Abstract
By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO2, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.
Highlights
Plenty of synthesizing organic dyes utilized in dye-sensitized solar cells (DSSCs), are based on the donor−linker−acceptor (D−π−A) architecture.[1−5] Generally, these photosensitizers utilized in DSSCs have carboxylic acid units (COOH) as anchoring groups for strong adsorption on semiconductor surfaces
We have shown that rhodanine moieties in the indoline dyes are responsible for the dye’s emission quenching by external protons
Quantum chemical calculations showed that ground-state complexation essentially happens through carboxylic acid groups, and that in the excited state, the carbonyl oxygens of the rhodanine rings are liable to strongly interact with proton donors and undergo protonation
Summary
Plenty of synthesizing organic dyes utilized in dye-sensitized solar cells (DSSCs), are based on the donor−linker−acceptor (D−π−A) architecture.[1−5] Generally, these photosensitizers utilized in DSSCs have carboxylic acid units (COOH) as anchoring groups for strong adsorption on semiconductor surfaces. The resulted free proton is expected to be transferred to an oxygen atom on the metal oxide surfaces, closely to the adsorbed photosensitizer.[6−10] the effect of such a close distance between the adsorbed proton and sensitizer on the excited-state dynamics of adsorbed sensitizers has not been investigated so far To study such an effect, we selected the indoline dyes as a case of study, as it has been shown that protons can dramatically quench the excited-state lifetime of these dyes in solution.[11] Indoline dyes consist of an indoline donor moiety, and different acceptor units; the better-known ones are D102, D149, D205, and D131 (see Figure 1 for molecular structures). The D149 is the best-performing dye within its family (9%),[14,15] and its excited-state properties have been studied before by several spectroscopic techniques.[16−20] In our Received: August 3, 2020 Revised: September 4, 2020 Published: September 8, 2020
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
