The photophysical properties of a julolidene-based molecular rotor

Abstract

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.

Fluorescence Anisotropy of Molecular Rotors

“…For perhaps the most widely studied molecular rotor, 9-(2,2-dicyanovinyl)julolidine (DCVJ) it was initially proposed that upon photoexcitation a twisted intramolecular charge transfer (TICT) state is formed, [14] with investigations to determine the nature of the intramolecular rotation and about which bond it occurs. [15] More recently there has been a compelling description of the photophysics of DCVJ and a related malononitrile which describe the formation of a zwitterion via photoisomerization following excitation. This was proposed as the dominant mechanism for non-radiative de-excitation through internal conversion rather than the formation of a TICT state.…”

Apparent Shear Sensitivity of Molecular Rotors in Various Solvents

“…In the latter case, emission is either red-shifted [9] or completely nonradiative [10]. Simulations have shown that a representative molecular rotor, 9-(dicyanovinyl)-julolidine (DCVJ) has a preferred (i.e., lowest-energy) planar conformation in the ground state, whereas it assumes a lowestenergy twisted conformation at 90 • in the excited state [11]. The reason for the nonradiative deexcitation lies in the low S 1 − S 0 energy gap in the twisted state.…”

Vapochromic features of new luminogens based on julolidine-containing styrene copolymers

“…For DCVJ, indeed, the twisted-state S1-S0 energy gap is three times smaller than the LE energy gap, as a consequence, fluorophores of this class exhibit only a single emission band. 41 The most noteworthy feature of FMRs is the dependency of the twisted state formation rate on the local microenvironment, predominantly the microviscosity of the solvent. This in turn affects the optical behaviour of the molecule.…”

“…As a consequence, isomerisation is likely to compete very well with radiative decay of the excited state. 41 It is worth noting that computational calculations do not take in account solvent influences on the energy levels, however DCVJ emission exhibit only a slight solvatochromic shift whereas its fluorescence quantum yield is strongly influenced by the local microviscosity. 15 It has been proved that the ICT also occurs in case of one cyano group as electron accepting unit, thus is it possible to modify the chemical structure of DCVJ aiming to confer additional chemical properties to the molecule without adversely affecting its behaviour.…”

“…39 The molecule assumes a non-planar conformation, with a lower excited state energy but higher ground state energy, therefore the relaxation from this twisted state is associated with either a red-shifted fluorescence emission or non-radiative decay depending on the structure of the considered fluorophore. [39][40][41][42][43] As aforementioned, relaxation from the TICT state may occur in a radiative rather than non-radiative way depending on the molecular structure. In the case of DMABN, for example, the S1-S0 gap in the twisted state is large enough to allow a radiative emission when the molecule returns to the ground state in the twisted conformation.…”