Subtle Structural Change in the Exciton-Forming Dye Drives Plasmon–Exciton Interaction Involving Gold Nanorods into the Strong Coupling Regime: Implications for Enhanced Optical Processes

Abstract

Coherent energy exchange between plasmons and excitons has attracted considerable interest from researchers worldwide because of their potential applications in enhanced optical processes, quantum information processing and quantum devices, artificial light harvesting, digital data storage, chemical and biological sensing, and so forth. The energy exchange between a plasmon and an exciton shows a strong dependence on various plasmonic and excitonic parameters. Here, we investigate the role of a subtle change in the chemical structure of an exciton-forming dye in controlling plasmon–exciton interaction at the nanoscale. The only difference between the two exciton-forming dyes used in this study is that one of them (Cy78) has an ethyl group on its polymethine chain, but the other (Cy75) does not. The two plexciton hybrids of the J-aggregates of Cy78 and Cy75 with the same gold nanorod are expected to show very similar plasmon–exciton interactions. However, using ensemble-level extinction as well as single-particle-level scattering spectroscopy, we reveal that, contrary to our expectations, two plexciton systems show drastically different plasmon–exciton coupling strengths. Further investigations into the origin of this unexpected observation using fluorescence spectroscopy and atomic force microscopy, complemented by density functional calculations, reveal that Cy75 exclusively exists in trans-conformation, while the presence of the ethyl substituent in Cy78 gives rise to the coexistence of cis- and trans-conformations. The presence of cis-isomers leads to the enhancement of the overall dipole moment of Cy78 J-aggregates and thereby enhancing the Rabi splitting of its plexciton hybrid by ∼100% compared to Cy75-plexciton. This comprehensive study clearly demonstrates how even an apparently “innocent” and extremely subtle change in the structure of an exciton-forming dye can drive nanoscale plasmon–exciton coupling from one coupling regime to another.

Tuning nanoscale plasmon–exciton coupling via chemical interface damping

“…The LSPR maxima of all three Au NRs were chosen to be ∼650 nm because the exciton used (J-aggregates of Cy78) shows a plexciton transparency dip at ∼650 nm when coupled to a plasmonic system. 44 Choosing the LSPR of all the nanorods identical and coinciding the plexcitonic dip wavelength allows us to measure the Rabi splitting directly from the extinction spectra.…”

“…31 Next, we prepared the plexciton hybrids of each Au nanorod (CTAB-capped as well as 4-NTP modified) sample using a J-aggregate forming cyanine dye, (5-chloro-2-[2-[5-chloro-3-(4sulfobutyl)-3H-benzothiazol-2-ylidenemethyl]-but-1-enyl]-3-(4sulfobutyl)-benzothiazol-3-ium hydroxide, triethylammonium salt) or Cy78, following our previously developed protocol ( please see section S6 of the ESI † for a brief description). 3,19,22,44,45 The concentration of Cy78 was chosen such that a saturated coverage of dye J-aggregates on the nanorod surface is ensured. The saturated coverage of the nanorod surface with excitons allows us to rationally compare the coupling strengths of different plexciton hybrids.…”

Plasmon–Exciton Interaction at the Nanoscale: Silver Is More “Precious” than Gold!

“…Since the LSPR positions of both Au NP and Ag NP were chosen to be resonant with the transparency dip position shown by Cy78 J-aggregate while coupled to a plasmonic system, our ensemble-level extinction measurements of the plexciton hybrids directly allows us to estimate the Rabi splitting (satisfying the zero-detuning condition). ,, Using different gold nanorods it has been shown earlier that an increase in plasmon linewidth due to radiation damping weakens the plasmon–exciton coupling . Increase in mode-volume too has been found to have the same effect on plasmon–exciton coupling strength. ,, In different studies we demonstrated that effective plasmon mode-volume ( V m ) dominates over plasmon decay effects (radiation damping or electron surface scattering damping) in controlling plasmon–exciton coupling. ,, If the geometric volume accurately represents V m , then the plasmon mode-volume of the silver nanoprism should be greater than that of the gold nanoprism. , On the other hand, if the mode-volume is only a fraction of the geometric volume, a precise calculation of the actual plasmon mode-volume is necessary. If the larger geometric volume-induced larger radiation damping alone would control the plasmon–exciton coupling strength, then our Ag nanoprism would show much weaker plasmon–exciton coupling compared to that of its gold counterpart.…”

“…At this point we also numerically simulated the optical spectra of the experimentally studied plasmonic and plexcitonic systems using the commercially available software package “FDTD solutions” (Ansys Lumerical 2022 R1; details of simulation technique can be found elsewhere, ,,,, and a brief description has been provided in section S5 of the Supporting Information) to complement our experimental results. We used the dimensional parameters obtained from the TEM images of the nanoprisms to calculate the optical spectra of Au and Ag nanoprisms as well as their plexciton hybrids.…”

“…We chose nanoprism structures as a case study and due to ease of synthesis. We utilized molecular J-aggregates of the cyanine dye “Cy78”, i.e., 5-Chloro-2-[2-[5-chloro-3-(4-sulfobutyl)-3H-benzothiazol-2-ylidenemethyl]-but-1-enyl]-3-(4-sulfobutyl)-benzothiazol-3-ium hydroxide, triethylammonium salt as the excitonic component, which shows transparency dip at around 650 nm whenever coupled to a plasmonic system. , Silver and gold nanoprisms were synthesized by following already established methods (see section S1 in the Supporting Information), having their LSPR maxima position at ∼650 nm. We prepared individual plexciton hybrids of both silver and gold nanoprisms separately with J-aggregates of Cy78 to investigate their plasmon–exciton coupling strengths.…”