Electron Transfer Reactions in the Excited Singlet States of Dimethyl Substituted Phenol−2-Nitrofluorene Systems: Evidence for the Marcus Inverted Region and Concurrent Occurrence of Energy Transfer Processes

Nonradiative inter- and intramolecular energy transfer from the aromatic donor anisole to a synthesized photoswitchable acceptor system

We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N'-1,2-phenylene di-p-tosylamide (B) and the multichromophore (M) where A and B are connected by a spacer containing both rigid triple (acetylenic) and flexible methylene bonds. Both steady state and time correlated single photon counting measurements demonstrate that though intermolecular energy transfer, of Forster type, between the donor and acceptor moieties occurs with rate 10(8)s(-1) but when these two reacting components are linked by a spacer (multichromophore, M) the observed transfer rate ( approximately 10(11)s(-1)) enhances. This seemingly indicates that the imposition of the spacer by inserting a triple bond may facilitate in the propagation of electronic excitation energy through bond. The time resolved fluorescence measurements along with the theoretical predictions using Configuration interaction singles (CIS) method by using 6-31G (d,p) basis set, implemented in the Gaussian package indicate the formations of the two excited conformers of B. The experimental findings made from the steady state and time resolved fluorescence measurements demonstrate that, though two different isomeric species of the acceptor B are formed in the excited singlet states, the prevailing singlet-singlet nonradiative energy transfer route was found from the donor A to the relatively longer-lived isomeric species of B.

Primary electron-transfer dynamics in 2-phenylindole-9-cyanoanthracene system. A comparative study with 2-methylindole

Electrochemical measurements by cyclic voltammetry predict the possibility of occurrence of photoinduced electron-transfer (PET) reactions between the ground state of 2-phenylindole (2PI) (electron donor) and the excited singlet of 9-cyanoanthracene (9CNA) molecule acting as an electron acceptor. However, 2PI should be expected to behave as a relatively weaker electron donating agent than the structurally related donor 2-methylindole (2MI) as it possesses higher oxidation potential value. Both steady-state and time-resolved spectroscopic measurements in the polar acetonitrile (ACN) and ethanol (EtOH) solvents show that the fluorescence quenching phenomenon of 9CNA in presence of 2PI is primarily due to the involvement of dynamic process which in high probability should be PET. Nevertheless, in less polar tetrahydrofuran (THF) medium, the quenching of 9CNA results from the combined effect of dynamic and static modes. The transient absorption spectra, measured by using nanosecond laser flash photolysis, of 9CNA in presence of 2PI exhibit the signature of the bands of the anionic species of 9CNA, cation of the donor 2PI and the contact neutral radical. Observations of the transient absorption at the different delays infer that ion-recombination mechanism is responsible for production of the monomeric triplets of both 9CNA and 2PI. From the transient absorption decays in ACN medium, it has been demonstrated that the diffusional separation of ions from geminate ion-pair is facilitated in the case of 2MI-9CNA pair whereas for 2PI-9CNA system the energy wasting charge recombination dominates over the process of charge dissociation. From the above observations, the possibility of developing much potential photosynthetic model compounds with the donor 2MI, rather than with the other donor 2PI molecule has been hinted.

Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S1) and triplet (T1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D+-H...A-), causes the formation of the neutral radical, 3(D+H-A)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.

Nature of non-radiative processes involved in the excited state of 9-cyanoanthracene in presence of 2-methylindole/2-methylindoline quenchers. A laser flash photolysis study to reveal the medium effects

By using steady state and time-resolved (laser flash photolysis and single photon counting) spectroscopic techniques the quenching of the lowest excited singlet (S1) state of 9-cyanoanthracene (9CNA) by the donors (quenchers) 2-methylindole (2MI) and 2-methylindoline (2MIN) in solvents of different polarity has been studied. Both the transient absorption, by laser flash photolysis technique, and photobleaching measurements were made at the ambient temperature both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) solvents. The photobleaching efficiency (alpha) was found to depend significantly on the polarity of surrounding solvents and also on the molecular structures of the quenchers. In NH the values of alpha are found to be larger than the corresponding values observed in ACN for both 2MI and 2MIN which possess highly reactive H atom bound to the heterocyclic N atom. Following the results obtained from the transient absorption spectra of the present donor-acceptor molecules in the different polarity solvents, a scheme describing the overall reaction mechanisms of the different photoreactions involved has been proposed. The probable causes for the changes observed in the mechanisms of the photoreactions involved in the cases of 2MI and 2MIN donors have been discussed in the light of their canonical structures.

Studies on unimolecular and bimolecular photoprocesses of a newly synthesized selenium compound, 7-chloro-2-phenyl-9H-[1]-benzopyrano[3,2-b]-selenophene-9-one (SeP)

Using steady state/time resolved spectroscopic and electrochemical techniques the spectroscopic and photophysical studies were made on a novel synthesized selenophene compound SeP in nonpolar methylcyclohexane (MCH), polar aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) solvents at the ambient temperature as well as at 77 K. Both from the studies on unimolecular and bimolecular photoprocesses this selenophene compound was found to possess several electronic levels, 1Bb, 1La, 1Lb (all are of pi pi* nature and 1Lb is hidden within 1La band envelop like the characteristics of most of the acenes) and 1(nO pi*) state arising due to carbonyl oxygen atom. In polar ACN environment this nO pi* state disappears because it moves within the envelop of intense 1La band due to large destabilization. Large overlapping of different band systems within the 1La band of SeP was confirmed from the observed depolarization effect. The lack of phosphorescence of SeP both in MCH and EtOH rigid glassy matrix at 77 K has been inferred due to large vibronic interactions between closely lying triplets of the corresponding 1nO pi* and 1Lb states. From the bimolecular investigations, it reveals that SeP acts as a good electron donor in presence of the well known electron acceptor 9 cyanoanthracene (9CNA). Transient absorption spectra measured by laser flash photolysis technique demonstrate the formation of ion-pair when the acceptor is excited. From the analysis of the fluorescence quenching data it seemingly indicates that the major contribution in the diminution of the fluorescence intensity of the acceptor 9CNA in presence of SeP is not only due to the photoinduced electron transfer (ET) but also originates from static type (instantaneous) quenching processes along with external heavy atom effect. The possibility of occurrence of photoinduced ET reaction in Marcus inverted region is hinted.

Electron transfer compatible molecular design of benzothiophene-acetophenone bichromophores: a theoretical approach

Computational work has been done for a bichromophore (4MBA) comprising a donor 4-methoxy-benzo[b]thiophene (4MBT) and an acceptor molecule p-chloro-acetophenone (pclA) linked together by a HC=CH bond which shows large hyperpolarizability. The charge transfer in this bichromophoric system is computed by semiemperical theoretical calculation. Ground state and excited state dipole moment difference of the bichromophore 4MBA indicates a large electron transfer probability.

Investigations on nonradiative transitions in different environments using excited (or ground state) 1,2,3,4-tetrahydroquinoline (THQ) as donor and ground state (or excited) 9-fluorenone (9FL) or 2-nitro-9-fluorenone (2N9FL) as acceptors

By using steady state and time resolved spectroscopic techniques, investigations were made on the nature and mechanisms of different nonradiative processes involved within the excited donor (or acceptor) and ground state acceptor (or donor) in nonpolar n-heptane (NH), polar protic ethanol (EtOH), and polar aprotic acetonitrile (ACN) solvents at the ambient temperature as well as in EtOH rigid glassy matrix at 77 K. The bicyclic molecule 1,2,3,4-tetrahydroquinoline (THQ) was used as a donor (D) in the present investigation whereas 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) were chosen as electron acceptors (A). When the donor chromophore was excited in presence of an acceptor, highly exothermic electron transfer (ET) reactions seemed to occur from observed large negative driving energy (deltaG0) values, measured by electrochemical techniques, Förster's type energy transfer, static quenching, etc. An attempt was made to estimate separately the contributions of static and dynamic quenching modes in the overall quenching mechanism of donor fluorescence by using a model of modified Stern-Volmer (SV) relation. From this relation it seemingly indicated that the major contribution in quenching originated from the static mode. When excited acceptors react with the ground state donor THQ it is primarily ET reactions that seem to occur. Observations of the transient absorption spectra, by laser flash photolysis techniques, of contact ion-pairs of the present D-A systems along with the triplet absorption spectra of the monomeric acceptor (9FL and 2N9Fl) corroborate our views regarding the occurrence of photoinduced ET reactions within the present reacting systems. At 77 K the combined effect of Förster type energy transfer and static quenching seems to be responsible for observed lowering of donor fluorescence emission intensity in presence of acceptors (9FL or 2N9FL). However, the phosphorescence lifetime measurements reveal that the triplet donor might be involved in photoinduced ET reaction with the unexcited acceptor in rigid glassy matrix at 77 K and this possibly causes the reduction in the phosphorescence band intensity of the donor THQ in presence of the latter one. Key words: electron transfer, static quenching, fluorescence quenching, phosphorescence, energy transfer.