**2.5 What is understood about DCM dye?**

The ground state and dipole moments of DCM are estimated to be very high (5.6 D and 26.6 D) which suggests that the charge is highly polarized even in the ground state. The steady-state absorption and fluorescence spectra of DCM reveal that the molecule exhibit solvatochromic shift and large Stokes shifts depending on the polarity of the solvent [10, 16, 24]. Solvatochromic shift of the electronic absorption is due to high ground-state dipole moment. The dramatic Stokes shift is attributed to the change of the dipole moment upon photoexcitation and fluorescent emitting state to a charge-transfer (CT) state [23, 24]. The fluorescence lifetime of DCM is measured to be of the order of a few nanoseconds, and the solvent relaxation occurs in between sub-picoseconds and picoseconds [9, 10, 23, 28–33]. Both fluorescence lifetime and relaxation depend on the solvent polarity.

Photoexcitation of DCM to its first absorption band put the excited molecule in the S1/LE state, and subsequently two conformational changes may happen. Firstly, –C=C bond rotation leading to trans and cis isomerization via a phantom singlet state which is a typical photochemical process occurring on trans-stilbene [36] and many olefin molecules [37]. Secondly, twisting of the N,N-dimethylamino group may give rise to a highly polar twisted intramolecular charge-transfer (TICT) state which can be stabilized in polar media like 4-dimethyl-aminobenzonitrile (DMABN) molecule [37, 38]. However, the transition from the LE state to the CT (or TICT) state is under debate, and from both experimental and theoretical calculations [39], the following widely accepted dynamical behavior has been proposed to understand the excited-state dynamics of DCM dye. The potential energy surface (PES) of the LE state (S1) for twisting motion of the central C=C bond (which bridges N,N-dimethylamino group with pyran group) is calculated to be very small (0.2 eV), and the barrier height is insensitive to the polarity of solvent. However, the shape of excited-state PESs of for the twisting motion of the CN single bond of the N,N-dimethylamino group of DCM is strongly influenced by the polarity of the solvent [39]. Moreover, in a polar media, the energy of the S1/LE state increases, whereas the energy of the S2/CT state decreases by twisting the CN single bond of the dimethylamino group and leads to a nonadiabatic curve crossing between the two states. Therefore, the formation of an emissive TICT state along the amino group twisting coordinate is more favored with increasing the polarity of the solvent. Trans and cis isomerization is dominated in polar solvents because of the increased the energy barrier in the TICT state along the torsional coordinate of the C=C double bond when the TICT state is formed at the perpendicular geometry where the energy of the S1/LE state is higher than that of the S2/CT state.
