Bulky Substituents Promote Triplet-Triplet Annihilation Over Triplet Excimer Formation in Naphthalene Derivatives

Abstract

Visible-to-ultraviolet (UV) triplet-triplet annihilation photochemical upconversion (TTA-UC) has gained a lot of attention recently due to its potential for driving demanding high-energy photoreactions using low-intensity visible light. The efficiency of this process has rapidly improved in the past few years, in part thanks to the recently discovered annihilator compound 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (N-2TIPS). Despite its beneficial TTA-UC characteristics, the success of N-2TIPS in this context is not yet fully understood. In this work, we seek to elucidate what role the specific type and number of substituents in naphthalene annihilator compounds play to achieve the characteristics sought after for TTA-UC. We show that the type of substituent attached to the naphthalene core is crucial for its performance as an annihilator. More specifically, we argue that the choice of substituent dictates to what degree the sensitized triplets form excimer complexes with ground state annihilators of the same type, which is a process competing with that of TTA. The addition of more bulky substituents positively impacts the upconverting ability by impeding excimer formation on the triplet surface, an effect that is enhanced with the number of substituents. The presence of triplet excimers is confirmed from transient absorption measurements, and the excimer formation rate is quantified, showing several orders of magnitude differences between different derivatives. These insights will aid in the further development of annihilator compounds for solar energy applications for which the behavior at low incident powers is of particular significance.

Figure 1

(a) Molecular structures of the TXS substituents used for the syntheses of N-1TXS and N-2TXS. (b) Simplified synthesis scheme and molecular structures of N-1TXS and N-2TXS compounds. (c) Molecular structure of the sensitizer 4CzBN and normalized steady-state absorption and emission (filled in) spectra of 4CzBN. Purple spectrum is fluorescence in toluene, and red spectrum is phosphorescence in methyl tetrahydrofuran at 100 K. (d) Normalized steady-state absorption and emission (filled in) spectra of optically dilute samples of N-1TXS and N-2TXS in toluene.

Figure 2

Jablonski diagram depicting the photophysical processes involved in TADF-sensitized TTA-UC. Experimentally determined singlet and triplet excited state energies are indicated for 4CzBN (black), N-1TXS (deep purple), and N-2TXS (light purple).

Figure 3

Time-resolved upconverted emission of 25 μM 4CzBN and N-2TMS for different [¹A]₀. (a) [¹A]₀ = 100 μM, (b) 1 mM, (c) 2 mM, and (d) 3 mM. Emission measured at 375 nm upon 405 nm pulsed excitation at different intensities ranging from 56 mW cm^–2 to 17.5 W cm^–2. Black lines are the best global fits of eq 1 using a shared τ_T. Note that the time scales are different for each panel.

Figure 4

Linear fittings of eq 2 to obtained triplet lifetime data at varying [¹A]₀ for (a) N-1TXS and (b) N-2TXS. Note that the y axis scales are different for panels (a) and (b).

Figure 5

Transient absorption spectra of (a) 8 mM N-1TMS and 25 μM 4CzBN in deaerated toluene and (b) 0.1 mM N-1TMS and 25 μM 4CzBN; l_ex = 410 nm, 1.4 mJ/pulse. Insets show the absorption features of relevance.