RESUMO
Exciton delocalization relates to many important photophysical processes such as excitation energy transfer, charge separation, and singlet fission. Here, we analyze the exciton delocalization through the photophysical measurements of the molecular crystal 2,2'-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-methylphenol) (m-MTTM), which is the segregated HJ-aggregate confirmed by the calculation of exciton coupling along each direction in the crystal structure. Linearly polarized steady-state absorption spectroscopy verifies that the red-shifted optical transition majorly arises from the aggregates unparalleled to the a-axis. In addition, the temperature-dependent emission spectra show the increase of 0-0 versus 0-1 vibronic emission ratio as the temperature decreases with the coherence number equaling 2.2-1.0 at 140-200 K, which is the characteristic behavior of J-aggregates. To elaborate these observations, we carry out the simulation with the Holstein-type Hamiltonian considering short-range charge-transfer-mediated couplings (perturbative regime) under the two-particle approximation, showing that the 3 × 3 laminar-like aggregates in the ac-plane and the 3 × 3 × 2 three-dimensional aggregates fit well with the emission spectrum at 140 K. In the 3 × 3 aggregates, the coherence function in the ac-plane shows the in-phase correlation along (1,0,-1), elucidating how J-aggregates form in segregated HJ-aggregates with dominant positive coupling. Under the strong intralayer out-of-phase correlation, the 3 × 3 × 2 aggregates demonstrate that the vibronic coupling has a great impact on the interlayer correlation. Furthermore, the coherence function along (0,1/2,-1/2) and (-1,1/2,-1/2) exhibits the thermal-activated phase flipping. These discoveries pave the ways for further manipulations of exciton delocalization in three-dimensional molecular solids.
RESUMO
We report O-H----S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-dimethyl-2,3-dihydro-1 H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond. Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study found that the lowest lying excited state (S1) of DM-7HIT is a sulfur nonbonding (n) to π* transition, which undergoes O-H bond flipping from S1(nπ*) to the non-H-bonded S'1(nπ*) state, followed by intersystem crossing and internal conversion to populate the T'1(nπ*) state. Fast H-bond on/off switching then takes place between T'1(nπ*) and T1(nπ*), forming a pre-equilibrium that affords both the T'1(nπ*, 685 nm) and T1(nπ*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, was rigorously evaluated with a variety of other H-bonded thiones, and these results open a new chapter in the chemistry of hydrogen bonds.
RESUMO
The compound 6-azaindole undergoes self-assembly by formation of N(1)-Hâ â â N(6) hydrogen bonds (H bonds), forming a cyclic, triply H-bonded trimer. The formation phenomenon is visualized by scanning tunneling microscopy. Remarkably, the H-bonded trimer undergoes excited-state triple proton transfer (ESTPT), resulting in a proton-transfer tautomer emission maximized at 435â nm (325â nm of the normal emission) in cyclohexane. Computational approaches affirm the thermodynamically favorable H-bonded trimer formation and the associated ESTPT reaction. Thus, nearly half a century after Michael Kasha discovered the double H-bonded dimer of 7-azaindole and its associated excited-state double-proton-transfer reaction, the triply H-bonded trimer formation of 6-azaindole and its ESTPT reaction are demonstrated.
