Are you sure you are not seeing Raman scattering from the medium? That will show the effect you are describing. You can check for this by leaving out the fluorescent molecule.

We are sure. Emitted light depends on the nature of molecule and excitation wavelength. Actually, there are two maxima of emission and these bands gradually overlap each other while we are changing wavelength of excitation. An isosbestic point is also presented.

Figure is attached.

Dear Mikhail

 I am quite sure that it is a phenomena known as Red Edge Excitation Shift (REES), also called as Edge Excitation Red Shift (EERS) or Red Edge Effect. But it is observed only for very polar molecules in highly polar and viscous medium in which the molecule is heavily solvated. Red shift is observed when excitation is at the longer wavelength edge of the lowest energy absorption band. In molecules having the reorientation relaxation time larger than the fluorescence time, the fluorescence emission spectrum comes from differently solvated species governed by F-CESD that is a function of excitation energy. If the excitation energy is smaller, only limited configurations of the ground state may be excited.  Thus, the resultant total emission lacks some high energy components and this causes the red shifted emission.

Dear Gopal Singh,

Thanks a lot for the excellent answer!

Dear Mikhail,

you have received excellent answers  (Adam and Gopal) but your posted question IS NOT EXCELLENT !! It remains too vague. Why??

We observed that certain molecules..

What a kind of molecules?? (you can use the message (above right) part if your dyes  or paper are confidential.

Please note that your dyes (e.g. certain molecules) must be very pure and your solvents too. Fluorescence is very sensitive .

Please give the nature of solvent(s) , viscosity   and quality (spectranal...)

Please give the typ of your spectrometer . Are you qorking at R.T  or 77° K?

You must not insert a isosbestic graph (see above)  if you don 't post a graph of your dye(s) with Stokes 'shift.

Sorry , but you posted your question like a student and NOT LIKE A SCIENTIST !

If you want a general answer to a vague question, please consult the old masters

(e.g Theodor Förster  and other experts) in your Uni library  PRIOR you post a question.

JRG

"What a kind of molecules??" -  Cyano-substituted pyridines (I'll give an exact structure, if it will be necessary)

"Please note that your dyes must be very pure and your solvents too."  - It's obvious, all structures are pure (confirmed by TLC and NMR spectral data), as well as we use spectrophotometric solvents (MeOH in this case exactly).

"Please give the typ of your spectrometer . Are you qorking at R.T or 77° K?" - We work on Fluorat-02-Panorama (Russian device) at rt. 

"You must not insert a isosbestic graph (see above) if you don 't post a graph of your dye(s) with Stokes 'shift." - My pic describes an behaviour of the emission band under the action of various excitation wavelegths. The point of intersection  was called as 'isosbestic' due to it really looks alike.

Just now, we have got an additional info. Various excitation wavelengths (chosen in the red-edge of absorption band) also give different maxima of emission in linear dependence. Graph is attached (We just changed MeOH to DMF in this case).

Дороой Mikhail Yur'evich Ievlev!

Я смущён Вишим вопросом и согласен с Jean Rene Grezes · 33.12 · 12.6 · Laboratoire Prive.

Если у Вас всё "гладко и сладко" проведите хотя бы температурные измерения.

На основе Ваших данных можно только спекулировать. Нет предмета для научного анализа.

С наилучшими пожеланиями Иван И. Долгов 20160715

Dear Ivan,

It is uncorrect to post in Russian language. Please translate it.

JRG

Ivan wrote about the following:

"Dear Mikhail!

I'm confused by your question and agreed with Jean Rene Grezes.

If you really have the data which are so good as you have described, you should do measurements at different temperatures.

On the basis of your data, only speculate is possible. There is no subject for scientific analysis.

Best regards, Ivan"

Therefore, I request a necessary procedure technique for such measurements or ref. where I can read about. Also, I'm intrested in what this experiment exactly should give us? Sorry for probably stupid questions, this area is really novel for our research group.

Good morning, dear Mikhail Yur'evich.

You’ve woken me up for nothing.

I’m amazed and quite annoyed with your question and your translation of my comment as well.

Your translation and interpreting my comments were taken out of the essence of your question and answer. It does not reflect the context, semantics and vocabulary of the Russian language.

As regards to (I'm confused by your question).

It is not me confused, made a mistake (to embarrass). That You made a mistake (embarrassed), asking an empty question.

Я смущён ≈ I am flabbergasted, surprised and disappointed.

I am flabbergasted, surprised and disappointed

in this context it is equivalent to saying that - You neglected the elementary conventional experimental data reporting rules:

- purpose of the study, a description of the object of research, measurement technique, the external conditions of measurement, the parameters influencing factor, the absolute values of influencing factors and the response to them an object of research in absolute terms.

2. I am flabbergasted, surprised and disappointed (Я смущён) in this context it is equivalent to saying that - because if you do not have a competent head chef, first go to Wikipedia. This is not a site with nicknames.

3. Я смущён ≈ With Your empty question you dropped on your level of your co-authors in other works and quite of the old University and his professors.

4. I am disappointed, because your answer, Jean Rene Grezes, whom I respect, was written in an unacceptable manner.

No one was interested in the name of your scientific instruments, if this is not PR manufacturer (see. Also claim 1).

You can carry out their "great experiment" with the gum and slinky and get a Nobel prize, but must submit their technical characteristics at a time (the time of) their use.

Regarding (“It's obvious, all structures are pure”) The expression "It's obvious" is not for the scientific literature. You must specify a specific impurity concentration (see. Item 1). That purely for you, for others it may be soiled.

5. I wrote, "if you are have all smoothly and sweetly", keeping in mind that you have expensive equipment (NMR), which were to serve a competent professionals. They could explain to you why and how to measure the temperature photoluminescence. I did not write that you have presented well-crafted data.

6. If you do not know for what the temperature measurements carried photoluminescence output, but understand the physical meaning of isobestic graph, having the monochromator and the photometr, you could measure the light transmission of the sample. When measuring the light transmittance also use the concept of isobestic graph.

In addition, scientists who respect readers build isosbestic graph depending on the photon energy or reverse wavelength.

However, I am not your research supervisor to develop a program of research and distracted from the essence of my answer.

Conclusion.

I do not know you, I'm not your follower, etc., etc.

You have no reason to call me Ivan nor when referring to me, or when referring to of my name while interacting with others in a public place.

I did not ask you, and no one else asked you to translate my comment.

You unwittingly or deliberately distorted my answer.

I demand an apology for your unethical behavior towards me at the ResearchGate both in Russian and English languages.

Доброе утро, дорогой Mikhail Yur'evich.

Зря Вы меня разбудили.

Я удивлён и недоволен Вашим вопросом и переводом моего комментария.

Ваш перевод и интерпретация моего комментария были вырваны из сути вопроса и ответа. Они не отражает контекст, семантику и словарь русского языка.

Относительно (I'm confused by your question).

Это не я запутался, сделал ошибку (оконфузился). Это Вы сделали ошибку (оконфузились), задав пустой вопрос.

1. Я смущён ≈ в данном контексте эквивалентно тому, что – Вы пренебрегли элементарными общепринятыми правилами представления экспериментальных данных:

- цель исследования, описание объекта исследования, методика измерений, внешние условия проведения измерений, параметры воздействующего фактора, абсолютные величины воздействующего фактора и отклик на них объекта исследований в абсолютных величинах.

2. Я смущён ≈ в данном контексте эквивалентно тому, что, если у Вас нет грамотного руководителя, зайдите сначала на Википедию. Здесь не сайт с никами.

3.  Я смущён ≈ потому, что своим пустым вопросом Вы опустили на свой уровень Ваших соавторов по другим работам и вполне старый Университет и его преподавателей.

4. Я разочарован, потому что ваш ответ, Jean Rene Grezes, которого я уважаю, был написан неприемлемым способом.

Никого не интересует название Ваших приборов для исследований, если это не PR производителя (см. п.1 также). Вы можете проводить свои «великие эксперименты» с помощью палочки и верёвочки и получить Нобелевскую премию, но необходимо представить их технические характеристики на время (момент) их использования.

Относительно («It's obvious, all structures are pure»). Выражение «Это очевидно» не для научной литературы. Необходимо указывать конкретные концентрации примесей (см. п.1). То, что для Вас чисто, для других может быть нечисто.

5 Я написал: «если у Вас всё гладко и сладко», имея в виду, что у Вас есть дорогое оборудование (ЯМР), которое должны обслуживать грамотные специалисты. Они могли бы вам объяснить, для чего и как проводить температурные измерения фотолюминесценции. Я не писал, что Вы представили хорошо обработанные данные.

6. Если Вы не знаете, для чего проводят температурные измерения выхода фотолюминесценции, но понимаете физический смысл изобестического графика, то, имея монохроматор и фотометр, Вы могли бы измерить светопропускание образца. При измерении светопропускания тоже используют понятие изобестический график.

Кроме того, уважающие читателей и себя учёные строят изобестические графики в зависимости от энергии фотонов или обратной длины волны.

Однако я не Ваш научный руководитель, чтобы разрабатывать Вам программу исследований, и отвлёкся от сути моего ответа.

Заключение.

Я Вас не знаю, я не Ваш последователь и т.д. и т.п.

У Вас нет никаких оснований называть меня Иван ни при обращении ко мне, ни при упоминании моего имени во время общения с другими лицами в общедоступном месте.

Я Вас не просил и никто другой не просил Вас переводить мой комментарий.

Вы по недомыслию или умышленно исказили мой ответ.

Я требую принести извинения за Ваше неэтичное поведение по отношению ко мне на ResearchGate как на английском, так и на русском языке.

Jean Rene Grezes · 33.82 · Laboratoire Prive

Dear Rene,

My apologies for the late reply. I’m on vacation. It’s summer after all.

I didn’t expect that my reply dedicated for Mr. Mikhail Yur'evich only will get your attention.

I was quite annoyed by how the question was put and by their reply to you.

I hope that in my reply to him I was clear as day and explained what I wanted to say.

I always take pleasure in reading your replies and comments.

Sincerely yours, Ivan Dolgov.

Dear Ivan Ivanovich Dolgov,

This site is dedicated to help researchers in solving the various problems.

But the only that I got from your comment is an information that you like bulky and not informative discussions.

Of course I apologize, if you feel that I gave the wrong translation of your answer. It is my fault, my English is not so well. Also I am sorry for the probably uncompleted data have been provided because of my inexperience.

Actually, I reached a goal, when I had recieved the really good and informative answer about the phenomenon of Red Edge Excitation Shift, it really helped our team.

Therefore, now, I suggest to close the topic and wish all the best to the followers and to you especially!

Have a good day!