Synthesis, excitation energy transfer and singlet oxygen photogeneration of covalently linked N-confused porphyrin-porphyrin and Zn(II) porphyrin dyads

Dyads of a N-confused porphyrin (NCP) moiety covalently linked to a porphyrin free-base (H₂P) or a zinc(II) porphyrinate (ZnP) moiety via a flexible alkyl chain of variable length have been synthesized. Photoluminescence study demonstrated an efficient excitation energy transfer from H₂P/ZnP moiety to the NCP moiety. Measurement of the near-IR emission of singlet oxygen produced by these dyads via photosensitization showed that the NCP-ZnP dyads (Ф_Δ = (0.61-0.65) ± 0.13) were better ¹O₂ generators than the NCP-P dyads (Ф_Δ = (0.36-0.41) ± 0.08).     

Femtosecond excitation energy transport in triarylamine dendrimers

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.     

Insights into the isomerization of photochromic oxazines from the excitation dynamics of BODIPY-oxazine dyads

We synthesized five BODIPY-oxazine dyads in one to four synthetic steps from known precursors. They differ in the nature of the unsaturated spacer linking the oxazine photochrome to either the conjugated framework or the boron center of the BODIPY fluorophore. Despite the π-character of the linkers, the two functional components are electronically isolated in the ground state and the BODIPY fluorophore maintains its absorption and, with one exception, emission properties unaltered. Instead, the photochemical response of the photochromic component is completely suppressed within all dyads. Rather than the expected opening of the oxazine ring, the laser excitation of these molecular assemblies results in the effective population of the BODIPY triplet in four of the five dyads. Control experiments with appropriate model compounds indicate that the local excitation of the oxazine component results first in intersystem crossing and then energy transfer to the BODIPY component. In fact, the transfer of energy from the triplet state of the former to the triplet state of the latter competes successfully with the opening of the oxazine ring and prevents the isomerization of the photochromic component. These observations demonstrate, for the very first time, that the photoinduced opening of these photochromic oxazines occurs along the potential energy surface of their triplet state. Such valuable mechanistic insights into their excitation dynamics can guide the design of novel members of this family of photochromic compounds with improved photochemical properties.     

Fluorescence excitation spectra and exited state dynamics of jet-cooled oxalyl halides

We have observed the $ {\tilde{\text{A}}}^{1} {\text{A}}_{\text{u}} \leftarrow {\tilde{\text{X}}}^{ 1} {\text{A}}_{\text{g}} $ fluorescence excitation spectra of jet-cooled oxalyl halides, (COR)₂, where R = F, Cl, and compared them with corresponding gas-phase absorption spectra obtained earlier. As a result, we have found some peculiarities of the excited state dynamics of the molecules under study: high effective fluorescence for oxalyl fluoride molecules excited to the single vibronic levels of b _g symmetry and high efficiency of radiationless transitions for molecules excited to the single vibronic levels of a _g symmetry. For oxalyl chloride, it has been found very intensive 7 ₀ ² 8 ₁ ⁰ (but not 8 ₀ ¹ or 8 ₁ ¹ ) hot transition. These results are compared with data for glyoxal, (COH)₂, obtained earlier.     

Forward and reverse excitation energy transport in concentrated two-component systems

The experimental and numerical study of the forward and reverse energy transport in concentrated two-component solutions is presented. Experimental results are compared with those of diagrammatic and hopping theories. It is shown that the effect of direct acceptor excitation by the light beam is important in the presence of the reverse transfer and it must be taken into account to describe correctly the experimental results within the framework of any theoretical model. This effect leads in particular to the decrease in the donor emission anisotropy and to the increase in its quantum yield. Our analysis indicates the advantage of the diagrammatic model over the hopping approach.     

Ultrafast photoinduced electron transfer in directly linked porphyrin-ferrocene dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.     

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.     

Synthesis and controlled self-assembly of covalently linked hexa-peri-hexabenzocoronene/perylene diimide dyads as models to study fundamental energy and electron transfer processes

We report the synthesis and photophysical characterization of a series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide (PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the two components and the conjugation of the bridging element are systematically modified to study the influence on self-assembly and energy and electron transfer between electron donor HBC and acceptor PDI. STM and 2D-WAXS experiments reveal that both in solution and in bulk solid state the dyads assemble into well-ordered two-dimensional supramolecular structures with controllable mutual orientations and distances between donor and acceptor at a nanoscopic scale. Depending on the symmetry of the dyads, either columns with nanosegregated stacks of HBC and PDI or interdigitating networks with alternating HBC and PDI moieties are observed. UV-vis, photoluminescence, transient photoluminescence, and transient absorption spectroscopy confirm that after photoexcitation of the donor HBC a photoinduced electron transfer between HBC and PDI can only compete with the dominant F?rster resonance energy transfer, if facilitated by an intimate stacking of HBC and PDI with sufficient orbital overlap. However, while the alternating stacks allow efficient electron transfer, only the nanosegregated stacks provide charge transport channels in bulk state that are a prerequisite for application as active components in thin film electronic devices. These results have important implications for the further design of functional donor-acceptor dyads, being promising materials for organic bulk heterojunction solar cells and field-effect transistors.     

Modulating charge separation and charge recombination dynamics in porphyrin-fullerene linked dyads and triads: Marcus-normal versus inverted region.

Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.     

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.     

Fluorescence study on dynamics of liquid-crystalline polymers

Fluorescence measurements of various main-chain liquid-crystalline (LC) polyesters, poly[oxybiphenyl-4, 4'-diyloxy(2, 5-dihexadecyloxy)-1, 4-pyromellitoyl] (B-C16), poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60), poly(oxybiphenyl-4, 4'-diyloxy-1, 10-decanedioyl) (PB-10), low-molecular-weight LC bis(p-hexyloxyphenyl) terephthalate (PP6), and a lyotropic LCP, poly(sulfo-1, 4-phynelene 3'-nitroterephthalate) (PSPNT), were carried out to investigate their dynamics and intermolecular interaction changes.     

Theoretical study of excitation energy transfer in DNA photolyase

Photolyase (PL) is a DNA repair enzyme which splits UV light-induced thymine dimers on DNA by an electron transfer reaction occurring between the photoactivated FADH(-) cofactor and the DNA dimer in the DNA/PL complex. The crystal structure of the DNA/photolyase complex from Anacystis nidulans has been solved. Here, using the experimental crystal structure, we re-examine the details of the repair electron transfer reaction and address the question of energy transfer from the antenna HDF to the redox active FADH(-) cofactor. The photoactivation of FADH(-) immediately preceding the electron transfer is a key step in the repair mechanism that is largely left unexamined theoretically. An important butterfly thermal motion of flavin is identified in ab initio calculations; we propose its role in the back electron transfer from DNA to photolyase. Molecular dynamics simulation of the whole protein/DNA complex is carried out to obtain relevant cofactor conformations for ZINDO/S spectroscopic absorption and fluorescence calculations. We find that significant thermal broadening of the spectral lines, due to protein dynamics, as well as the alignment of the donor HDF and the acceptor FADH(-) transition dipole moments both contribute to the efficiency of energy transfer. The geometric factor of F?rster's dipolar coupling is calculated to be 1.82, a large increase from the experimentally estimated 0.67. Using F?rster's mechanism, we find that the energy transfer occurs with remarkable efficiency, comparable with the experimentally determined value of 98%.     

Intramolecular electronic excitation energy transfer in donor/acceptor dyads studied by time and frequency resolved single molecule spectroscopy

Electronic excitation energy transfer has been studied by single molecule spectroscopy in donor/acceptor dyads composed of a perylenediimide donor and a terrylenediimide acceptor linked by oligo(phenylene) bridges of two different lengths. For the shorter bridge (three phenylene units) energy is transferred almost quantitatively from the donor to the acceptor, while for the longer bridge (seven phenylene units) energy transfer is less efficient as indicated by the occurrence of donor and acceptor emission. To determine energy transfer rates and efficiencies at the single molecule level, several methods have been employed. These comprise time-correlated single photon counting techniques at room temperature and optical linewidth measurements at low temperature (1.4 K). For both types of measurement we obtain broad distributions of the rate constants of energy transfer. These distributions are simulated in the framework of Forster theory by properly taking into account static disorder and the flexibility of the dyads, as both effects can substantially contribute to the distributions of energy transfer times. The rate constants of energy transfer obtained from the calculated distributions are smaller on average than those extracted from the experimental distributions, whereby the discrepancy is larger for the shorter bridge. Furthermore, by plotting the experimentally determined transfer rates against the individual spectral overlaps, approximately linear dependencies are found being indicative of a Forster-type contribution to the energy transfer. For a given single molecule such a linear dependence could be followed by spectral diffusion induced fluctuations of the spectral overlap. The discrepancies between measured energy transfer rates and rates calculated by Forster theory are briefly discussed in light of recent results of quantum chemical calculations, which indicate that a bridge-mediated contribution is mainly responsible for the deviations from Forster theory. The availability of the inhomogeneous distributions of donor and acceptor electronic transition frequencies allows for comparing the energy transfer process at liquid helium and room temperature for the same set of molecules via simple simulations. It is found that on average the energy transfer is by a factor of approximately 3 faster at room temperature, which is due to an increase of spectral overlap.     

Photoinduced electron transfer dynamics in porphyrin donor dyads

Intramolecular photoinduced electron transfer (PET) processes occurring in dyads with a free base porphyrin-tetraazaanthracene donor (P) and either a tetracyanonaphthoquinidodimethane (TCQ) or benzoquinone (BQ) acceptor linked by a rigid six σ-bond polynorbornane bridge ([6]) have been investigated. For P[6]BQ, PET in the polar solvent benzonitrile (_s = 25.9) occurs with a rate constant (k_PET) of 1.6 × 10⁸ s⁻¹ but is not evident in solvents less polar than tetrahydrofuran (_s = 7.52). For P[6]TCQ, highly efficient forward PET occurs in both polar and non-polar solvents (k_PET > 2 × 10¹⁰ s⁻¹). For P[6]TCQ the lifetime of the resulting charge-separated state decreases markedly with increasing solvent polarity. The results are discussed in the context of the likely mechanisms for electronic coupling and current theories for PET processes in such linked molecular systems.     

A model calculation on the transport of excitation energy in a molecular crystal

We report a model calculation of the transport of a local (site) excitation in a doped molecular crystal containing one impurity. We do not consider the impurity as a direct trap for electronic excitations (zero trap depth) but assume that exciton-phonon interaction is exclusively given by the coupling of excitons with the vibrational displacement of the impurity. The dynamical problem is solved by using a time-dependent effective potential consisting of equilibrium average exciton-phonon interaction and fluctuations around this average. Two correlation functions are computed using the slow phonon limit and assuming that the temperature of the system is 300 K. Transmission of the excitation energy over a distance of eight spacings takes place, electronically, within a few picoseconds. With the exciton-phonon interaction switched on, calculated correlation functions diminish very rapidly with increasing time, indicating that an irreversible transfer of excitonic energy to the thermal bath takes place. Thus transmission of the excitation energy over such a distance (and without a high rate of trapping) is not an efficient process.     

Synthesis of directly linked zinc(II) porphyrin-imide dyads and energy gap dependence of intramolecular electron transfer reactions

A series of zinc(II) porphyrin-imide dyads (ZP-Im), in which an electron donating ZP moiety is directly connected to an electron accepting imide moiety in the meso position, have been prepared for the examination of energy gap dependence of intramolecular electron transfer reactions with large electronic coupling. The nearly perpendicular conformation of the imide moiety towards the porphyrin plane has been revealed by Xray crystal structures. The energy gap for charge separation, 1ZP* - Im --> ZP+ - Im-, is varied by changing the electron accepting imide moiety to cover a range of about 0.8 eV in DMF. Definitive evidence for electron transfer has been obtained in three solvents (toluene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed us to determine the rates of photoinduced charge separation, 1ZP* - Im --> ZP+ - Im-, and subsequent thermal charge recombination ZP+ - Im- --> ZP - Im. The free-energy gap dependence (energy gap law) has been probed from the normal to the nearly top region for the charge separation rate alone, and only the inverted region for the charge recombination rate. Although both of the energy gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, when we take into consideration the effect of the high frequency vibrations replaced by one mode of averaged frequency, many features, including the effects of solvent polarity and the electron tunneling matrix element on the energy gap law, differ considerably from those of the previously studied porphyrin-quinone systems, which have weaker interchromophore electronic interactions.     

Photoinduced electron transfer and excitation energy transfer in directly linked zinc porphyrin/zinc phthalocyanine composite

Photoinduced electron transfer (ET) and excitation energy transfer (ENT) reactions in monomer and slipped-cofacial dimer systems of a directly linked Zn porphyrin (Por)-Zn phthalocyanine (Pc) heterodyad, ZnPc-ZnPor, were investigated by means of the picosecond and femtosecond transient absorption spectroscopies. In the dimer dyad system of two heterodyads connected through the coordination bond between two imidazolyl-substituted ZnPor bearing ZnPc, ZnPc-ZnPor(D), the rapid ENT from the ZnPor to ZnPc in the subpicosecond time region was followed by photoinduced charge separation (CS) and charge recombination (CR) with time constants of 47 and 510 ps, respectively. On the other hand in the monomer dyad system, no clear charge-separated state was observed although the CS with a time constant of 200 ps and CR with < or =70 ps were estimated. These results indicated that the dimer slipped-cofacial arrangement of pair porphyrins is advantageous for the effective production of the CS state. This advantage was discussed from the viewpoint of a decrease in the reorganization energy of the dimer relative to that of the monomer system. In addition, the electrochemical measurements indicated that the strong interaction between ZnPc and ZnPor moieties also contributed to the fast CS process despite the marginal driving force for the CS process. The dimer dyad of ZnPc-ZnPor provides full advantages in efficiencies of the light harvesting and the CS state production.     

Excited-state energy-transfer dynamics of self-assembled imine-linked porphyrin dyads

Toward the development of new strategies for the synthesis of multiporphyrin arrays, we have prepared and characterized (electrochemistry and static/time-resolved optical spectroscopy) a series of dyads composed of a zinc porphyrin and a free base porphyrin joined via imine-based linkers. One dyad contains two zinc porphyrins. Imine formation occurs under gentle conditions without alteration of the porphyrin metalation state. Five imine linkers were investigated by combination of formyl, benzaldehyde, and salicylaldehyde groups with aniline and benzoic hydrazide groups. The imine-linked dyads are quite stable to routine handling. The excited-state energy-transfer rate from zinc to free base porphyrin ranges from (70 ps)(-)(1) to (13 ps)(-)(1) in toluene at room temperature depending on the linker employed. The energy-transfer yield is generally very high (>97%), with low yields of deleterious hole/electron transfer. Collectively, this work provides the foundation for the design of multiporphyrin arrays that self-assemble via stable imine linkages, have predictable electronic properties, and have comparable or even enhanced energy-transfer characteristics relative to those of other types of covalently linked systems.     

Directly meso-meso linked porphyrin rings: synthesis, characterization, and efficient excitation energy hopping

Directly meso-meso linked porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins have been synthesized in a stepwise manner from a 5,10-diaryl zinc(II) porphyrin building block. Symmetric cyclic structures have been indicated by their very simple (1)H NMR spectra that exhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplit Soret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31G* level indicate that a dihedral angle between neighboring porphyrins decreases in order of CZ6 > CZ8 > CZ4, which is consistent with the (1)H NMR data. Photophysical properties of these molecules have been examined by the steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within the porphyrin rings, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Forster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates have been estimated for CZ4 (119 +/- 2 fs)(-)(1), CZ6 (342 +/- 59 fs)(-)(1), and CZ8 (236 +/- 31 fs)(-)(1), which reflect the magnitude of the electronic coupling between the neighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light harvesting antenna model in light of very efficient excitation energy hopping along the ring.     

Excited-state energy relaxation dynamics of triply linked Zn(II) porphyrin arrays

In this study, we have investigated the excited-state energy deactivation dynamics of extended π-conjugated molecular systems that consist of competitive electronic and vibrational relaxation processes.