Autofluorescence is a major challenge in complex tissue imaging when molecules present in the biological tissue compete with the fluorophore. This issue may be resolved by designing organic molecules with long fluorescence lifetimes. The present work reports the two-photon absorption (TPA) properties of a thermally activated delayed fluorescence (TADF) molecule with carbazole as the electron donor and dicyanobenzene as the electron acceptor (i.e., 4CzIPN). The results indicate that 4CzIPN exhibits a moderate TPA cross-section ($9 Â 10 À50 cm 4 s photon À1), high fluorescence quantum yield, and a long fluorescence lifetime ($1.47 ls). 4CzIPN was compactly encapsulated into an amphiphilic copolymer via nanoprecipitation to achieve water-soluble organic dots. Interestingly, 4CzIPN organic dots have been utilized in applications involving two-photon fluorescence lifetime imaging (FLIM). Our work aptly demonstrates that TADF molecules are promising candidates of nonlinear optical probes for developing next-generation multiphoton FLIM applications.
In this review, we summarize the computationally efficient time-dependent approaches to calculating the vibronic spectra including one-photon/two-photon absorption (OPA/TPA), emission, circular dichroism (CD), and resonance Raman/hyperRaman scattering (RRS/RHRS) spectra with inclusion of the mode-mixing, Frank-Condon (FC) and Herzberg-Teller (HT) effects. At first, a unified dependency of spectral differential cross sections (SDCSs) on a generalized linear polarizability is derived in view of the relationship between SDCSs and the linear or nonlinear polarizabilities. Then, the Green's function technique is adopted to calculate the polarizability to avoid the tedious summation over the large number of vibrational states. To demonstrate the computational accuracy and efficiency, we apply the generalized time-dependent approaches to calculate the OPA(E)/TPA, RRS, and RHRS spectra of a series of radicals and organic molecules. The computational accuracy is demonstrated by a close comparison between the simulated spectra and the available experimental data. In the spectral calculations, we apply our recently developed analytical energy-derivative approaches for the excited states within the framework of time-dependent density functional theory (TDDFT) to calculate the electronic-structure parameters which enter the expressions of SDCSs, such as the excited-state geometries, energy gradient, harmonic vibrational frequencies, the geometrical derivatives of transition dipole moments, and so forth. It is found that the integrated approach which combines the time-dependent approach to SDCSs with the analytical energy-derivative approaches for TDDFT excited-state structure parameters breaks the bottlenecks of vibronic spectroscopy calculation down.
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