Effects of Proaromaticity on Excited-State Lifetimes and Charge Separation in Near-Infrared Sensitizer Dyes in Solution and on TiO₂

Abstract

The influence of proaromaticity on the excited state dynamics of near-infrared (NIR)-absorbing sensitizer dyes is explored using a combination of ultrafast transient absorption spectroscopy (TAS) and computational chemistry. The addition of a proaromatic π-bridge was found to stabilize the excited state and lead to lower excitation energies in solution and longer excitedstate lifetimes, contrary to what is expected by the energy gap law. When studied under standard device conditions on TiO 2 , it was found that the dye structure plays a significant role in determining excited-state dynamics. Computational chemistry results confirm the proaromatic nature of the dyes through both bond length analyses and nucleus-independent chemical shift (NICS) calculations. Through incorporation of excited-state aromaticity, a 10× increase in excited-state lifetime was observed for dyes with a near 0.5 V lower energy excited state.

Phospha-RosIndolizine Dye with Shortwave Infrared (SWIR) Absorption and Emission

“…NICS ZZ (avg) values reported in Figure 2c are the average of the NICS ZZ (+1) and NICS ZZ (−1) values (at 1 Å above and below each ring). Consistent with previous reports, 57,71 the T 1 state is used as an approximation of the excited state since the magnetic shielding tensors are not available via TD-DFT for the S 1 state with our computational This shift in ring D from antiaromatic in the ground state to more aromatic in the excited state follows closely with Baird's rule, 72 since the central ring can be drawn as a homoaromatic 4n π-electron cationic system consistent with weak antiaromaticity (Figure 2c). Also, this trend is seen when comparing PRos1450 to our previous NICS calculations 47 on tol RosIndz and SiRos1300, where tol RosIndz changes from being aromatic in the ground state (−4.8 ppm) to antiaromatic in the excited state (+11.0 ppm) and SiRos1300 remains antiaromatic in both the ground state (+9.1 ppm) and the excited state (+7.5 ppm).…”

“…NICS ZZ (avg) values reported in Figure c are the average of the NICS ZZ (+1) and NICS ZZ (−1) values (at 1 Å above and below each ring). Consistent with previous reports, , the T 1 state is used as an approximation of the excited state since the magnetic shielding tensors are not available via TD-DFT for the S 1 state with our computational software. The indolizine rings are aromatic in the ground state (A and G = −10.9 ppm, B and F = −15.8 ppm) and remain aromatic in the excited state but slightly less so (A and G = −7.5 ppm, B and F = −10.4 ppm).…”

“…Similar to previous studies of comparable NIR and SWIR dyes, ,,, all computational calculations were conducted at the B3LYP/6-311G­(d,p) − level of theory using a DCM polarizable continuum model − as implicit solvent with Gaussian16 software. Density functional theory (DFT) calculations (Table S1 and Figure b) show that the highest occupied molecular orbital (HOMO) is raised significantly (643 mV) switching from an amine donor ( 1b ) to an indolizine donor ( PRos1450 ), which helps to shrink the HOMO–LUMO energy gap ( E g H‑L ).…”

Switch-on near infrared emission in albumin behind dark fabric: toward application in forensic latent bloodstain detection

“…The relatively long lifetimes observed for this spectral region could improve visibility in NIR photography. 47–51…”

“…The relatively long lifetimes observed for this spectral region could improve visibility in NIR photography. [47][48][49][50][51] An important aspect of applicability in eld work is stability of the dye or dye-albumin complex. Two samples were prepared with SO3C7 in HSA: one was kept in a dark box and one was kept in ambient indoor lighting for 5 days and emission was collected each day.…”