Synthesis, photophysical, structural and electronic properties of novel regioisomerically pure 1,7-disubstituted perylene-3,4,9,10-tetracarboxylic monoimide dibutylester derivatives

Three new isomerically pure 1,7-disubstituted perylene-3,4,9,10-tetracarboxylic monoimide dibutylester derivatives namely; N-[2-(diethylamino)ethyl]-1,7-dibromoperylene-3,4,9,10-tetracarboxy monoimide dibutylester (PMD-1), N-[2-(diethylamino)ethyl]-1,7-di(4-tert-butylphenoxy)perylene-3,4,9,10-tetracarboxy monoimide dibutylester (PMD-2) and N-[2-(diethylamino)ethyl]-1,7-di(pyrrolidinyl)perylene-3,4,9,10-tetracarboxy monoimide dibutylester (PMD-3), have been synthesized and their electronic absorption spectra, and steady-state fluorescence were investigated experimentally as well as by using density functional theory (DFT) calculations. All three compounds show good solubility in toluene and chloroform. Attachment of two tert-butylphenoxy groups at the 1,7-positions in PMD-2 resulted in a red shifted absorption band with an absorption maximum at 518 nm. UV/Vis absorption spectrum of PMD-3 bearing electron donating pyrrolidinyl groups at the 1,7-positions shows a broad absorption band within the visible region, extending to red region. Absorption maximum of lowest energy transition now shifts to 653 nm. In addition to the S0 → S1 absorption bands, bands belonging to S0 → S2 electronic transitions were observed at 378, 386, and 411 nm for PMD-1, PMD-2, and PMD-3 respectively. Compounds PMD-1, PMD-2 and PMD-3 display low fluorescence quantum yields of 0.027, 0.040 and 0.001, respectively in chloroform. DFT calculations revealed that the attachment of electron donating groups at 1,7-positions of perylene core, results in an increase in frontier orbitals energy levels. Observed energy increase in HOMO level is larger in each case, compared to the energy increase in LUMO levels, due to the higher contribution of side groups to HOMO. DFT calculated band gaps for PMD-1, PMD-2 and PMD-3 are 2.68, 2.63 and 2.29 eV respectively.

The energy dependence of CO(v,J) produced from H2CO via the transition state, roaming, and triple fragmentation channels.

The dynamics of CO production from photolysis of H2CO have been explored over a 8000 cm-1 energy range (345 nm-266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S1←S0 absorption band, covering almost 8000 cm-1 of excitation energy. The S0 radical and TS molecular channels are the most important over this energy range. The absolute quantum yield of roaming is fairly constant ∼5% at all energies. The T1 radical channel is important (20%-40%) between 1500 and 4000 cm-1 above the H + HCO threshold, but becomes unimportant at higher energy. Triple fragmentation increases rapidly above its threshold reaching a maximum of 5% of the total product yield at the highest energy.

Conformer-specific geminate recombination following methyl nitrite photolysis in solution

The dynamics of the ultraviolet (UV) photoexcitation of methyl nitrite in weakly interacting perfluoromethylcyclohexane solution are investigated using transient absorption spectroscopy. UV excitation in the structured S1←S0 absorption band induces dissociation, with geminate recombination and vibrational cooling of syn and anti-conformers of methyl nitrate on a timescale of ∼56ps. Solvent-induced vibrational cooling favours relaxation to the higher-energy anti-conformer on entropic grounds, and subsequent inter-conversion to the lower-energy syn-conformer is prevented by a 3500cm–1 barrier. UV excitation to the S2 state produces a transient electronic absorption band resembling the absorption spectrum of NO2.

1-vinylazulenes – potential host molecules in ligands for metal ion detectors

Several vinylazulenes containing metalophilic groups were prepared as potential host molecules in ligands for metal ion detectors. The azulene-1-carbaldehyde derivatives were condensed with methylene active compounds in the presence of piperidine. The reaction is influenced by both the nature of the methylene activating groups and by the presence of the alkyl groups on the 4-,6- or 8- position of the azulene moiety. Some unusual reagents, as nitroacetate were condensed with imines activated by acetic anhydride. Several autocondensation azulenic products were evidenced in the presence of weak activated methylenes. Considerations regarding S1 and S2 absorption bands of the azulene compounds were revealed and correlated with frontier molecular orbitals. At the same time, the prepared ligands redox potentials were investigated by differential pulse voltammetry (DPV).

Understanding the torsion effects on optical properties of azobenzene derivatives

Most azobenzene derivatives require the use of ultraviolet light for photoisomerization, this limits the utilization of them in biological systems, energy-storing material fields and so on. It’s useful to develop azo-based photo-switches triggered with visible light. Three synthesized azobenzene derivatives (tetra-o-methoxyl-4,4′-diacetamidoazobenzene, 5,6-dihydrodibenzo[c,g][1,2]diazocine and 3,5-dimethyl-bisazo) who can be driven by visible light were studied by DFT calculations in this article. We found that they all have the same structural features of the twisted azobenzene trans geometries. The planarity of azobenzene, which can be reflected by ϕCNNC (the dihedral angle connecting the azo moiety and two phenyl rings) and θCCNN (the dihedral angle connecting the azo moiety and one of the phenyl rings). To better understand the relationship between structure and optical properties, absorption spectra of azobenzene in vacuum as the function of ϕCNNC and θCCNN rotation were investigated by using DFT and TDDFT methods. Calculations demonstrated that distorting ϕCNNC and θCCNN can both result in the red shift of the absorption bands to the visible light wavelength, and ϕCNNC has a greater impact than θCCNN. Furthermore, the influence of twisted structure on the absorption band of 4-(4-aminophenylazo) nitrobenzene (a D-π-A type azobenzene derivative) in DMSO solution was also studied. Our results indicated that both ϕCNNC and θCCNN(NH2) rotation decreased the HOMO–LUMO gap, causing a red shift of the S0→S1 absorption band. However, the θCCNN(NO2) rotation increased the HOMO–LUMO gap slightly, resulting in a little blue shift of the whole bands. This work may provide useful theoretical foundation for fabricating of new visible light driven azobenzene-based photo-switches.

Ultrafast Dynamics of the Indoline Dye D149 on Mesoporous ZnO and Al2O3 Thin Films

Abstract Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a “non-injecting” mesoporous thin film with large band-gap, consisting of Al2O3 nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250–350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0 → S1 absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+ [N(CN)2] – show that D149 is ideally suited for solar cell applications because of its long S1 lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential.

Synthesis, photophysical and electrochemical properties of aza-boron-diquinomethene complexes

A series of aza-boron-diquinomethene (aza-BODIQU) complexes with different aryl-substituents (B1–B6) were synthesized and characterized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. All complexes exhibit strong 1π–π* absorption bands and intense fluorescent emission bands in the visible spectral region at room temperature. The fluorescence spectra in solution show the mirror image features of the S0→S1 absorption bands, which can be assigned to the 1π–π*/1ICT (intramolecular charge transfer) emitting states. Except for B6, all complexes exhibit high photoluminescence quantum yields (ΦPL = 0.47–0.93). The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these aza-BODIQUs can be tuned by the appended aryl-substituents, which would be useful for rational design of boron–fluorine complexes with high emission quantum yield for organic light-emitting applications.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 μs, 170 μs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.

Photophysics of hydrogen bonded diarylethene dimers in the liquid phase

Hydrogen bonded diarylethene dimers are formed through a carboxylic acid ligand, as is evident from concentration dependent OH stretching infrared spectra. Dimer formation leads to a red shift, enhancement and broadening of OH stretching absorption characteristic for cyclic carboxylic acid dimers. We compare the electronic spectra and photoinduced ring-opening and -closing kinetics of dimers and monomers. The S0–S1 absorption band of the closed form undergoes a slight red-shift when going from the monomer to the singly closed dimer and the doubly closed dimer. The photoreaction kinetics of dimer and monomer are identical. Consequently, the photochrome retains its original photophysical properties in the dimer state.

Photophysics of fluorinated benzene. III. Hexafluorobenzene

A theoretical study of the photoabsorption spectroscopy of hexafluorobenzene (HFBz) is presented in this paper. The chemical effect due to fluorine atom substitution on the electronic structure of benzene (Bz) saturates in HFBz. State- of-the-art quantum chemistry calculations are carried out to establish potential energy surfaces and coupling surfaces of five energetically low-lying electronic (two of them are orbitally degenerate) states of HFBz. Coupling of these electronic states caused by the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type of interactions are examined. The impact of these couplings on the nuclear dynamics of the participating electronic states is thoroughly investigated by quantum mechanical methods and the results are compared with those observed in the experiments. The complex structure of the S(1) ← S(0) absorption band is found to originate from a very strong nonadiabatic coupling of the S(2) (of πσ* origin) and S(1) (of ππ* origin) state. While S(2) state is orbitally degenerate and JT active, the S(1) state is nondegenerate. These states form energetically low-lying conical intersections (CIs) in HFBz. These CIs are found to be the mechanistic bottleneck of the observed low quantum yield of fluorescence emission, non overlapping absorption, and emission bands of HFBz and contribute to the spectral width. Justification is also provided for the observed two peaks in the second absorption (the unassigned "c band") band of HFBz. The peaks observed in the third, fourth, and fifth absorption bands are also identified and assigned.

The Kinetics of Exogenous Phosphors Delayed Fluorescence in Tissues

The kinetics of delayed fluorescence (DF) and phosphorescence (Ph) of xanthene dyes in tumourous and normal mammary tissues of the BYRB line mice was investigated. Spontaneous mammary cancer tumours are characteristic of the mice of this line with the possible exogenous MMTV- retrovirus, found in leukocytic fraction. The kinetics of DF and Ph molecules of the dyes was measured by means of laser flash-photolysis. The molecules in basic S0→S1 absorption band were excited by the second harmonic of pulsed laser to YAG:Nd3+ with wave length 532 nm. At the expiration of the process S1→T1 intersystem crossing (IC) and the formation of triplet T1 molecule state, the registration system of delayed luminescence started. The luminescence was registered by PMT and monochromator. The parameters of the exciting pulse: pulse duration 10 ns, density of energizing power not more than 5 MWt/cm2. The DF was registered at the wave length of 560 nm, and Ph at the wave length of 680 nm. The DF of the examined molecules can occur both as the result of termoactivated reverse to T1→S1 IC and as the result of the two T1 states annihilation with the sequent formation of S1 states. Moreover under the certain conditions in the presence of oxygen the cross – annihilation of the dye and oxygen stimulated molecules can occur. It was demonstrated that the most effective quencher of the triplet T1 states of phosphors in the tissues is the molecular oxygen 3!g"(O2). In the result of the interaction of the dye T1 molecules with the molecular oxygen singlet 1∆g (O2) oxygen is formed: T1+3!g"(O2)#$#S0+1%g(O2). Then as the result of singlet-triplet T1→1∆g(O2) annihilation of the rest triplet T1 states of the dyes with 1∆g(O2): T1+1!g(O2)"#"S1+3$g%(O2) the singlet S1 states of dyes are formed, thus contributing to the DF. As a result the registered kinetics of the DF of the dyes is made up of the three signals of various nature: termoactivated DF; T1-T1 annihilated DF; and luminescence due to the singlet-triplet T1→1∆ g (O2) cross–annihilation. The kinetic curve transforms and becomes no monotonously dependent on time. It was specified that in the tissues during short periods of time the most significant contribution to the total signal was made by singlet-triplet T1→1∆ g (O2) annihilation. However, other things equal, the delayed luminescence kinetics in normal and pathogenous tissues differ. The contribution of the singlet-triplet annihilation to the total signal of the DF in the normal tissues is significantly less than in tumour, which indicates the less effectiveness of the triplet states of the dyed molecules interaction with the singlet oxygen. Unlike DF the Ph intensity of dye molecules decreases monotonously with the period of time. Phosphorescence kinetics as well as DF differs within different tissues. The luminescence peculiarity depends on the phase of the tumour, the biotissue condition and other factors. However in all our experiments the common regularity is evident, namely the Ph lifetime in tumours is shorter than that in normal tissues. Reliable registered differences in the dye tissue delayed luminescence kinetics can be used when developing an alternative method of optical diagnosis of biotissues. We assume that the method based on the measurement of lifetime of the delayed luminescence phosphor is fairly promising. Any combinations possessing delayed luminescence and meeting the requirements set to such specimen may serve as exogenous phosphor.

Photophysical properties and vibrational structure of ladder-type penta p-phenylene and carbazole derivatives based on SAC-CI calculations

The π-conjugated ladder-type molecules constitute an attractive field of organic photoactive materials. In this work, the photophysical properties of ladder-type penta-p-phenylene (LPP) and carbazole derivatives (bisindenocarbazole and diindolocarbazole) have been investigated theoretically using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The equilibrium geometries in the ground (S 0) and first excited (S 1) states were calculated to be planar, and the excitation is delocalized over the molecules. SAC-CI/DZP calculations have been applied to the absorption and emission spectra of these molecules. The absorption spectra were well reproduced in both peak positions and the shape of the absorption bands. The strong absorption is attributed to the highest occupied molecular orbital to the lowest unoccupied molecular orbital (H–L) transition; however, in carbazoles, the H–1→L transition is located below the H–L transition. The vibrational structure in the S 0–S 1 absorption band of LPP was analyzed by calculating the Franck–Condon (FC) factors based on the potential energy surfaces (PESs) along the normal coordinates that are relevant to the geometry change. The vibrational structure was well reproduced by the theoretical simulation. The C–C stretching mode dominantly contributes to the vibrational structure, while the breathing motion of the molecular frame does not influence the structure. The emission energies calculated by the SAC-CI method also agree well with the experimental values. The vibrational structure in the fluorescence band was also examined by the FC analysis; the theoretical spectrum is satisfactory for the two carbazoles, while the 0–0 transition is overestimated in LPP. In diindolocarbazole, the S 2 state has a large oscillator strength, while the S 1 state has a small oscillator strength.

Syntheses, crystal structures, photophysical and theoretical studies of 1,3,2-benzodiazaborolyl-functionalized diphenylacetylenes

A series of diphenylacetylenes with one 1,3,2-benzodiazaborolyl end group (BDB) and a second end group X (X = H, OMe, NMe(2), SMe, CN and BDB) were synthesized using established 1,3,2-benzodiazaborole methodologies. The 1,3,2-benzodiazaborolyldiphenylacetylenes with X = p-H (4), p-OMe (5), p-NMe(2) (6), p-SMe (7) and p-CN (8) end groups are functionalized with cyano groups at the central ring in an ortho-position to the triple bond. Molecular structures of 2, 3, 5, 6 and 7 were determined by X-ray diffraction. These borylated systems show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran, whereas green luminescence was observed in acetonitrile solutions. Thereby Stokes shifts in the range 1700-8600 cm(-1) and quantum yields of 0.60-1.00 were observed in cyclohexane solutions. The absorption maxima (308-380 nm) are well reproduced by TD-DFT computations (B3LYP/G-311G(d,p)) and arise from strong HOMO-LUMO transitions. The LUMOs in all the molecules under study are mainly located on the diphenylacetylene bridge, while with the exception of the dimethylamino derivative 6, the HOMO is largely benzodiazaborolyl in character. Thus, the S1←S0 absorption bands are assigned to π(diazaborolyl)-π*(diphenylacetylene) transitions. In contrast to this, in compound 6 the HOMO is mainly represented by the terminal dimethylaminophenyl unit. While calculated ground state dipole moments μ(g) are small (1.1-7.5 D), experimentally determined changes of the dipole moments upon excitation are large (14.8-19.7 D) and reflect a significant charge transfer upon excitation. NLO activities of the rod-structured compounds 2, 4, 6 and 8 are indicated by calculated static first-order hyperpolarizabilities β up to 76.8 × 10(-30) esu.