Excited State Tuning of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores by Push–Pull Effects and Bite Angle Optimization: A Comprehensive Experimental and Theoretical Study

Abstract

The synergy of push-pull substitution and enlarged ligand bite angles has been used in functionalized heteroleptic bis(tridentate) polypyridine complexes of ruthenium(II) to shift the (1) MLCT absorption and the (3) MLCT emission to lower energy, enhance the emission quantum yield, and to prolong the (3) MLCT excited-state lifetime. In these complexes, that is, [Ru(ddpd)(EtOOC-tpy)][PF6 ]2 , [Ru(ddpd-NH2 )(EtOOC-tpy)][PF6 ]2 , [Ru(ddpd){(MeOOC)3 -tpy}][PF6 ]2 , and [Ru(ddpd-NH2 ){(EtOOC)3 -tpy}][PF6 ]2 the combination of the electron-accepting 2,2';6',2''-terpyridine (tpy) ligand equipped with one or three COOR substituents with the electron-donating N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine (ddpd) ligand decorated with none or one NH2 group enforces spatially separated and orthogonal frontier orbitals with a small HOMO-LUMO gap resulting in low-energy (1) MLCT and (3) MLCT states. The extended bite angle of the ddpd ligand increases the ligand field splitting and pushes the deactivating (3) MC state to higher energy. The properties of the new isomerically pure mixed ligand complexes have been studied by using electrochemistry, UV/Vis absorption spectroscopy, static and time-resolved luminescence spectroscopy, and transient absorption spectroscopy. The experimental data were rationalized by using density functional calculations on differently charged species (charge n=0-4) and on triplet excited states ((3) MLCT and (3) MC) as well as by time-dependent density functional calculations (excited singlet states).

Engineering Tunable Single and Dual Optical Emission from Ru(II)–Polypyridyl Complexes through Excited State Design

“…[18][19][20][21][22][23][24][25][26] The further design and control of optical materials based on Ru-polypyridyl will require an explanation and predictive prescription for, perhaps, their most intriguing optical aspect -the observation of single and dual optical emission. [27][28][29][30][31][32][33][34][35][36][37][38] In general, dual emission properties in organometallic complexes may arise from two different 3 MLCT states involving different ligands, from types of states e.g. 3 MLCT, 3 IL or 3 LL'CT or based on different metals in dissymmetric bimetallics complexes.…”

Push‐Pull Design of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores as Deep Red Light Emitters in Light‐Emitting Electrochemical Cells

“…[3](PF 6 ) 2 is oxidized at the highest potential because of the electron-withdrawing effect of the three ester groups. [46] The first reduction potential, E 1/2 red , follows the same order [1] [3](PF 6 ) 2 seems to be only quasireversible. [35,36,46] Thus, for all complexes sufficient reversibility in oxidation and reduction processes is given for the application in LECs.…”

“…[35][36][37]46] The oxidation potential, E 1/2 ox , follows the order Table 1). [1](PF 6 ) 2 is oxidized at the lowest potential because of the electron-donating effect of the NH 2 substituent.…”

“…[1](PF 6 ) 2 -[3](PF 6 ) 2 are photostable and chemically stable as a result of their tridentate coordination mode; they feature a sophisticated push-pull substitution with high directionality due to electron-withdrawing ester and electron-donating amino groups. [35][36][37][46][47][48] The push-pull substitution lowers the HOMO-LUMO gap (and hence increases the energy difference between the 3 MLCT and 3 MC states) and leads to emission in the red spectral region (λ em = 729-744 nm, Table 1). [35,36,46,47] Red light and NIR emission are particularly favorable for noninvasive bioimaging, telecommunication, night-vision-readable displays, downconversion and triplet-triplet annihilation upconversion.…”

“…[35][36][37][46][47][48] The push-pull substitution lowers the HOMO-LUMO gap (and hence increases the energy difference between the 3 MLCT and 3 MC states) and leads to emission in the red spectral region (λ em = 729-744 nm, Table 1). [35,36,46,47] Red light and NIR emission are particularly favorable for noninvasive bioimaging, telecommunication, night-vision-readable displays, downconversion and triplet-triplet annihilation upconversion. [1,2,[49][50][51][52][53][54][55][56][57] However, according to the energy gap law, low-energy emission is correlated to low luminescence quantum yields and short luminescence lifetimes due to effective radiationless deactivation into the ground state.…”