Pyromellitimide Research Articles

Direct Measurement of Singlet−Triplet Splitting within Rodlike Photogenerated Radical Ion Pairs Using Magnetic Field Effects: Estimation of the Electronic Coupling for Charge Recombination

Determining the electronic coupling matrix element, V, for an electron transfer reaction is challenging both experimentally and theoretically. The magnitude of the singlet−triplet splitting (spin−spin exchange interaction), 2J, within a radical ion pair (RP) is directly related to the sum of the squares of the matrix elements that couple the RP state to the ground state and to other energetically proximate excited and ionic states. Each term in this sum is weighted by the reciprocal of the energy gap between the RP state and the particular state to which it is coupled. We present here a series of intramolecular triads with linear, rodlike structures that undergo very efficient two-step electron transfer following direct excitation of a 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophore. Attachment of a p-methoxyaniline (MeOAn) donor by means of the piperazine bridge and naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI) acceptors, either directly or through a 2,5-dimethylph...

Effects of Solvent and Structural Dynamics on Electron Transfer Reactions of 4-Aminonaphthalene-1,8-dicarboximide Donor−Acceptor Molecules in Nematic Liquid Crystals and Isotropic Solvents

Photoinduced charge separation and thermal charge recombination reactions in a series of structurally related donor−acceptor molecules based on the 4-(N-pyrrolidinyl)naphthalene-1,8-dicarboximide (5ANI) and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophores were studied in the nematic liquid crystals 4-cyano-4‘-(n-pentyl)biphenyl (5CB) and N-(4‘-methoxybenzylidene)-4-(n-butyl)aniline (MBBA) as well as in the isotropic solvents pyridine, 2-methyltetrahydrofuran, and pentyl propionate over a temperature range of 293−353 K. The photoexcited 5ANI and 6ANI chromophores donate an electron to pyromellitimide (PI), which is attached to the donors via a N−N single bond between their imide groups to give 5ANI−PI and 6ANI−PI, respectively. The photoexcited 6ANI chromophore also accepts electrons from p-methoxyaniline, which is attached to the 4-position of the naphthalene-1,8-dicarboximide as a N‘-(4‘-methoxyphenyl)piperazinyl substituent to give MeOAn−6ANI. Attachment of a secondary PI acceptor to M...

Electronic structure and conduction mechanism of donor–bridge–acceptor systems where PPV acts as a molecular wire

Abstract p -Phenylenevinylene (PPV) oligomers behave as molecular wires, although in different ways in different experiments. The donor–bridge–acceptor systems of Davis et al., where a tetracene (TET) donor and a pyromellitimide (PI) acceptor are linked by PPV oligomers of varying lengths, have revealed an unusual donor–acceptor distance dependence in the rate constant for photo-induced electron transfer (ET). Sikes et al. in an electrochemical experiment have found fast tunneling over 28 A through PPV oligomers. In the present paper, we have applied density functional theory and other quantum chemical methods to study the electronic structure of PPV-systems and used these data to make conclusions regarding the conduction mechanisms. Consistent with the earlier interpretations we find that the PPV bridges act as a tunneling medium for ET in the thermal case and for the shorter bridges in the photo-induced case. In the three longer molecules the photo-induced ET is coherent to the bridge molecule, and is followed by fast draining to the acceptor. The electron transfer does not appear to be connected to torsional activation of bridge plane or side groups.

Dependence of electron transfer dynamics in wire-like bridge molecules on donor–bridge energetics and electronic interactions

Electron transfer (ET) reactions in donor–bridge–acceptor (DBA) molecules that occur by means of superexchange interactions between the donor (D) and the bridge (B) molecules depend on the vertical energy gap separating D and B. This dependence modulates the electronic coupling matrix element for ET, and hence the ET rate. However, when the energy gap between D and B is small, the assumptions intrinsic to the simplest superexchange model break down and charge injection from D to B may occur. To investigate this range of possibilities, we synthesized a family of DBA molecules based on a 2,5-bis(2 ′-ethylhexyloxy)-1,4-distyrylbenzene (OPV3) bridge. Three electron donors, zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), perylene (PER) and tetracene (TET) as well as two electron acceptors naphthalene-1,8:4,5-bis(dicarboximide) (NI) and pyromellitimide (PI) were attached to the OPV3 bridge. The observed ET dynamics of these molecules are sensitive not only to the donor–bridge energy gap, but also to the excited state torsional dynamics between the donor and bridge.

Biomimetic Electron Transfer Using Low Energy Excited States: A Green Perylene-Based Analogue of Chlorophylla

We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a), yet unlike Chl a, it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor systems. The N,N‘-dicyclohexyl derivative (5PDI) absorbs strongly (e = 46 000 M-1 cm-1) at 686 nm in toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced in CH2Cl2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and 1,7-bis(3,5-di-tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond. Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, an...

Electron Transfer Involving Nonbonded Superexchange Interactions in Rigid Donor−Acceptor Arrays

Rigid intramolecular trichromophoric arrays based on a 4,5-diaminoxanthene bridge B were prepared in which the xanthene bridge orients A2 relative to D, so that nonbonded interactions between 1*D and A2 can be studied. The electron donor, D, is 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (ANI), and the acceptors A1 and A2 are pyromellitimide (PI) and naphthalene-1,8:4,5-bis(dicarboximide) (NI), respectively, and X is either n-C8H17 or H. The torsional angles of the xanthene−imide single bonds orient the NI and PI acceptors approximately cofacial to one another. Femtosecond transient absorption measurements show that electron transfer from 1*ANI to NI occurs by nonbonded superexchange interactions that include contributions from both the n-C8H17 substituent on the NI acceptor and nearby solvent molecules. The structural rigidity of these compounds allows evaluation of the distance dependence of both charge separation and recombination. For charge separation, β = 1.1−1.3 A-1, and evidence exists for solv...

Ultrafast electron transfer reactions initiated by excited CT states of push–pull perylenes

Two new chromophores that absorb in the visible spectrum, the 9-( N-pyrrolidinyl)- and 9-( N-piperidinyl)perylene-3,4-dicarboximides, 5PMI and 6PMI, respectively, were synthesized and shown to possess lowest excited singlet states with about 70% charge transfer (CT) character. Changing the ring size of the cyclic amine from 5 to 6 significantly changes the energies of the CT states, as well as the redox potentials of the chromophores. These chromophores were linked to pyromellitimide (PI) and 1,8:4,5-naphthalenediimide (NI) electron acceptors using a single N–N bond between their respective imides to yield the corresponding donor–acceptor dyads 5PMI–PI, 5PMI–NI, 6PMI–PI, and 6PMI–NI. The donors and acceptors in these molecules are positioned relative to one another in a rod-like arrangement at fixed distances and restricted orientations. The rates of charge separation and recombination were measured using transient absorption spectroscopy. These chromophores were also used to prepare rigid donor–acceptor triads 5PMI–PI–NI and 6PMI–PI–NI, which display one- or two-step electron transfer mechanisms that depend on solvent polarity. These compounds exhibit a broad range of structure and media driven changes in electron transfer mechanism.

Coordination control of intramolecular electron transfer in boronate-bridged zinc porphyrin-diimide molecules.

Three sets of dyads, in which a zinc-porphyrin (ZP) electron donor is connected to an aromatic diimide electron acceptor,either pyromellitimide (PI) or naphthalene-1,8:4,5-tetracarboxylic acid diimide (NI), via a boronate-ester bridge, a piperidine bridge, and a 1,3-dioxolane bridge, respectively, were prepared for the purpose of control of intramolecular electron transfer (ET) by acid-base reactions at the connecting bridge. Boronate-ester bridge is a Lewis acidic site and confers a chance to regulate intramolecular ET reaction upon base coordination. This has been demonstrated by suppression of photoinduced ET from ZP to PI or NI in highly electron-pair donating solvents or upon addition of a fluoride anion. To extend this strategy to control of ET-path selectivity, we prepared triad 18, which consists of a ZP donor bearing NI and PI acceptors at similar distances through a boronate-ester bridge and an acetal bridge, respectively. Photoexcitation of 18 in a free form led to intramolecular ET from (1)()ZP preferentially to NI, but the ET path was completely switched toward PI in F(-)-coordinated form without a serious drop in the rate, constituting a novel ET-switching molecular system.

Novel Mechanism for Triplet State Formation in Short Distance Covalently Linked Radical Ion Pairs

We report on a series of electron donor−acceptor (D-A) dyads that undergo singlet-initiated charge separation to produce a strongly spin coupled radical ion pair that subsequently undergoes charge recombination to produce a triplet state with unusual spin polarization. The molecules consist of either a 4-(N-piperidinyl)naphthalene-1,8-imide (6P) or 4-(N-pyrrolidinyl)naphthalene-1,8-imide (5P) donor and a 1,8:4,5-naphthalenediimide (NI) or pyromellitimide (PI) acceptor. Selective photoexcitation of D within D-−A produces the radical ion pair 1[D•+-A•-] quantitatively. This is followed by the formation of 3[D•+-A•-] via singlet−triplet mixing within the radical pair. Radical pair intersystem crossing (RP-ISC) leads to charge recombination to yield [D-3*A] or [3*D-A]. Time-resolved optical absorption and emission spectroscopy is coupled with EPR spectroscopy to characterize the mechanism of the nearly quantitative initial charge separation reaction and the subsequent radical ion pair recombination reaction leading to the unusually spin polarized triplet state. These radical pairs also possess charge transfer emission bands that aid in the data analysis. The small number of previously reported covalent donor−acceptor systems that yield a triplet state from radical ion pair recombination use multistep charge separation reactions to achieve a ≥20 Å spacing between the oxidized donor and reduced acceptor. These examples have small exchange couplings, J, within the radical pair, so that S-T0 mixing between the radical pair energy levels occurs. In the strongly coupled systems described here, we show that the triplet states are formed by means of both S-T0 and S-T-1 mixing, producing novel spin-polarized EPR spectra characterized by anisotropic spin lattice relaxation.

Factoring through-space and through-bond contributions to rates of photoinduced electron transfer in donor—spacer—acceptor molecules

Contributions from direct orbital overlap (through-space interactions) and superexchange (through-bond interactions) to the electronic coupling matrix elements for photoinduced charge separation and recombination in a series of linked donor—spacer—acceptor molecules were studied. The molecules consisted of a 4-piperidinyl-naphthalene-1,8-dicarboximide (ANI) electron donor and a N-(n-octyl)pyromellitimide (PI) electron acceptor attached to the 1,5- and 1,8-positions of either anthracene (ANC) or dibenzobicyclo(2.2.2)octatriene (DBO) spacers. For the 1,8-disubstituted compounds, ANI and PI are held approximately cofacial with a center-to-center distance of 5.3 Å, whereas for the 1,5-disubstituted compounds, the center-to-center distance increases to 13.5 Å. The through-bond interaction was investigated by replacing the ANC spacer with DBO. The charge separation and recombination reactions were examined in both toluene and tetrahydrofuran (THF). The results show that for the 1,8-disubstituted DBO system in both toluene and THF, charge separation and recombination is dominated by through-space interactions. For the 1,8-disubstituted ANC system in both toluene and THF, charge separation occurs by means of a direct, through-space interaction, while charge recombination occurs through the intermediacy of the ANI ANC + PI - ion pair in toluene and directly from ANI + ANC PI - to ground state in THF. For the 1,5-disubstituted molecules, possessing either the ANC or the DBO spacers, electron transfer from 1*ANI to PI was not kinetically competitive with the decay of 1*ANI to ground state in either toluene or THF.

Picosecond excited-state dynamics of fixed-distance triads consisting of metalloporphyrin, doubly strapped metal-free porphyrin, and pyromellitimide

Upon photoexcitation of fixed-distance triads cosisting of zinc-porphyrin (ZnP) or magnesium-porphyrin (MgP), a doubly strapped metal-free porphyrin (SH 2P), and pyromellitimide (PIm), (ZnP) +SH 2P(PIm) − and (MgP) +SH 2P(PIm) − ion pair states are produced in high yields by two-step electron-transfer reactions: (1) charge separation between 1(SH 2P)* and PIm and (2) hole transfer from (SH 2P) + to the metalloporphyrin. A (MgP) +SH 2P(PIm) − ion pair state is fairly long-lived, while a (ZnP) +SH 2P(PIm) − state is only short-lived due to a decay pathway to the ground state through a reverse hole transfer to give ZnP(SH 2P) +(PIm) − followed by charge recombination.

Synthesis and Intramolecular Charge Separation of Fixed-Distance Triads Consisting of Zinc Porphyrin, Metal-Free Porphyrin, and Electron-Accepting Diimide Moiety

Abstract Synthesis of fixed-distance triads consisting of zinc porphyrin (ZnP), metal-free porphyrin (H2P), and pyromellitimide (PIm) or 1,4 : 5,8-naphthalenetetracarboximide (NIm) is described. The ZnP and H2P moieties are bridged by aromatic spacers such as 1,4-phenylene, 4,4′-biphenylylene, methylenebis(1,4-phenylene), and bicyclo[2.2.2]octane-1,4-diylbis(1,4-phenylene) groups. The steady-state fluorescence spectra indicated the occurrence of intramolecular singlet-singlet energy transfer form the ZnP to the H2P. Formation of long-lived charge-separated states (ZnP)+–H2P–(PIm)− and (ZnP)+–H2P–(NIm)− with lifetimes of 0.14—80 μs in THF was observed by nanosecond to microsecond transient absorption spectroscopy. The lifetimes of the (ZnP)+–H2P–(Im)− states depend on the distances between the charged sites as well as the energy gaps between the ion pair and the ground state.

Effect of molecular orientation and crystallization on mechanical properties of oriented polypyromellitimides

By the action of a temperature field of force a highly oriented crystalline structure could be obtained for four polymers prepared from pyromellitic acid dianhydride and unable to crystallize during imidization by heat. Results are given of a study of the structure and mechanical properties of fibres of this group of polymers. It was shown that, in contrast to pyromellitimide (PM), diamine fragments and not dianhydride fragments are responsible for intermolecular packing of chains. It was assumed that increasing the cross section of the diamine fragment of the recurrent unit, compared with the dianhydride one, weakens intermolecular contacts in the dianhydride layer and is the main cause of noticeably poorer mechanical properties of polymers, compared with similar properties of PM.

Thermodynamics of synthesis of poly-( p, p′-dipenyleneoxide)pyromellitimide

The heat capicity has been investigated and the thermodynamic functions calculated for pyromellitic dianhydride (PMDA), 4,4′-diaminodiphenyl ether (DAPE), polyamido-acid (PAA) based on PMDA and DAPE, and poly-( p, p′-diphenyloxide) pyromellitimide (PI). The enthalpies of combustion have been determined and the standard thermochemical characteristics calculated for reactions of formation of the studied compounds from the original materials at 298·15°K. The equilibrium water vapour pressure over the PAA during its cyclodehydration has been measured. The results were used as a basis for calculating thermodynamic parameters of the synthesis of PAA from PMDA and DAPE and of the process PAA → PI+2H 2O. Similar measurements and calculations were carried out for the synthesis of N-phenylphthalimide via intramolecular cyclodehydration of phthalanilic acid.

Molecular weight characteristics of a polyimide prepared by polycyclization of a polyamidoester

A comparative study has been made of the molecular weight characteristics of poly(4′,4″-diphenylenephthalide) pyromellitimide (PI) prepared by polycyclization of the appropriate polyamidoester (PAE) by different methods. Viscosity measurements along with fractionation results and sedimentation analysis show that transition from PAE to PI by solid phase thermal cyclization is accompanied by a reduction in the intrinsic viscosity [η] and in the weight average degree of polymerization X w , and the MWD broadens to become the most probable. No rise in polydispersity occurs in the case of thermal cyclization of PAE to PI in nitrobenzene, but increases in [η] and X w are observed. In the conversion of PAE to PI in solution in presence of acetic anhydride and pyridine, no changes in [η], X w or in the polydispersity were detectable.