Biomedical subjects

Jahur A Mondal

Publications and source records attributed to Jahur A Mondal.

4 recordsLinked to original sources

PubMed2006

Ultrafast relaxation dynamics of the excited states of Michler's thione.

Ultrafast relaxation dynamics of the S2 and S1 states of 4,4'-bis(N,N-dimethylamino)thiobenzophenone (Michler's thione, MT) have been investigated in different kinds of solvents, using steady-state absorption and emission as well as femtosecond transient absorption and fluorescence up-conversion spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation to the S2 state of MT, reveal weak fluorescence from the S2 state (phi F approximately 10(-3) in nonpolar and 10(-4) in polar solvents) but much weaker fluorescence from the S1 state. Yield of fluorescence from the S2 state is reduced in polar solvents because of reduced energy gap between the S2 and S1 states, Delta E(S2-S1), as well as interaction with the solvent molecules. Occurrence of S2-fluorescence in polar solvents, despite small energy gap, suggests that symmetry allowed S2(1A1) --> S0 (1A1) radiative and symmetry forbidden S2(1A1) --> S1 (1A2) nonradiative transitions are the factors responsible for the S2 fluorescence in MT. Lifetime of the S2 state is shorter (varying in the range 0.28-3.5 ps in different solvents) than that predicted from the Delta E(S2-S1) value and this can be attributed to its flexible molecular structure, which promotes an efficient intramolecular radiationless deactivation pathways. The lifetime of the S1 state (approximately 1.9-6.5 ps) is also very short because of small energy difference between the S1 and T1 states (Delta E(S1-T1) approximately 300 cm(-1)) in cyclohexane and hydrogen-bonding interaction as well as the presence of the isoenergetic T1(pipi*) state to enhance the rate of the intersystem crossing process from the S1(npi*) state in protic solvents.

Journal Article↗

PubMed2006

Twisting dynamics in the excited singlet state of Michler's ketone.

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.

Journal Article↗

PubMed2005

S2 fluorescence and ultrafast relaxation dynamics of the S2 and S1 states of a ketocyanine dye.

Dynamics of the excited singlet (both the S2 and S1) states of a ketocyanine dye, namely, 2,5-bis[(2,3-dihydroindolyl)-propylene]-cyclopentanone (KCD), have been investigated in different kinds of media using steady-state absorption and emission as well as femtosecond transient absorption spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation of KCD to its second excited singlet state, reveal dual fluorescence (emission from both the S2 and S1 states) behavior. Although the intensity of the S2 --> S0 fluorescence is weaker than that of the S1 --> S0 fluorescence in solutions at room temperature (298 K), the former becomes as much as or more intense than the latter in rigid matrixes at 77 K. The lifetime of the S2 state is short and varies between 0.2 and 0.6 ps in different solvents. After its creation, the S2 state undergoes two simultaneous processes, namely, S2 --> S0 fluorescence and S2 --> S1 internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S1 state. A good correlation between the lifetime of this component and the longitudinal relaxation times (tauL) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S1 state in alcoholic solvents in the ultrafast time domain has been explained by the occurrence of the repositioning of the hydrogen bonds around the carbonyl group in the excited state of KCD. In 2,2,2-trifluoroethanol, a strongly hydrogen bond donating solvent, it has even been possible to establish the existence of two distinct forms of the S1 state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.

Journal Article↗

PubMed2005

Ultrafast intermolecular electron transfer dynamics: Perylene in electron-accepting micellar medium.

The dynamics of ultrafast photoinduced intermolecular electron transfer (ET) from the excited singlet (S1) state of perylene (Pe) to an electron-accepting cationic surfactant molecule, N-cetylpyridinium chloride (CPC), in aqueous micellar solutions has been investigated using the femtosecond transient absorption spectroscopic technique with temporal resolution of 120 fs. The Pe molecule is localized at or near the micellar surface, where it coexists with the pyridinium moieties (headgroups of the micelle) of the surfactant molecule. Following photoexcitation of Pe, an electron is transferred to the neat and geometrically restricted headgroup of the micelle. Dynamics of the forward ET process as well as the geminate recombination or back ET (BET) process have been followed by monitoring the temporal evolution of the S1 state of Pe and the cation radical of Pe (Pe*+), respectively. The multiexponential forward ET process indicates that the ET dynamics is highly correlated with the spatial distributions of the micellar headgroups around a donor Pe molecule and thus dependent on the donor-acceptor distance. The distance-dependent ET and BET rates have been calculated following the method of Weidemaier and Fayer (J. Chem. Phys. 1995, 102, 3820) to get the best fit parameters for the multiexponetial temporal profiles for the S1 state of Pe as well as Pe*+. Because the acceptor is a constituent of the neat micellar medium, their confinement on the surface of the microheterogeneous medium provides a very large concentration such that, even though the forward transfer rate is 0.06 ps(-1) at the distance of closest approach, the ET process is complete within a 200-ps time domain. If the concepts of distribution of ET distances are utilized, the possible role of material diffusion on the kinetics of forward ET is ruled out. This is an experimental study to show, for the first time, the ultrafast distance-dependent light-induced ET dynamics following both the excited state of the donor and the cation radical formed in an ET process using the transient absorption spectroscopic technique in a self-reactive restrictive environment.

Electrons↗