Mihir Chowdhury

Spectra of Benzil

The polarized low‐temperature absorption spectrum, emission spectrum, and excitation spectrum of benzil and benzil‐doped stilbene crystals have been recorded. It has been found that the 3800 A (n–π*) band system of benzil is split by the weak interaction between the two halves of the molecule. The excitation spectrum reveals the presence of a S→T transition hidden underneath the S→S transition. The crystal spectrum of benzil‐doped stilbene is different from the benzil spectrum. The new sharp band at 4850 A has been interpreted in terms of a trans–planar configuration of benzil. A weak (π*→n) fluorescence has been detected in this crystal.

Effect of environment on the magnetic field modulation of exciplex luminescence

Abstract The effect of a magnetic field on the pyrene-N,N-dimethylaniline exciplex luminescence in methanol has been studied using an improved phase-sensitive detection technique. The wavelength dependence of B 1/2 , the effect of isotopic substitution of the solvent, and the consequences of using mixed donors have been investigated. The results throw light on the environmental perturbation of the intersystem crossing process in solvent-separated radical pairs.

Effect of variation of dielectric constant on the magnetic field modulation of exciplex luminescence

The effect of variation of dielectric constant on the relative magnetic field effect in singlet luminescence has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In contrast to alcohols where relative singlet magnetic field effect is of the order of 2% only, the magnetic field effect in non-alcoholic medium reaches as high as 9%. Moreover, dielectric constant variation in alcohols yields curves which are distinctly different from those in non-alcoholic media. It turns out that this dependence of magnetic field effect on dielectric constant is similar in all non-alcoholic solvent mixtures. An analytical study based on Hong and Noolandi’s solution of Smoluchowski equation has been made. Derived expressions can interpret experimental curves reasonably well.

Excited-state proton transfer kinetics of carbazole

Abstract Rate constants for the excited-state proton transfer reaction of carbazole in aqueous alkaline solution have been determined using picosecond single photon counting. Fluorescence decay measurements show that the back reaction is slow compared to the fluorescence decay time and therefore equilibrium is not attained in the excited state. The validity of a pK value for the lowest excited state determined from steady-state fluorescence measurements assuming equilibrium is discussed. It is concluded that the thermodynamic pK* value for carbazole is 10.98.

An excited state proton transfer reaction in micellar media

The excited state proton transfer reaction was studied for carbazole (CAZL) in non-ionic (Triton X-100), cationic (cetyltrimethylammonium bromide (CTAB)) and anionic (sodium dodecylsulphate (SDS) micelles. The micelles strongly affect the equilibrium quotient of the process. Taking the excited state proton transfer reaction as a probe, the critical micelle concentrations (CMCs) for the three micellar solutions were estimated. Furthermore, a phase transition was observed at high ionic strengths for Triton X-100.

Conformational relaxation in the excited electronic states of benzil and naphthyl

Abstract A time-resolved study of the emission from benzil and naphthyl in semi-solid glasses (e.g. alcoholic glass near the melting point) using a pulsed N2-laser as an excitation source is reported. The emission from the relaxed excited triplet shows a growth followed by a decay. This growth provides a convincing proof of geometrical relaxation occurring in the excited states of benzil and naphthyl.

Nature of s‐Tetrazine Emission Spectra

Fluorescence is established as the unique emission spectrum of s‐tetrazine and dimethyl s‐tetrazine in several ways—notably by polarization measurements (out of plane) and life‐time measurements of the emitting state (4×10—9 sec for dimethyl s‐tetrazine at 300°K). The vibrational features (which indicate an allowed transition) are not fully understood on the basis of emission solely from the lowest singlet state. Experiments involving H‐bonding solvents suggest that the hydrogen bond exists in the (n, π*) singlet state in s‐tetrazine as contrasted to the situation in the (n, π*) triplet state in pyrazine and pyrimidine, where the H bond is broken. Polarization measurements on the emission as a function of the exciting wavelength confirms Masons observation of a second n→π* band system at about 3200 A. The polarization of the 3200‐A band system is out of plane, thereby supporting Masons assignment of this band as an allowed n→π* transition. Different viewpoints on the mechanism of the fluorescence are su...

Effect of inclusion of cyclodextrin on excited state proton transfer: Carbazole—γ-cyclodextrin

Abstract The excited state proton transfer reaction of carbazole was investigated in the presence of γ-cyclodextrin (γ-CD) using steady state and time-resolved fluorometry. The deprotonation rate was enhanced for the carbazole—γ-CD inclusion complex compared with free carbazole, but the rate of reverse protonation was unaffected.

Fluorescence excitation spectrum of jet-cooled dihydroanthracene

Abstract The lowest excited singlet site of 9,10-dihydroanthracene has been studied by supersonic-jet laser spectroscopy. The spectrum consists of a number of prominent low-frequency vibronic bands near the 0—0 band followed by a structureless continuum. The low-frequency vibronic bands are nicely fitted with a quadratic Gaussian-type potential for the excited stated. The height of the barrier of the double-minimum potential is found to be 94 cm −1 .

S1 (n,π*), T1 (n,π*) and S2(n,π*) emissions in 3,6-diphenyl-s-tetrazine

Abstract Simultaneous emissions from S1(n,π*) and S2(n,π*) states in 3,6-diphenyl-s-tetrazinc (DPT) have been observed along with weak luminescence from T1 (n,π*). The occurrence of the S2(n,π*) fluorescence has been justified on the basis of the slow S2 XXX S1 internal conversion resulting from the large energy gap between the two states. This is the first case of dual fluorescence where both the emitting states are of (n,π*) nature.