Lifetime Of Excited Singlet State Research Articles

Presenting a new fluorescent probe, methyl(10-phenylphenanthren-9-yl)sulfane sensitive to the polarity and rigidity of the microenvironment: applications toward microheterogeneous systems.

A molecule, methyl(10-phenylphenanthren-9-yl)sulfane (MPPS), with a straightforward structure, has been synthesized, characterized, and explored as a new fluorescent probe for microheterogeneous systems. The photophysical properties of MPPS have been studied through experimental and theoretical calculations using the range-separated hybrid functional CAM-B3LYP in conjunction with a 6-311++g(d,p) basis set. Theoretical calculations show that the freely rotating phenyl ring forms a 94° dihedral angle with the phenanthrene ring in the ground state. Experimentally found two absorption bands correspond to the n → π* and π → π* transitions supported by the frontier molecular orbital calculations. Two excited singlet states, E-1 and E-2 (the former being more stable than the latter in the gas phase), exist with dihedral angles between the phenyl and phenanthrene rings as 142° and 133°, respectively, in the gas phase. Two emitting states in a condensed medium of varying polarities are supported by the steady-state fluorescence and fluorescence intensity decay data. Emission energies, fluorescence intensities, and excited singlet state lifetimes change with the polarity of the solvents. To support that the free rotation in the molecule is responsible for these changes, the fluorescence properties of another molecule, methyl(10-(o-tolyl)phenanthren-9-yl)sulfane (MTPS), with restricted rotation of the substituted benzene, i.e., o-tolyl ring have been studied. The fast-intensity decay component of MPPS is ascribed to the conformer in the E-1 state. The molecule has proved to be an excellent polarity probe explored to determine the critical micelle concentrations (cmc) values of different surfactants, which agree well with the literature reports. Different regions of binding isotherm (specific, non-cooperative, cooperative, and massive binding) of a gemini surfactant, 12-6-12,2Br- with bovine serum albumin (BSA) have been successfully demonstrated by the steady-state and time-resolved fluorescence and fluorescence anisotropic properties of MPPS. Docking results show that MPPS resides in the hydrophobic pocket of BSA. The fluorescence quenching of BSA by MPPS reveals the location of Trp residues of BSA. Thus, a polarity and molecular rigidity-sensitive fluorescent molecule, MPPS has been presented here that can potentially be used to monitor the changes in the microenvironment of biomolecules in different processes.

Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands.

The reaction of two equivalents of N,N'-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (H2bppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda)2. Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (ΦPL = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by 3O2 and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S1, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T1. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25-30% for the S1 and T1 states. A comparison of Zr(bppda)2 to related zirconium pyridine dipyrrolide complexes, Zr(PDP)2, revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda)2, which were correlated to differences in the photophysical properties between the two compound classes.

Modulation of lanthanide luminescence by carbamoylmethylphosphine oxide ligand: A theoretical study

The antenna capacity of Phenacyldiphenylphosphine oxide (Phenac), combining C=O and P=O donor groups was established by theoretical examination of its chromophore properties, binding ability to encapsulate and stabilize lanthanide complexes and potential to sensitize the EuIII and TbIII luminescence in Eu(Phenac)2(NO3)3(H2O) (1), Eu(Phenac)2(NO3)3 (2), Eu(Phenac)3(NO3)3 (3) and Tb(Phenac)2(NO3)3(H2O) (4) complexes in vacuum, solution and solid phase. The ligand-centered singlet and triplet excited state energies, the rates and lifetimes for the rival nonradiative (S1→T1,2 intersystem crossing (ISC)) and radiative and nonradiative S1→S0 and T1→S0 relaxation were predicted by means of DFT/TDDFT/ωB97XD calculations. Similar relaxation paths were found for all complexes. The absorbed energy by Sn > 1(π-π*) states relaxed to T1min by major ISC mechanism S1→T2→T1. The long radiative lifetime of S1→S0 due to n(p(C)(O))π-π*,p(P) character facilitated the forward ISC process. The environment, Phenac's number and LnIII size affect the T1 energy ∼0.1 eV. The calculated (Phenac→EuIII) energy transfer rates suggested the most likely T1 → 5D1/5D0 channels for a population of emitting metal-centered states. The small Φ of EuIII and larger luminescence of TbIII were discussed and explained. The importance of the P=O and C=O groups for the coordination ability and absorption properties of Phenac has been elucidated.

Carbazole appended trans-dicationic pyridinium porphyrin finds supremacy in DNA binding/photocleavage over a non-carbazolyl analogue.

A carbazolyl appended trans-pyridyl porphyrin (1) was synthesized and its dicationic form 2 was obtained by methylation of the pyridyl group. Copper and zinc complexes of porphyrin 2 (Cu(II), 3; Zn(II), 4) were isolated and characterized by various modern spectroscopic techniques. The DNA binding properties of 2, 3, and 4 have been explored against calf thymus-DNA (CT-DNA). DNA binding was quantized using the intrinsic binding constant (Kb) that was calculated by UV-visible absorption spectroscopy, and the value Kb = 1.6 × 106 M-1 for compound 2 reveals a better interaction of 2 towards CT-DNA than those of 3 (3.1 × 105 M-1) and 4 (3.4 × 105 M-1), which follows the order 2 > 4 > 3. The fluorescence quenching efficiency and ethidium bromide quenching assay also indicated a good binding affinity of all the compounds towards CT-DNA. Furthermore, the spectroscopic data suggest that the possible mode of interaction is intercalation. The docking studies were in accordance with the experimental results. Notably, DNA cleavage studies reveal that 2 shows better damage than 3 and 4 which is in accordance with the binding affinity order 2 > 4 > 3. The observed quantum yield (2: 0.65, 3: 0.33, and 4: 0.97) and no change in DNA cleavage in the presence of NaN3 reveal the involvement of singlet oxygen. The singlet excited state lifetimes were in the range of 6.3-1.2 ns. Furthermore, these porphyrins can be investigated as interesting photosensitizers in photodynamic therapy and photochemotherapy.

Fundamental electronic changes upon intersystem crossing in large aromatic photosensitizers: free base 5,10,15,20-tetrakis(4-carboxylatophenyl)porphyrin.

Free base 5,10,15,20-tetrakis(4-carboxylatophenyl)porphyrin stands for the class of powerful porphyrin photosensitizers for singlet oxygen generation and light-harvesting. The atomic level selectivity of dynamic UV pump – N K-edge probe X-ray absorption spectroscopy in combination with time-dependent density functional theory (TD-DFT) gives direct access to the crucial excited molecular states within the unusual relaxation pathway. The efficient intersystem crossing, that is El-Sayed forbidden and not facilitated by a heavy atom is confirmed to be the result of the long singlet excited state lifetime (Qx 4.9 ns) and thermal effects. Overall, the interplay of stabilization by conservation of angular momenta and vibronic relaxation drive the de-excitation in these chromophores.

Toward efficient photochemistry from upper excited electronic states: Detection of long S2 lifetime of perylene.

A long 0.9ps lifetime of the upper excited singlet state in perylene is resolved by femtosecond pump-probe measurements under ultraviolet (4.96eV) excitation and further validated by theoretical simulations of transient absorption kinetics. This finding prompts exploration and development of novel perylene-based materials for upper excited state photochemistry applications.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl-β,β'-dioxo-bacterio- and -isobacteriochlorin Series.

It is well-known that treatment of β-octaethylporphyrin with H2 O2 /conc. H2 SO4 converts it to a β-oxochlorin as well as all five constitutional isomers of the corresponding β,β'-dioxo-derivatives: two bacteriochlorin-type isomers (β-oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin-type isomers (β-oxo-groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled β-oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X-ray structures of all free-base diketone isomers and a comparative description of their UV-vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying β-oxo functionalities. This comparative study lays the basis for their further study and utilization.

Electronic Structure and Excited-State Dynamics of Rylene-Tetrapyrrole Panchromatic Absorbers.

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (τS) and increased fluorescence quantum yield (Φf) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a π-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Φf and shortened τS values. The PMI-P-TMI triad in toluene exhibits Φf = 0.038 and τS = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φf = 0.26 and τS = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.

Conjugated-linker dependence of the photophysical properties and electronic structure of chlorin dyads

The synthesis, photophysical properties and electronic structure of seven new chlorin dyads and associated benchmark monomers are described. Each dyad contains two identical chlorins linked at the macrocycle [Formula: see text]-pyrrole 13-position. The extent of electronic communication between chlorin constituents depends on the nature of the conjugated linker. The communication is assessed by modification of prominent ground-state absorption and redox properties, rate constants and yields of excited-state decay processes, and molecular-orbital characteristics. Relative to the benchmark monomers, the chlorin dyads in toluene exhibit a substantial bathochromic shift of the long-wavelength absorption band (30 nm average), two-fold increased radiative rate constant [average (10 ns)[Formula: see text] vs. (22 ns)[Formula: see text]], reduced singlet excited-state lifetimes (average 5.0 ns vs. 8.2 ns), and increased fluorescence quantum yields (average 0.56 vs. 0.42). The excited-state lifetime and fluorescence yield for the chlorin dyad with a benzothiadiazole linker are reduced substantially in benzonitrile vs. toluene due largely to [Formula: see text]25-fold accelerated internal conversion. The results aid design strategies for molecular architectures that may find utility in solar-energy conversion and photomedicine.

Solvent Effects and Side Reactions in Organocatalyzed Atom Transfer Radical Polymerization for Enabling the Controlled Polymerization of Acrylates Catalyzed by Diaryl Dihydrophenazines.

Investigation of the effects of a solvent on the photophysical and redox properties of the photoredox catalyst (PC), N,N-di(2-naphthyl)-5,10-dihydrophenazine (PC 1), revealed the opportunity to use tetrahydrofuran (THF) to modulate the reactivity of PC 1 toward achieving a controlled organocatalyzed atom transfer radial polymerization (O-ATRP) of acrylates. Compared with dimethylacetamide (DMAc), in tetrahydrofuran (THF), PC 1 exhibits a higher quantum yield of intersystem crossing (ΦISC = 0.02 in DMAc, 0.30 in THF), a longer singlet excited-state lifetime (τ Singlet = 3.81 ns in DMAc, 21.5 ns in THF), and a longer triplet excited-state lifetime (τ Triplet = 4.3 μs in DMAc, 15.2 μs in THF). Destabilization of 1 •+, the proposed polymerization deactivator, in THF leads to an increase in the oxidation potential of this species by 120 mV (E 1/2 0 = 0.22 V vs SCE in DMAc, 0.34 V vs SCE in THF). The O-ATRP of n-butyl acrylate (n-BA) catalyzed by PC 1 proceeds in a more controlled fashion in THF than in DMAc, producing P(n-BA) with low dispersity, Đ (Đ < 1.2). Model reactions and spectroscopic experiments revealed that two initiator-derived alkyl radicals add to the core of PC 1 to form an alkyl-substituted photocatalyst (2) during the polymerization. PC 2 accesses a polar CT excited state that is ~40 meV higher in energy than PC 1 and forms a slightly more oxidizing radical cation (E 1/2 0 = 0.22 V for 1 •+ and 0.25 V for 2 •+ in DMAc). A new O-ATRP procedure was developed wherein PC 1 is converted to 2 in situ. The application of this method enabled the O-ATRP of a number of acrylates to proceed with moderate to good control (Đ = 1.15-1.45 and I* = 83-127%).

Photophysical Properties and Electronic Structure of Zinc(II) Porphyrins Bearing 0-4 meso-Phenyl Substituents: Zinc Porphine to Zinc Tetraphenylporphyrin (ZnTPP).

Six zinc(II) porphyrins bearing 0-4 meso-phenyl substituents have been examined spectroscopically and theoretically. Comparisons with previously examined free base analogues afford a deep understanding of the electronic and photophysical effects of systematic addition of phenyl groups in porphyrins containing a central zinc(II) ion versus two hydrogen atoms. Trends in the wavelengths and relative intensities of the absorption bands are generally consistent with predictions from time-dependent density functional theory calculations and simulations from Gouterman's four-orbital model. These trends derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital. The calculations reveal additional insights, such as a progressive increase in oscillator strength in the violet-red (B-Q) absorption manifold with increasing number of phenyls. Progressive addition of 0-4 phenyl substituents to the zinc porphyrins in O2-free toluene engenders a reduction in the measured lifetime of the lowest singlet excited state (2.5-2.1 ns), an increase in the S1 → S0 fluorescence yield (0.022-0.030), a decrease in the yield of S1 → T1 intersystem crossing (0.93-0.88), and an increase in the yield of S1 → S0 internal conversion (0.048-0.090). The derived rate constants for S1 decay reveal significant differences in the photophysical properties of the zinc chelates versus free base forms. The unexpected finding of a larger rate constant for internal conversion for zinc chelates versus free bases is particularly exemplary. Collectively, the findings afford fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins, which are widely used as benchmarks for tetrapyrrole-based architectures in solar energy and life sciences research.

Theoretical Investigation on Copper(I) Complexes Featuring a Phosphonic Acid Anchor with Asymmetric Ligands for DSSC

A series of heteroleptic copper­(I) complexes featuring a phosphonic acid anchor with asymmetric ligands has been studied using density functional theory (DFT) and time-dependent DFT (TD-DFT). Results showed that the absorption spectra of all copper­(I) complexes covered the visible light region with typical metal-to-ligand charge transfer characteristics. The modification of functionalized asymmetric ligands by introducing phenyl, anisole, N,N-diethyl-4-vinylaniline, and bromobenzene groups can reduce the energy gap (ΔH–L), form an effective charge-separated state, and lead to red-shifted absorption within the 350–650 nm range compared to the reference complex. The copper­(I) complexes with functionalized N,N-diethyl-4-vinylaniline ligands displayed the smallest ΔH–L and highest light-harvesting efficiency (LHE), exhibiting excellent light-harvesting capabilities. The introduction of isoquinoline ligands increased the excited singlet state lifetimes, the number of transferred electrons, and electron transfer distances. The electron injection time and driving force revealed efficient interfacial electron injection and regeneration of oxidized dyes. The photoelectronic properties of copper­(I) complexes featuring a pyridine ring with asymmetric ancillary ligands were superior to those of the reference complex. These copper­(I) complexes exhibited desirable electronic and spectral characteristics for future DSSC applications.

Improving triplet excited-state absorption and lifetime of cationic iridium(III) complexes by extending π-conjugation of the 2-(2-quinolinyl)quinoxaline ligand

The synthesis and photophysical properties (UV−vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C^N)2Ir(R-quqo)+ (HC^N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno[4,5-d]imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV–vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending π-conjugation of the C^N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized 1π,π transitions, broad and structureless metal-to-ligand charge transfer (1MLCT) / ligand-to-ligand charge transfer (1LLCT) transitions, and very weak spin-forbidden 3MLCT/3LLCT/3π,π transitions in their UV–vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer (1ILCT) / 1π,π band at 430–550 nm. The predominant 3ILCT/3π,π characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520−800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 >Ir-2 ≈ Ir-3 >Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.

Excited-State Properties of Metal-Free ((Z)-2-Cyano-3-(4-((E)-2-(6-(4-methoxyphenyl)-9-octyl-9H-carbazol-3-yl)vinyl)phenyl)acrylic Acid and (E)-2-Cyano-3-(4-((E)-4-(diphenylamino)styryl)phenyl)acrylic Acid) and Ru-Based (N719 and Z907) Dyes and Photoinduced Charge Transfer Processes in FTO/TiCl4/TiO2/Dye Photoanodes Fabricated by Conventional Staining and Potential-Assisted Adsorption.

Excited-state properties of two novel metal-free custom-made dyes D2d [(Z)-2-cyano-3-(4-((E)-2-(6-(4-methoxyphenyl)-9-octyl-9H-carbazol-3-yl)vinyl)phenyl)acrylic acid] and T-SB-C [(E)-2-cyano-3-(4-((E)-4-(diphenylamino)styryl)phenyl)acrylic acid] and two commercially available Ruthenium-based N719 and Z907 dyes were investigated with application of time-resolved absorption and emission. Singlet excited state lifetimes of D2d and T-SB-C were determined in acetonitrile and are 1.4 and 2.45 ns, respectively. The 3MLCT state lifetimes of N719 and Z907 dyes determined in methanol are 9.25 and 8.85 ns, respectively. Subsequently, photoexcited processes like electron injection and charge recombination were studied for those dyes adsorbed on the FTO/TiCl4/TiO2 photoanodes and fabricated via a conventional staining technique and innovative potential-assisted fast dye staining method. The dynamics of the spectro-temporal data was determined with application of single-wavelength and global fitting. All dye-TiO2 systems showed fast picosecond injection of excited electrons to the conduction band of the TiO2 layer and in complex multiphasic charge recombination processes. The dynamics of those processes is not altered by the dye adsorption method.

Spectroscopic and photophysical properties of mono- and dithiosubstituted BODIPY dyes

A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes, substituted at the 3 and 5 positions was synthesized and their spectral and photophysical properties were investigated in solvents of different polarity and polarizability. All compounds are characterized by strong absorption bands at 500–580 nm, corresponding to S0 → S1 transition, the typical for the BODIPY chromophore. Emission spectra are mirror images to absorption with maxima at 520–600 nm. Substitution of the BODIPY chromophore by Cl or thioalkyl substituents induces batochromic shifts in both absorption and emission spectra for 50–60 nm and induces higher quantum yields of fluorescence (Φf = 0.1–0.7). The findings were rationalized by TD-DFT computations. The dyes exhibit weak solvatochromic properties which could not be correlated to solvent polarizability, contrary to majority of BODIPY derivatives. Furthermore, estimated singlet excited state lifetimes (τ = 2–6 ns) reveal that radiative and non-radiative rate constants for deactivation from S1 do not vary with changes in solvent polarity/polarizability. However, the highest values of Φf and τ are generally found in solvents of high polarizability. Presented spectroscopic and photophysical study is important for the use of dyes as chemodosimeters for thiols and cystein, as well as for the rational design of new molecular probes.

Molecular 5,8-π-extended quinoxaline derivatives as chromophores for photoluminescence applications

The 5,8-π-extended quinoxaline derivatives are widely studied due to their well-known photophysical and electrochemical properties. In order to investigate the structure-property relationship, a novel series of fluorescent calamitic liquid crystals based on the quinoxaline heterocycle was successfully synthesized and characterized. The final molecules presented calamitic mesomorphism with nematic and smectic phases. These compounds displayed intense green photoluminescence under UV light excitation in solution and in the solid state. In chloroform solution, the fluorescence quantum yields (ΦFL = 0.54–0.62) of the quinoxaline-based derivatives were significantly higher than those previously described for similar benzothiadiazoles. The maximum emission peaks were between 511 and 520 nm with singlet excited-state lifetimes in the nanosecond timescale. The solvatochromism studies showed a significant dependence of the emission on the polarity of the solvent. Doping of the quinoxalines with fullerene C60 suggests a charge transfer process, this being dependent on the π-conjugate core. The energy band gaps predicted with DFT calculations are in excellent agreement with the experimental data.

Photophysics of BODIPY-Based Photosensitizer for Photodynamic Therapy: Surface Hopping and Classical Molecular Dynamics.

Halogenated BODIPY derivatives are emerging as important candidates for photodynamic therapy of cancer cells due to their high triplet quantum yield. We probed fundamental photophysical properties and interactions with biological environments of such photosensitizers. To this end, we employed static TD-DFT quantum chemical calculations as well as TD-DFT surface hopping molecular dynamics on potential energy surfaces resulting from the eigenstates of the total electronic Hamiltonian including the spin-orbit (SO) coupling. Matrix elements of an effective one-electron spin-orbit Hamiltonian between singlet and triplet configuration interaction singles (CIS) auxiliary wave functions are calculated using a new code capable of dealing with singlets and both restricted and unrestricted triplets built up from up to three different and independent sets of (singlet, alpha, and beta) molecular orbitals. The interaction with a biological environment was addressed by using classical molecular dynamics (MD) in a scheme that implicitly accounts for electronically excited states. For the surface hopping trajectories, an accelerated MD approach was used, in which the SO couplings are scaled up, to make the calculations computationally feasible, and the lifetimes are extrapolated back to unscaled SO couplings. The lifetime of the first excited singlet state estimated by semiclassical surface hopping simulations is 139 ± 75 ps. Classical MD demonstrates that halogenated BODIPY in the ground state, in contrast to the unsubstituted one, is stable in the headgroup region of minimalistic cell membrane models, and while in the triplet state, the molecule relocates to the membrane interior ready for further steps of photodynamic therapy.

Effects of Intra- and Intermolecular Hydrogen Bonding on O-H Bond Photodissociation Pathways of a Catechol Derivative.

The catechol functional motif is thought to play both a structural and photochemical role in the ubiquitous natural pigment, eumelanin. Intramolecular and intermolecular hydrogen bonding interactions lead to a variety of geometries involving the two O-H groups in catechol, but its photophysical behavior in these situations has not been comprehensively characterized. Toward this end, we monitor the UV-induced O-H bond photodissociation reaction in an exemplar catechol derivative, 4- tert-butylcatechol, possessing different intramolecular and intermolecular hydrogen bonding geometries using femtosecond transient absorption spectroscopy measurements in the UV-visible and mid-infrared regions following 265 nm photoexcitation. Three different hydrogen bonding arrangements are obtained by tuning solution complexation equilibria of the catechol with the hydrogen bond acceptor, diethyl ether (Et2O), and are verified computationally. We find that intermolecular hydrogen bonding to the free O-H group in catechol increases its first excited singlet state (S1) lifetime by 2 orders of magnitude (i.e., ∼ 16 to 1410 ps), and that O-H bond dissociation is prevented because Et2O is a poor hydrogen atom acceptor. Complexation of both O-H groups with multiple Et2O molecules further elongates the S1 lifetime to 1670 ps due to shifting of the solution equilibria that describe complex formation. Weakening of the characteristic, intramolecular hydrogen bond of the catechol derivative by intermolecular hydrogen bonding to one or more Et2O molecules does not enhance the rate of O-H bond dissociation.

Magnesium Tetra(phenylethynyl)porphyrin: Stepwise Synthetic Route, Crystal Structures, and Longer Singlet Excited-State Lifetime than Zinc Congener.

Magnesium tetra(arylethynyl)porphyrins (aryl=Ph or 4-CF3 C6 H4 ) were synthesized via 5,15-di(triisopropylsilylethynyl)-10,20-di(arylethynyl)porphyrin to ensure good solubility and high synthetic yields. Magnesium tetra(phenylethynyl)porphyrin was subjected to structural analyses and physico-chemical characterization. Single-crystal X-ray analysis revealed porous crystal structures featuring solvent molecules in their pores. From femtosecond transient absorption measurements we concluded that the singlet excited-state lifetime of magnesium tetra(phenylethynyl)porphyrin is with 7.4 ns substantially longer than that of its zinc congener with 2.8 ns; this is attributed to the lower atomic weight of magnesium compared with zinc.

Boosting Transport Distances for Molecular Excitons within Photoexcited Metal-Organic Framework Films.

Herein, we describe the fabrication of porphyrin-containing metal-organic framework thin films with 1,4-diazabicyclo[2.2.2]octane (DABCO) pillaring linkers and investigate exciton transport within the films. Steady-state emission spectroscopy indicates that the exciton can traverse up to 26 porphyrin layers when DABCO is used as a pillaring linker, whereas on average only 9-11 layers can be traversed when either 4,4'-bipyridine (a pillaring linker) or pyridine (a nonpillaring, layer-interdigitating ligand) is used. These results can be understood by taking into account the decreased separation distances between transition dipoles associated with chromophores (porphyrins) sited in adjacent layers. Shorter distances translate into faster Förster-type exciton hopping and, therefore, more hops within the few nanosecond lifetime of the porphyrin's singlet excited-state. The findings have favorable implications for the development of MOF-based photoelectrodes and photoelectrochemical energy-conversion devices.