Secondary through-space interactions facilitated single-molecule white-light emission from clusteroluminogens

Clusteroluminogens refer to some non-conjugated molecules that show visible light and unique electronic properties with through-space interactions due to the formation of aggregates. Although mature and systematic theories of molecular photophysics have been developed to study conventional conjugated chromophores, it is still challenging to endow clusteroluminogens with designed photophysical properties by manipulating through-space interactions. Herein, three clusteroluminogens with non-conjugated donor-acceptor structures and different halide substituents are designed and synthesized. These compounds show multiple emissions and even single-molecule white-light emission in the crystalline state. The intensity ratio of these emissions is easily manipulated by changing the halide atom and excitation wavelength. Experimental and theoretical results successfully disclose the electronic nature of these multiple emissions: through-space conjugation for short-wavelength fluorescence, through-space charge transfer based on secondary through-space interactions for long-wavelength fluorescence, and room-temperature phosphorescence. The introduction of secondary through-space interactions to clusteroluminogens not only enriches their varieties of photophysical properties but also inspires the establishment of novel aggregate photophysics for clusteroluminescence.


N-tritylethane-1,2-diamine (TPM-NH 2 )
7.48 mL ethylenediamine (112 mmol, 6.74 g) was added into a 250 mL two-necked flask with 70 mL dichloromethane (DCM) and cooled in ice water to keep the temperature at 0 °C. 3.90 g triphenylmethyl chloride (14 mmol) dissolved in 20 mL DCM was added into the flask drop by drop for 20 mins. The mixture was stirred at 0 °C for 2 h and at room temperature for another 7 h under nitrogen gas (Supplementary Scheme 1). Then the reaction mixture was washed with saturated NaHCO3 solution three times. The organic layer was separated and dried with enough anhydrous sodium sulfate. After filtration, the filtrate was evaporated under reduced pressure, and the crude product was purified on a silica gel column using DCM/MeOH mixture (10/1, v/v) as the eluent.
3.06 g of TPM-NH 2 was obtained as light-yellow oil in a 73% yield. 1

(E)-2-[(4-chlorobenzylidene)amino]-N-tritylethan-1-amine (TPMI-Cl)
TPM-NH 2 (1.00 g, 3.31 mmol) and 4-chlorobenzaldehyde (0.51 g, 3.64 mmol) with a molar ratio of 1:1.1 were added into a 100 mL two-necked flask with 30 mL ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas (Supplementary Scheme 1). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was purified through recrystallization in an ethanol/water mixture at least three times. 0.96 g of TPMI-Cl was obtained as colorless crystals in a 69% yield. 1

(E)-2-[(4-bromobenzylidene)amino]-N-tritylethan-1-amine (TPMI-Br)
TPM-NH 2 (1.00 g, 3.31 mmol) and 4-bromobenzaldehyde (0.67 g, 3.64 mmol) with a molar ratio of 1:1.1 were added into a 100 mL two-necked flask with 30 mL ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas (Supplementary Scheme 1). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was purified through recrystallization in an ethanol/water mixture at least three times.

(E)-2-[(4-iodobenzylidene)amino]-N-tritylethan-1-amine (TPMI-I)
TPM-NH 2 (1.00 g, 3.31 mmol) and 4-iodobenzaldehyde (0.84 g, 3.64 mmol) with a molar ratio of 1:1.1 were added into a 100 mL two-necked flask with 30 mL absolute ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas (Supplementary Scheme 1). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was purified through recrystallization in an ethanol/water mixture at least three times.

Recrystallization protocol
The obtained products (TPMI-Cl, TPM-Br, and TPMI-I) were added to a 250 mL two-necked flask with 80 mL absolute ethanol. The mixture was heated to 80 °C in the oil bath, and about 20 mL of deionized water was added to form a clear and saturated solution. Then the temperature was slowly decreased to room temperature within 6 hours, and the flask was left to stand for another 6 hours. Bulk crystals were washed with absolute ethanol three times and dried in a vacuum oven at 40 °C for 6 hours. Each compound was recrystallized at least three times, and about 10% of the product was lost during each recrystallization process. Figure 2. Synthetic routes to model compound Me-TPMA and Me-PMI-X (X = Cl, Br, and I).

N-methyl-1,1,1-triphenylmethanamine (Me-TPMA)
3.99 mL aminomethane in absolute ethanol (33 wt. %, 32 mmol, 3.01 g) was added into a 100 mL two-necked flask with 30 mL dichloromethane (DCM) and cooled in ice water to keep the temperature at 0 °C. 1.11 g triphenylmethyl chloride (4 mmol) dissolved in 10 mL DCM was added into the flask drop by drop for 10 mins. The mixture was stirred at 0 °C for 2 h and at room temperature for another 7 h under nitrogen gas (Supplementary Scheme 2). Then the reaction mixture was washed with saturated NaHCO3 solution three times. The organic layer was separated and dried with enough anhydrous sodium sulfate. After filtration, the filtrate was evaporated under reduced pressure, and the crude product was purified on a silica gel column using DCM/hexane mixture (3/5, v/v) as the eluent. 0.59 g of Me-TPMA was obtained as white powder in a 54% yield. 1

(E)-1-(4-chlorophenyl)-N-methylmethanimine (Me-PMI-Cl)
0.75 mL aminomethane in absolute ethanol (33 wt. %, 6 mmol, 0.56 g) and 4-chlorobenzaldehyde (4 mmol, 0.56 g) with a molar ratio of 1.5:1 were added into a 100 mL two-necked flask with 30 mL ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas until the aldehyde was completely reacted (Supplementary Scheme 2). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was washed with absolute ethanol and evaporated under reduced pressure three times. Due to the low boiling points of aminomethane and acetic acid, they were easily removed. 0.38 g of Me-PMI-Cl was obtained as pale-yellow oil in a 58% yield. 1 H NMR (400 CDCl 3 ),  (ppm): 161.1, 136.4, 134.7, 129.1, 128.9, 48.2

(E)-1-(4-bromophenyl)-N-methylmethanimine (Me-PMI-Br)
0.75 mL aminomethane in absolute ethanol (33 wt. %, 6 mmol, 0.56 g) and 4-bromobenzaldehyde (4 mmol, 0.74 g) with a molar ratio of 1.5:1 were added into a 100 mL two-necked flask with 30 mL ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas until the aldehyde was completely reacted (Supplementary Scheme 2). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was washed with absolute ethanol and evaporated under reduced pressure three times. Due to the low boiling points of aminomethane and acetic acid, they were easily removed. 0.47 g of Me-PMI-Br was obtained as pale-yellow oil in a 60% yield. 1

(E)-1-(4-iodophenyl)-N-methylmethanimine (Me-PMI-I)
0.75 mL aminomethane in absolute ethanol (33 wt. %, 6 mmol, 0.56 g) and 4-iodobenzaldehyde (4 mmol, 0.93 g) with a molar ratio of 1.5:1 were added into a 100 mL two-necked flask with 30 mL ethanol. Several drops of acetic acid were added to the mixture as the catalyst. The mixture was stirred at 70 °C for 12 h under nitrogen gas until the aldehyde was completely reacted (Supplementary Scheme 2). The mixture was cooled to room temperature and evaporated under reduced pressure. The product was washed with absolute ethanol and evaporated under reduced pressure three times. Due to the low boiling points of aminomethane and acetic acid, they were easily removed. 0.50 g of Me-PMI-I was obtained as pale-yellow solid in a 51% yield. 1