Photo-initiated intramolecular charge transfer (ICT) processes play a pivotal role in the excited state reaction dynamics in donor-bridge-acceptor systems. The efficacy of such a process can be improved by modifying the extent of π-conjugation, relative orientation/twists of the donor/acceptor entities and polarity of the environment. Herein, 4-dimethylamino-4'-cyanodiphenylacetylene (DACN-DPA), a typical donor-π-bridge-acceptor system, was chosen to unravel the role of various internal coordinates that govern the extent of photo-initiated ICT dynamics. Transient absorption (TA) spectra of DACN-DPA in n-hexane exhibit a lifetime of > 2 ns indicating the formation of a triplet state while, in acetonitrile, a short time-constant of ~2 ps indicates the formation of charge transferred species. Ultrafast Raman loss spectroscopy (URLS) measurements show distinct temporal and spectral dynamics of Raman bands associated with C�C and C=C stretching vibrations. The appearance of a new band at ~1492 cm À 1 in acetonitrile clearly indicates structural modification during the ultrafast ICT process. Furthermore, these observations are supported by TD-DFT computations.
The compound 4-dimethylamino-4′-carbomethoxydiphenylacetylene (DACM-DPA) is a donor-π-acceptor system that exhibits significant charge transfer character in the excited state in polar solvents. Femtosecond transient absorption measurements in n-hexane, acetonitrile, and methanol exhibit excitedstate lifetimes of >2 ns, ∼500 ps, and ∼30 ps, respectively. Femtosecond Raman measurements of DACM-DPA elucidated the intricate structural dynamics during the excited-state evolution by revealing distinct vibrational signatures of DACM-DPA in these solvents. In the case of n-hexane solvent, vibrational bands at 2075 cm −1 (C�C) and 1600 cm −1 (C�C) stretch exist throughout the excited-state lifetime. However, in the case of polar solvents, the amplitudes of vibrational frequencies at 2098 cm −1 (C�C), 1618 cm −1 (C�C, acetonitrile), and 1603 cm −1 (C�C, methanol) diminish in a time period of 2.5 ps (acetonitrile) and 6.5 ps (methanol) along with the appearance of new bands at 2082 cm −1 (cumulene stretch) and 1582 cm −1 (quinoid stretch), clearly revealing the formation of an intramolecular charge-separated state. Furthermore, these experimental observations are corroborated by computations using DFT and TD-DFT.
Colloidal quantum dots (QDs) have attracted much attention due to their optical tunability and application as a unique solution processable gain medium in lasers. Many research groups over the world have successfully attained amplification from Cd-based QDs through femtosecond (fs) and nanosecond (ns) pumping. However, for the wide application of QD-based lasers, an operation in the CW regime is much more favorable. Here, we present a critical step towards this goal by demonstrating CW laser (532 nm) pumped amplified spontaneous emission (ASE) in Cd1-xZnxSe/ZnSe alloyed core QDs dispersed in toluene at room temperature. Alloyed core QDs show lower non radiative transitions, which has been demonstrated by TA dynamics in this report. The threshold level for CW ASE is observed to be 31 W cm-2. These results indicate that these QDs can sustain CW-stimulated emission at room temperature, and can be widely applicable by further optimizing the QD material quality in order to have a low amplification threshold.
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