Solvent polarity and temperature effect on the photophysical properties of two 1,2-benzopyrone dyes, namely, coumarin-152 (C152) and coumarin-481 (C481), have been investigated using steady-state and time-resolved fluorescence measurements. In nonpolar (NP) solvents (e.g., hexane, cyclohexane, methylcyclohexane, 2-methylpentane, and 3-methylpentane), the Stokes' shifts (Δν̄) and fluorescence lifetimes (τf) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Δν̄ correlates linearly with the solvent polarity function, Δf = {(ε − 1)/(2ε + 1) − (n 2 − 1)/(2n 2 + 1)}, and the slopes of these plots indicate that the fluorescent states for both the dyes in these solvents are of intramolecular charge transfer (ICT) character. The unusually lower Δν̄ and τf values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields (Φf) and τf values of the two dyes also show an unusual reduction in higher polarity solvents (Δf > ∼0.2). Furthermore, unlike in all other solvents, in solvents with Δf > ∼0.2, the Φf and τf values are strongly temperature-dependent. These results indicated the involvement of a new activation-controlled nonradiative deexcitation channel in higher polarity solvents, assigned to the involvement of the nonfluorescent twisted intramolecular charge transfer (TICT) state. Though the activation barrier (ΔE a) for such processes usually decreases with solvent polarity, for the present systems the ΔE a is seen to increase with Δf. This unusual behavior has been rationalized assuming that the ΔE a arises because of the potential energy (PE) crossing of the TICT and ground states rather than that of the ICT and TICT states as is usually the case for most TICT molecules. On increasing Δf, since the highly polar TICT state not only gets better stabilization but also its PE surface becomes more steeper than the ICT state, the ΔE a effectively increases with solvent polarity.

The fascinating aspect of excitation dependent fluorescence in carbon nanodots has led to several hypotheses, starting from particle size distribution to the presence of different emissive states and even to sluggish solvent relaxation around nanodot. In this contribution we provide definitive evidence for the involvement of discrete multiple electronic states for the excitation dependent emission in carbon nanodots. The presence of different types of aggregates even at very dilute solutions used in ensemble fluorescence spectroscopy, where fluorescence intensity shows linear dependence with absorbance, is the origin of these multiple electronic states. Inhomogeneous broadening due to slow solvent relaxation leading to excitation dependent spectral shift has negligible influence in conventional solvents.

The fundamental process of bond twisting that is responsible for the fluorescence sensing activity of the most extensively used amyloid fibril sensor, Thioflavin T, has been revealed using ultrafast time-resolved fluorescence spectroscopy. From the wavelength-dependent fluorescence decay kinetics and the subsequently constructed time-resolved emission spectra (TRES), the dynamic Stokes shift and the change in the spectral width were observed. These results are rationalized on the basis of the proposition that, following photoexcitation, Thioflavin T undergoes ultrafast bond twisting to form a twisted intramolecular charge-transfer state that is weakly emissive in nature. Formation of the twisted state from the local excited state was found to occur in the subpicosecond time domain (time constant approximately = 570 fs). Quantum chemical calculations support the proposition of the bond twisting process in the photoexcited Thioflavin T and suggest that the twisting around the central C-C single bond, rather than the C-N single bond, of the Thioflavin T molecule is mainly responsible for the observed ultrafast dynamics in the excited state. Detailed time-resolved fluorescence studies of Thioflavin T incorporated in amyloid fibril show substantial retardation in the bond twisting dynamics, suggesting the involvement of this process in the sensor activity of the dye.

Despite numerous efforts, there are several fundamental ambiguities regarding the photoluminescence of carbon dots (CDs). Spectral shift measurements display characteristic of both π-π* and n-π* transitions for the main absorption or excitation band at ∼350 nm, contrary to common assignment of exclusive n-π* transition. Additionally, the generally perceived core-state transition at ∼250 nm, involving sp-networked carbogenic domains shielded from external environments, needs to be reassessed because it fails to explain the observed fluorescence quenching and spectral shift. These results have been explained based on the molecular origin of PL in CDs invoking the similarity between CD and citrazinic acid. Fluorescent derivatives of the latter are recognized to be produced during citric-acid-based CD synthesis. Concentration-dependent spectral splitting of the main excitation band in combination with the temperature-dependent PL results has been envisioned assuming self-assembly of CDs into various H-aggregates.

The temperature effect on solvation dynamics has been investigated in two neutral micelles, namely, Triton X-100 (TX100) and Brij-35 (BJ-35), using dynamic fluorescence Stokes' shift method, to explore the role of micellar size and hydration on the solvation process. In TX-100, the temperature effect on C(t) is not only very strong but shows an unusual inversion around 298 K. On the contrary, for the BJ-35 micelle, the temperature effect is not that significant. Present results on solvation dynamics in the two micelles have been rationalized on the basis of the temperature dependent changes in micellar size and hydration, which are reported to be very large for TX-100 but very marginal for BJ-35. Observed inversion in the solvation rate in TX-100 around 298 K is inferred to be arising due to the interplay of two factors, one is the largely reduced micellar size at lower temperature, which causes the bulk water to come reasonably closer to the probe and thus contribute to the solvation process, and the other is the largely increased hydration at higher temperature that makes the micellar structure loose and consequent enhancement in the solvation rates. With intermediate micellar size and hydration, the solvation dynamics is slowest at around 298 K. For the BJ-35 micelle, as the micellar size and hydration does not change to any significant extent, there is almost no temperature effect on the solvation dynamics for this micelle.